Polybenzoxazines: Chemistry and Properties

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Polybenzoxazines: Chemistry and Properties

Dr. K.S. Santhosh Kumar Dr. C.P. Reghunadhan Nair

iSmithers – A Smithers Group Company Shawbury, Shrewsbury, Shropshire, SY4 4NR, United Kingdom Telephone: +44 (0)1939 250383 Fax: +44 (0)1939 251118 http://www.ismithers.net

First Published in 2010 by

iSmithers Shawbury, Shrewsbury, Shropshire, SY4 4NR, UK

©2010, Smithers Rapra

All rights reserved. Except as permitted under current legislation no part of this publication may be photocopied, reproduced or distributed in any form or by any means or stored in a database or retrieval system, without the prior permission from the copyright holder. A catalogue record for this book is available from the British Library.

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ISBN: 978-1-84735-500-3 (hardback) 978-1-84735-501-0 (softback) 978-1-84735-502-7 (ebook)

Typeset by Argil Services Printed and bound by Lightning Source Inc.

P

reface

The chemistry of polybenzoxazines blossomed in the mid-1990s. These thermally stable polymeric materials have become very popular during the last decade. They received a lot of attention in view of their potential use as high-performance polymers. This necessitated meticulous investigation of their chemical, mechanical, thermal and surface characteristics. Even though >200 articles have been published and many patents applied for, a book detailing the potential research and development of polybenzoxazines is lacking. We hope that a book describing the up-to-date development of these high-tech polymers will be an important resource for researchers and those working in the polymer industry. Polybenzoxazines obviate many shortcomings associated with their ‘parental’ phenolic resins. They can form void-free materials (due to addition polymerisation) whereas phenolic resins form voids during curing (because of condensation polymerisation). For polybenzoxazines, no catalyst is needed for curing, the moisture uptake is very low, the glass transition temperature is high, and the char yield is high. This book is divided into four chapters. Chapter 1 describes the competency of the properties of polybenzoxazine against other state-of-the-art polymers. Recent developments in parent phenolic resins, epoxy resins, and cyanate esters are addressed. The hydrogen bonding aspects in the crosslinked network of polybenzoxazines, the regioselectivity and kinetics of polymerisation (curing) are also detailed in this chapter. Chapter 2 is devoted entirely to the diverse types of syntheses of benzoxzines and their properties. The syntheses are grouped into (i) benzoxazine with additional polymerisable groups (e.g., allyl, propargyl); and (ii) benzoxazines derived from various precursors (based on several different amines and diphenols) without additional polymerisable moieties. Tailored polymeric systems such as blends, alloys, copolymers, nano-, micro- and fibre composites are described in a systematic manner in Chapter 3. Finally, the stability and degradation of polybenzoxazines under chemical,

iii

Polybenzoxazines: Chemistry and Properties thermal and photochemical environments are discussed in Chapter 4. A concerted effort has been made to include all significant advancements in polybenzoxazine science. However, in this emerging area of high-temperature polymers, novel polymers are constantly being discovered. Suggestions from readers are welcome and will be acknowledged gratefully. We thank R. Sivaramakrishnan and Dr. P. Radhakrishnan Nair, VSSC, for reviewing the manuscript and for giving fruitful suggestions. Thanks are due to the Editorial Board and Director, VSSC, for giving permission to publish the book. We would like to thank iSmithers Publishers for inviting us to write this book. K.S. Santhosh Kumar C.P. Reghunadhan Nair India, 2010 to Vikram Sarabhai Space Centre, Thiruvananthapuram, India, 2010

iv

C

ontents

1

Polybenzoxazines and State-of-the-Art High-Temperature Polymers.................................................................................................1 1.1

Introduction.................................................................................1

1.2

Epoxy Resins...............................................................................1

1.3

1.4

1.2.1

Chemistry of Epoxy Resins..............................................2

1.2.2

Reactions of Epoxy Resins and Curing Mechanisms.......3

1.2.3

Recent Developments......................................................3

Bismaleimides..............................................................................5 1.3.1

Synthesis and Features of BMI.........................................5

1.3.2

Modified BMI..................................................................6

1.3.3

Recent Developments......................................................8

Cyanate Ester Resins..................................................................12 1.4.1

Thermal Curing.............................................................12

1.4.2

Thermal Stability...........................................................13

1.4.3

Modified Systems..........................................................14 1.4.3.1

1.5

Phenolic-triazine Resins.................................15

Phenolic Resins..........................................................................17 1.5.1

Allyl-functional Phenolics..............................................19

1.5.2

Bisoxazolineñphenolics..................................................19

1.5.3

Phenolic ResinñEpoxy Systems......................................20

1.5.4

Propargyl Ether and Ethynyl Novolacs..........................21

1.5.5

Recent Developments in Phenolics.................................23

v

Polybenzoxazines: Chemistry and Properties

1.6

1.7 2

3

Polybenzoxazines.......................................................................24 1.6.1

Features of PBZ.............................................................28

1.6.2

Regioselectivity..............................................................28

1.6.2

Hydrogen-bonding Aspects...........................................31

1.6.4

Cure Monitoring and Kinetics.......................................34

Conclusion.................................................................................36

Structure-Property Relationships..........................................................51 2.1

Introduction...............................................................................51

2.2

Functionalised BZ (BZ Containing Addition-curable Moieties)....................................................................................52 2.2.1

BZ-Containing Allyl Groups.........................................52

2.2.2

BZ Containing Maleimide Groups................................55

2.2.3

BZ-Containing Propargyl Groups..................................59

2.2.4

BZ-Containing Furan Groups........................................61

2.2.5

BZ-Containing Acetylene Groups..................................63

2.2.6

BZ-Containing Nitrile Groups.......................................65

2.2.7

Main-chain or High-molecular-weight PBZ...................66

2.3

BZ Derived from Various Precursors (Non-functionalised BZ)............................................................77

2.4

Conclusion.................................................................................91

Blends and Composites of Polybenzoxazines........................................95 3.1

Introduction...............................................................................95

3.2

PBZ-Epoxy Blends.....................................................................95

3.3

PBZ-Poly(e-Caprolactone) Blends..............................................97

3.4

PBZ-Polyimide Blends................................................................98

3.5

Other Blend Systems................................................................100

3.6

Nanocomposites of PBZ..........................................................112 3.6.1

vi

PBZ-Clay Systems.......................................................112

Contents

4

3.6.2

PBZ-Polyhedral Oligomeric Silsesquioxane (POSS) Systems........................................................................114

3.6.3

Miscellaneous Nanocomposites...................................115

3.7

Fibre Composites and Microcomposites..................................115

3.8

Conclusion...............................................................................126

Stability, Degradation Chemistry and Applications.............................131 4.1

Introduction.............................................................................131

4.2

Chemical Stability....................................................................132

4.3

Thermal Stability and Degradation..........................................133

4.4

UV Stability and Photochemical Degradation..........................140

4.5

Degradation Mechanism of PBZ: A Comparison with other Thermosets.............................................................141

4.6

Applications.............................................................................145

4.7

Conclusion...............................................................................149

Abbreviations................................................................................................153 Index ..........................................................................................................161

vii

Polybenzoxazines: Chemistry and Properties

viii

1

Polybenzoxazines and State-of-the-Art High-Temperature Polymers

1.1 Introduction High-temperature polymers are leading the polymer market because of their variety of applications. They find widespread use in areas such as adhesives, structural applications in aerospace, printed circuit boards, conductive polymer elements, and encapsulation materials for electronic applications [1]. The aerospace industry and space programmes have created new demands for high-temperature polymers which can withstand further higher temperatures owing to their excellent thermal and thermo-oxidative stability, high char yield, good chemical inertness, abrasion resistance and flame retardance. The best known members of the thermoset family are phenolic resins, epoxy resins, unsaturated polyesters, isocyanate-derived polymers, bismaleimides (BMI), acrylates, and cyanate esters [2]. Several challenges must be overcome regarding processing of these materials. This includes higher melting and processing temperatures as well as void formation during curing. In recent years, development of the benzoxazine (BZ)-based family of phenolic resins has attracted significant attention. Polybenzoxazines (PBZ) are a special type of phenolic resins that do not form voids during curing due to addition polymerisation. ‘Traditional’ phenolic resins result in void-full polymers because the polymerisation is of the condensation type. The attractive characteristics of BZ polymers include: low melt viscosity; no release of volatiles during cure and no need of harsh catalysts for curing; high thermal stability; good mechanical properties; excellent electrical properties; and wide flexibility with respect to molecular design. Before the discussion of PBZ chemistry, the state-of-the-art and other competing thermally stable polymeric candidates are addressed in this chapter. This is followed by discussion of the chemistry and fundamentals of PBZ.

1.2 Epoxy Resins Epoxies are the most versatile class of polymers and have diverse applications; they are probably the best-known thermoset polymers. The largest use of epoxies is in

1

Polybenzoxazines: Chemistry and Properties protective coatings; other applications include printed circuit boards, laminates, electronic materials and structural composites. Cured epoxies provide excellent mechanical strength; toughness; outstanding resistance to chemicals, moisture and corrosion; good thermal, adhesive and electrical properties; absence of volatiles and low shrinkage on cure; and good dimensional stability. This unique combination of properties is, in general, not found in any other plastic material. The patent literature indicates that the synthesis of epoxy resins was discovered as early as the late 1890s [3–5]. These materials are used as antenna reflectors, solar panel arrays, and optical support structures in satellites. Epoxy-polyaromatic composite motor cases have replaced the heavy metallic counterparts in modern launch vehicles. Satellites and other components are also made of epoxy/carbon and epoxy/polyaromatic composites.

1.2.1 Chemistry of Epoxy Resins The monomers of epoxy resins are characterised by more than one epoxy ring which undergoes ring opening to produce epoxy resins. Most commercially important epoxy resins are prepared by the coupling reaction of compounds containing at least two active hydrogen atoms (including polyphenolic compounds, monoamines and diamines, aminophenols, heterocyclic imides and amides, aliphatic diols and polyols and dimeric fatty acids) with epichlorohydrin followed by dehydrohalogenation. Epoxy resins derived from epichlorohydrin are termed as ‘glycidyl-based resins’. The most widely used epoxy resins are the diglycidyl ethers of bisphenol A (DGEBA) (Scheme 1.1).

HO

O

OH

CH 2 Cl

Epichlorohydrin

Bisphenol A

OH

O H2 C

O

O

CH 2

O CH 2 O

Typical bisphenol A epoxy resin Scheme 1.1 Typical synthesis of epoxy resins

2

O

CH 2

Polybenzoxazines and State-of-the-Art High-Temperature Polymers Approximately 75% of epoxy resins currently used worldwide are derived from DGEBA. Others include brominated bisphenol A (which imparts fire resistance), epoxy phenol novolac (EPN) resins, bisphenol F epoxy resins, epoxy cresol novolac resins, and tetraglycidyl-4,4′-diaminodiphenylmethane.

1.2.2 Reactions of Epoxy Resins and Curing Mechanisms Crosslinking of the resin can occur through the epoxide or hydroxyl groups. It proceeds by two types of curing mechanism, i.e., direct coupling of resin molecules by a catalytic homopolymerisation, or coupling through a reactive intermediate. The ability of this ring to react by several paths and with various reactants gives epoxy resins their great versatility. The chemistry of most curing agents currently used is based on polyadddition reactions that result in coupling as well as crosslinking. The most widely used curing agents are compounds containing active hydrogen such as polyamines, polyacids, polymercaptans and polyphenols [6–15].

1.2.3 Recent Developments The fully cured fluorinated epoxy resin 1,1-bis (4-glycidylesterphenyl)-1-(3trifluoromethylphenyl)-2,2,2-trifluoroethane was shown to have good thermal stability with a glass transition temperature (Tg) of 170–175 °C and temperature at 5% weight loss of 370–382 °C in N2. It also showed mechanical properties as good as the commercial epoxy resins. It also possessed low water absorption due to the presence of a hydrophobic fluorine atom [16]. A novel imide ring and siloxanecontaining cycloaliphatic epoxy monomeric compound 1,3-bis[3-(4,5-epoxy-1,2,3,6tetrahydrophalimido) propyl] tetramethyl disiloxane (BISE) was thermally cured with alicyclic anhydrides, hexahydro-4-methylphthalic anhydride (HMPA) and hexahydrophthalic anhydride (HHPA), respectively. The fully cured BISE epoxy resins have good thermal stability, high thermal decomposition temperature, and excellent mechanical and dielectric properties. However, they gave a relatively low Tg due to the presence of flexible propyl and siloxane segments in the epoxy backbone [17]. Blends of diethyl tolylene diamine-cured DGEBA epoxy resin have been modified by flexible diamines, which show that the addition of flexible diamines improves the elongation at break and impact strength [18]. New mercaptan-terminated polythiourethanes were used as curing agents for epoxy resin, which found use as effective surface-coating materials [19]. The effect of multiwalled carbon nanotubes (MWNT) on the cure reaction of the epoxy resin showed that, at the early stage of the cure reaction, a low mass fraction of MWNT reduces the activation energy of the reaction. However, excess MWNT hinder the contact

3

Polybenzoxazines: Chemistry and Properties between functional groups, which elevates the activation energy [20]. The epoxycardanol resin exhibits better properties as compared with epoxy resin (DGEBA) in terms of increase in tensile strength, elongation, bonding with steel, and lowering of water vapour transmission of the film [21]. Diallyl bisphenol A (DABA) was used as a curative for novolac epoxy resin (EPN). The reaction was catalysed by triphenyl phosphine (TPP). The activation energy of the system diminished from 91 kJ/mol to 67 kJ/mol on addition of 0.5 wt% TPP [22]. The matrix resin provided a low Tg (80 °C). On initiating further crosslinking by the 100% curing of the allyl groups, the mechanical properties were diminished even though Tg was increased (Table 1.1) [23]. However, the glass composites of the matrix system showed a marked increase in flexural strength from 369 MPa (40% allyl curing) to 458 MPa by complete curing of allyl groups.

Table 1.1 Effect of allyl curing on the properties of a DABA-EPN system Property

40% Allyl cured neat

100% Allyl cured neat

Tensile strength (MPa)

118

77

Flexural strength (MPa)

113

110

Tg (°C)

79

91

A polyhedral oilgomeric silsesquioxane containing eight functional hexafluorine groups called octakis (dimethylsiloxyhexafluoropropyl ether) silsesquioxane (OF) is a nanoporous additive. OF has been synthesised and blended with the ultraviolet (UV)-cured epoxy resin, which resulted in remarkable thermal characteristics. OF containing (10%) epoxy has a significantly lower dielectric constant (2.65) than the neat epoxy (3.71) [24]. Epoxy-phenolic-based foams have also been studied [25]. Epoxy-clay nanocomposites were prepared with organically modified layered clay with varying clay contents (1–8 wt%). The tensile modulus of the nanocomposites increased by 47%, but improvement in tensile strength and Tg was not observed due to the intercalated morphology of clay layers in the epoxy resin systems [26]. Epoxy-coiled carbon nanotube composites have shown high hardness and elastic modulus [27].

4

Polybenzoxazines and State-of-the-Art High-Temperature Polymers

1.3 Bismaleimides Polyimides are manufactured in various forms which find use in the electronic, medical, structural, and adhesive fields. Polyimide films are very popular, particularly in the aerospace field, and are known as ‘Kapton films’ (DuPont). In film form, they do not display a Tg, have good radiation resistance, and are used as bagging material for composites, belts, wire wrapping, and thermal insulation for satellites. Condensation and addition polyimides are used as high-performance matrices. Condensation polyimides are typically based on high-molecular-weight poly(amic acid) precursors that release voids during curing and decompose below their melt temperature. Addition polyimides are low-molecular-weight resins containing unsaturated moieties such as maleimido, ethynyl, and nadimido. They do not form voids when they undergo curing and a void-free matrix is theoretically possible. Among addition polyimides, BMI is the most important system used for advanced material applications due to its high performance-to-cost ratio. However, unmodified BMI show brittleness due to high crosslink density and lower toughness.

1.3.1 Synthesis and Features of BMI BMI resins are synthesised by reacting diamines with maleic anhydride in two steps: formation of bismaleimic acid and imidisation [28]. A typical synthesis is given in Scheme 1.2. O

O

O NH

O O

H2 N

Ar

R

NH

O

N H2

N C O

OH

OH

O

C

C

O

Ar

N O

O

Scheme 1.2 General synthesis of a bismaleimide monomer BMI resins undergo curing initiated at ~150–250 °C and peak temperatures ~230– 330 °C dependent upon the structure. In general, the onset and peak exotherm temperatures increase with the size of the bridging group between two maleimido groups. These results arise from the reduced concentration of the maleimido groups. Cured BMI have excellent heat and thermal resistance, and high Tg and decomposition temperatures. Bis(4-maleimidophenyl)methane and its modified resins are the most important advanced thermosetting materials. These resins possess a differential

5

Polybenzoxazines: Chemistry and Properties scanning calorimetry (DSC) exotherm peak temperature of 260 °C and melting point of 148–158 °C. The mechanical properties are good and have a Tg of 230–290 °C. They have high property retention at elevated temperatures. Electrical properties such as dielectric strength, volume resistivity, dielectric constant, and dissipation factor are generally good for the fabrication of electronic components [29]. However, low elongation to failure and poor toughness are concerns for structural applications. Even though basic BMI resins possess good performance after thermal curing, the toughness of the cured resin is a major concern with respect to material durability. BMI resins modified with soft segments are useful for flexibility improvement but drastically reduce their glass transition temperatures and property retention at elevated temperatures. Incorporation of ether linkages in a BMI resin reduces the brittleness of the cured thermoset. BMI resins are incorporated with aliphatic polyether to modify the fracture strength. Introduction of engineering thermoplastic segments between two bismaleimido groups is an attractive way to achieve high heat resistance and improved toughness. BMI-poly(arylene ethers) show significant increase in their Tg values after curing. Because of the improved solubility (processability) and toughness compared with simple BMI resins, these materials are useful for the preparation of structural composites. These composites have low tensile strength, elongation, good heat resistance and electrical properties similar to phenolic and epoxy composites [30]. BMI resins with low-molecular-weight compounds and oligomers require a significant increase in molecular weight and structure modification to become useful. A few curing reactions, such as thermal polymerisation, addition reactions, and the Diels–Alder reaction have been developed to increase the application performance of BMI resins.

1.3.2 Modified BMI In general, aromatic BMI resins produce high char yield at elevated temperatures and are self-extinguishable. The fire retardance of aromatic BMI can be enhanced by bridging with phosphorus-containing segments [31]. They have a Tg >380 °C and anaerobic char yield of 60–71% at 800 °C. Maleimide-terminated oligomers and maleimide end-capping agents have been investigated for high-temperature applications [32–34]. BMI undergo Michael additions with hydrogen-active moieties such as phenol, thiols, and amines. These reactions are used to prepare various high-performance thermosets. One important class of high-temperature thermoset is derived from BMI resins and allylphenyl oligomers. Upon thermal curing, the mixtures copolymerise to produce polymers with a high Tg. For example, the mixture of 3,3′-DABA and bis(4-maleimidophenyl)methane displays low- and high-temperature exotherms at ~130 °C and ~255 °C, respectively. The low-temperature exotherm was recognised as an ene-reaction between allyl and maleimido groups. The high-temperature exotherm

6

Polybenzoxazines and State-of-the-Art High-Temperature Polymers was related to a Diels–Alder reaction of the functional groups produced in the reaction [35]. The overall reaction, known as the ‘Alder-Ene reaction’, is illustrated in Scheme 1.3. A stiff, heat-resistant ring structure is formed by Diels–Alder polymerisation. It is an excellent approach to increase polymer performance. Soluble BMI with good electrical properties have been synthesised from siloxane-containing bis furans and BMI monomers [36]. The brittleness of the cured thermoset is a major concern for structural applications. Rubber toughening is an effective way to improve the impact resistance, but heat and thermo-oxidative resistance goes down due to the reduction in the Tg and the poor oxidative stability [37].

O HO

N

OH

O R

O

o,o′ diallyl bisphenol-A

O

Bismaleimide Ene addition O

O

HO

O

N

OH

R

O

N

N

HO

O

O

Ene adduct O

Wagner-Jauregg

N

O R

O R

O

O O

N

O

N OH

O

O R

N

HO

N

OH O

O

O

Ene and Diadduct Further crosslinking Crosslinked product

Scheme 1.3 Co-reactions in the blend of DABA and BMI 7

Polybenzoxazines: Chemistry and Properties Diamine-terminated amide resins were prepared to modify bis(4-maleimidophenyl) methane to lower the crosslinking density and to increase the toughness of the cured materials [38, 39]. The same resin has also been chain-extended with aliphatic amines [40]. Polyoxyalkyleneamines are effective chain extenders that significantly improve the flexibility and elasticity of modified resins [41]. Copolymerised BMI materials have been prepared to improve the processability and the performance of the final thermoset [42]. A technology using alkenyl phenols and alkenyl phenol ethers to react with BMI to form processable prepolymers was developed by Zahir and Renner to improve the toughness and humidity resistance of BMI resins [43]. During the crosslinking process, allylphenyl ethers undergo thermal Claisen rearrangements to the corresponding allylphenols, and then proceed with the crosslinking reactions through the Ene–Alder reaction and Diels–Alder reaction. A class of thermosetting resins, N-allyloxyphenyl maleimides, having both allyl and maleimide groups in a molecule have been developed [44]. Polybenzimidazole is a high-temperature thermoplastic polymer with a Tg of 435 °C that has been incorporated into BMI to increase the fractural energy and toughness [45]. Combination of the rigid rod benzimidazole structure, Michael addition of BMI resins, and BMI-allylphenol reactions have resulted in the preparation of molecular composites with the simultaneous improvement of tensile strength, modulus, and elongation [46]. Commercial addition-cure formulations based on co-reaction of diallyl phenols and BMI such as Matrimide-5292 from Ciba-Geigy which typically contain DABA and 4,4′-bismaleimido diphenyl methane (BMM) (Scheme 1.4) are leading matrix resins for carbon fibre composites for advanced aerospace applications.

O

O

N

CH 2

O

N

O

4,4′ Bismaleimido diphenyl methane (BMM)

Scheme 1.4 Structure of BMM

1.3.3 Recent Developments Interpenetrating polymer networks (IPN) of BMI-modified polyurethane-epoxy

8

Polybenzoxazines and State-of-the-Art High-Temperature Polymers systems were prepared by curing polyurethane-modified epoxy with aliphatic or aromatic BMI in the presence of 4,4′-diaminodiphenylmethane. Incorporation of aromatic BMI into the polyurethane-modified epoxy system increased the Tg, thermal stability, and electrical properties. Decreased values of the Tg and heat distortion temperature were obtained in the case of aliphatic BMI-modified polyurethaneepoxy systems [47]. Epoxy systems modified with cyanate ester (CE) were made with diaminodiphenyl methane as the curing agent. The cyanate ester-toughened epoxy systems were further modified with the BMI N,N′-bismaleimido-4,4′-diphenylmethane and N,N′-bismaleimido-4,4′-diphenylsulfone. Incorporation of BMI into unmodified epoxy and CE enhanced the thermal and mechanical properties according to its percentage content [48]. Epoxy-novolac resin modified with 4,4′-bismaleimido diphenyl methane showed that when BMI concentration was high, the adhesive force, degradation temperature and thermal stability were enhanced, whereas the shear strength showed a decreasing trend [49]. Epoxy-based laminate properties were improved with the incorporation of BMI [50]. The BMI modified-novolac resin was synthesised by allylation of the novolac resin and its ‘ene’ reaction with BMI. The BMI-modified allyl novolac resin with 48% degree of allylation has the best thermal properties and the highest dynamic modulus [51]. The curing process of BMI and dicyanate ester monomers containing a naphthalene ring involves copolymerisation between these monomers, and self-additional polymerisation may occur in the last course of the curing process. Investigation into the co-curing of bisphenol A-based CE and the BMI 2,2-bis[4-(4-maleimido phenoxy) phenyl]propane (BMP) indicates no co-reaction between the two, and the system finally formed a sequential IPN [52, 53]. The cured polymer blends were found to undergo two-stage decomposition, with each stage corresponding to the components of the blend. Compositions richer in BMP were found to be relatively brittle and possessed a high propensity to develop microvoids on curing. The system becomes brittle with BMP content of >40%. The 2,2-bis(4-cyanatophenyl)propane (BACY)-rich systems were good only for neat resin casting. The increase in BMP content decreased the tensile properties, whereas these blends possessed improved flexural strength and fracture toughness. The BACY-rich blends showed a single Tg which was observed at a slightly lower Tg than that of polycyanaurate (~250 °C). However, the blends with BMP content of >50% showed two glass transitions due to phase separation. Due to the formation of IPN structure, these blends showed low moisture absorption (1.491.25 wt%). The mechanical properties of BACY/BMP sequential IPN are shown in Table 1.2.

9

Polybenzoxazines: Chemistry and Properties

Table 1.2 Mechanical properties of BACY/BMP IPN matrix systems Content of Fracture BMP in the IPN toughness (MN/ (wt%) m3/2)

Tensile strength (MPa)

Tensile modulus (MPa)

Flexural strength (MPa)

0

3.8

70

3140

95

10

3.5

68

3100

100

20

3.4

64

3531

105

30

4.1

59

3825

114

40

5.3

46

3603

117

The BMI–triazine resins (effectively a blend of CE and BMI) with different proportions showed good thermal stability, their Tg values were >440 °C, and the residual char ratio at 700 °C was ~60% [54]. The onset cure temperatures of the blend of bis propargyl ether bisphenol A (PBPA) with 4,4′-bismaleimide diphenyl methane (BDM) resins were about 20–30 °C lower than that of pure PBPA, and the cure exothermic enthalpy of the resins also significantly reduced from 1320 J/g (PBPA) to 493 J/g (PBPA-BDM (1.0:2.0)) [55]. Thermosetting resin systems with very high Tg values were formulated on the basis of BMI and allylated novolac. When the allylation degree of the novolac resin was sufficiently high, the BMI proportion was not critical to the heat resistance of the cured resin [56, 57]. BMI and biscitraconimides with bisallyl groups and brominated BMI also showed enhanced thermal and mechanical features [58, 59]. A series of BMI monomers with amide groups were prepared and characterised [60]. A new type of BMI resin containing an epoxy unit and phosphorus in the main chain was synthesised [61]. The polymers, obtained through the reactions between BMI and diamine agents, also demonstrated excellent thermal properties and high char yield. Novolac resin based on 2,2′-diallyl bisphenol A when co-reacted with bisphenol A bismaleimide provided a resultant high-temperature resin with good adhesive strength at higher temperature. The moderately crosslinked blend was conducive for achieving optimum adhesive properties on aluminium substrates. Retention of adhesive properties was >100% at 150 °C [62]. Phenol-(4-hydroxy)phenylmaleimide-

10

Polybenzoxazines and State-of-the-Art High-Temperature Polymers formaldehyde (PMF) resins were prepared from phenol, hydroxyphenylmaleimide (HPM) and formaldehyde. These matrix resins were investigated for their adhesive properties by blending with EPN. The lap shear strength (LSS) of PMF resin was poor (~5 MPa) at ambient temperature. The PMF-29 resin (which contains 29% of HPM) with EPN in a 1:1 ratio provided optimum adhesive properties with 84% and 47% retention of the LSS at 150 °C and 175 °C, respectively, and could serve as a potential structural adhesive for moderately load-bearing applications. The cure reactions in the PMF-EPN system are shown in Scheme 1.5. The adhesive strength was improved by toughening the materials through blending with high-temperature thermoplastics [63, 64].

CH2 OH O

CH2

CH2

CH O

CH2

O

N

O

O

N

O

CH2

CH2

m

CH2

n

170 °C

CH2

OH

n

O

OH

CH2

EPN

PMF

HO

O

CH CH2

CH2

O O

CH2

m

CH2 HO

CH O

O

N

H2 C

CH2

CH2

m

CH2

0 °C -25 180 n

O CH2 HO

CH2 O

H2C

O

CH

CH2 HO

CH O

CH2

Scheme 1.5 Cure reactions in PMF-EPN blend

11

Polybenzoxazines: Chemistry and Properties

1.4 Cyanate Ester Resins CE resins have received considerable attention in the past few years due to their importance as thermosetting resins for use as encapsulants in electronic devices, hightemperature adhesives, and structural materials in aerospace applications because of their outstanding mechanical, thermal, and adhesive properties [65–70]. This new generation of thermoset resins encompasses the processability of epoxy resins, thermal characteristics of BMI, and the heat resistance and fire resistance of phenolic resins. CE resins have their own unique properties such as good strength, low dielectric constants, radar transparency, low water absorption, and superior metal adhesion, which make them the resin of choice in high-performance structural applications in the electronics and aerospace industries [71, 72]. CE are formed in excellent yields by the reaction of the corresponding phenols with cyanogen halides (Scheme 1.6).

R

OH

CNBr

HBr

R

OCN

Scheme 1.6 Synthesis of cyanate esters

1.4.1 Thermal Curing CE can undergo thermal or catalytic polycyclotrimerisation to give polycyanurates. The catalysts are usually Lewis acids, transition metal complexes or amines. In general, they are cured with a transition metal catalyst or chelate catalyst in the presence of a hydrogen donor such as nonyl phenol. It is accepted that the cyanate cure takes place through cyclotrimerisation to give polycyanurates. However, evidence for this mechanism is inconclusive. The thermal cure reaction of the bisphenol A cyanate BACY using a monofunctional model compound suggested that trimerisation was the major reaction (>80%) in the curing process [73]. There was very little outgassing during the polymerisation reaction, which allowed for easy fabrication of void-free composites. The thermal stability of the thermoset was much higher than that of most epoxy-based systems.

12

Polybenzoxazines and State-of-the-Art High-Temperature Polymers A widely studied CE known as BACY can be processed and cured above its melting point (80 °C). The resulting thermoset exhibits good thermal stability (95% weight retention at 430 °C) and a Tg as high as 290 °C if fully cured [67, 74]. However, the resulting polymer is brittle, which limits its use in many applications. Several factors such as impurities and environment [75, 76], solvent [77], and catalysts [78] influence the cure reaction. It has been reported that no reaction occurs if absolutely pure CE is heated [79]. In the absence of an externally added catalyst, the reaction is believed to be catalysed by water and residual hydrogen-donating impurities such as phenol [80]. It was found that the Tg of the network was lowered if it cured in the presence of solvent. Like other thermoset resins, CE are amenable to processing by a large variety of conventional techniques. Their processing versatility has gained them widespread acceptability in composites for various applications. The flexibility is further enhanced by blending with other resins such as epoxies, BMI, additives, and toughening agents. The cure cycle is dependent upon the catalytic level. Partially polymerised thermosetting resins retain the ability to fuse and to form further crosslinks achieving good tackiness.

1.4.2 Thermal Stability The cyanurate is a thermally stable crosslinking polymer responsible for the high mass loss temperature (450 °C) of these thermosets. Polycyanurates derived from phenol novolac cyanate esters known as phenolic-triazine (PT) resins have a high Tg (>350 °C), which approaches their thermal decomposition temperature [81]. In addition to high thermal stability, polycyanurates form a carbonaceous char during burning that protects the underlying material and further enhances the fire resistance. Thermogravimetric studies of polycyanurates in air have indicated that thermo-oxidative degradation proceeds via rapid hydrolysis of the ether oxygen bond between the phenyl and triazine rings in the presence of moisture at 350–420 °C [82, 83]. Purely thermal degradation under anaerobic conditions is claimed at higher temperature (545 °C) via homolytic cleavage of the hydrocarbon backbone over a narrow temperature range (450–500 °C) independent of the chemical structure of the linking groups between the cyanurate rings. Polycycanuarte structure is given in Scheme 1.7.

13

Polybenzoxazines: Chemistry and Properties

R

R OCN

R O

NCO

OCN

R

Heat

O

N N

N O

R R

Cyanate ester monomer

Polycyanurate

Scheme 1.7 Structure of cyanate ester monomer and polycyanurate

1.4.3 Modified Systems Even though most commercial CE possess good flammability and high-temperature properties, they are too brittle to be widely used in structural applications. Many additives have been used to strengthen the resulting CE thermoset, including epoxies [74, 76, 84], polyesters [85] and BMI [66, 86] with varying success. Co-curing the CE with these polymers could result in non-miscibility, which has desired and undesired effects on the physical and thermal properties of the polymeric matrix [87, 88]. CE could be substantially toughened by the addition of rubbery or rigid thermoplastic components [89–97]. The most effective toughening approach has been incorporation of thermoplastics with a high modulus and high Tg [89, 91, 94–98]. In recent years, the addition of nanoscale fillers such as layered silicates has been used as an alternative approach to enhance performance [99–105]. The major reason for introducing these fillers at the nanoscale level was the pronounced improvements in properties at low clay contents. CE resins have recently attracted the attention of composite fabricators due to their high Tg (typically ~290 °C) [99, 106] and relatively easy, epoxy-like processing. However, to obviate the brittleness of CE, thermoplastic resins with high Tg values were used for blending. Resins such as polyether ketones [107], polyether imides [108-110], polyethersulfone [111, 112] were also incorporated for improving properties.

14

Polybenzoxazines and State-of-the-Art High-Temperature Polymers

1.4.3.1 Phenolic-triazine Resins PT precursor resin is a reaction product between novolac resin and cyanogen halides. A PT network is formed by the thermal cyclotrimerisation of the cyanate ester of novolac [113] (Scheme 1.8). The networks have low melt viscosity, resinous consistency, long gel time, good thermal expansion and a Tg up to 399 °C dependent upon post-cure conditions. However, the ultimate cure temperature must be high (>300 °C) to achieve optimum cure and a higher Tg (>300 °C). PT resins possess better thermo-oxidative stability and char yield than conventional phenolics because they are mostly crosslinked by triazine groups. The decomposition starts at ~420 °C and the char yield is ~65–70%. PT resin is commercially available under the trade name Primaset PT-15, PT-30, PT-60 and PT-90. They essentially differ in their molar masses [114].

OH

O

OCN CH 2

CNBr

CH 2

Triethyl amine CH 2

Novolac

N

Heat O

N N

O

CH 2

Novolac cyanate ester

Phenolic-triazine network

Scheme 1.8 Synthesis of PT resins

Many structural alterations have been attempted to confer specific properties to polycyanurate matrices. Thus, a flame-retardant and atomic oxygen-resistant cyclomatrix phosphazene-triazine network was derived by employing Alder-ene chemistry. Co-reaction of a blend of allyl phenoxy triazine and allyl phenoxy phosphazene with BMI gave rise to a phosphazene-triazine network [115] (Scheme 1.9).

15

Polybenzoxazines: Chemistry and Properties NC O CH3 C CH3

O

P PhO N

CH3 C

N

P

N P O OPh

N

CH3 C CH3

O CN CH3

PNC-3

NCO

O P PhO N

OPh O

Heat

Ph O P O N

N

O

P O OPh

O

O

O N

N

N

N

O N

O

N

P

O Ph N

Ph P O O P O N Ph O

O

CH3

Where,

=

C CH3

Scheme 1.9 Formation of phosphazene-triazine network polymers

16

Polybenzoxazines and State-of-the-Art High-Temperature Polymers

1.5 Phenolic Resins Despite the emergence of several new classes of high-performance thermosets that are superior in some aspects, phenolic resins retain industrial and commercial interest one century after their introduction. These resins have several desirable characteristics such as superior mechanical strength, heat resistance, and dimensional stability, as well as high resistance against various solvents, acids and water. Although phenolics cannot be substituted for epoxies and polyimides in many engineering areas, their composites find a major market in thermo-structural applications in the aerospace industry due to good heat resistance and flame resistance, excellent ablative properties, and low cost. These key properties mean that phenolic resins can cope with the ever-changing requirements and challenges of advanced aerospace technologies [116–120]. The simple and inexpensive synthesis of phenolic prepolymers is well established. The commonest types of phenolic-based prepolymers are resoles and novolacs. Syntheses of resole and novolac prepolymers are carried out using phenol or its derivatives and formaldehyde under acidic or basic conditions. Novolacs are formed under highly acidic conditions with an excess of phenol. Resoles are synthesised under basic conditions with an excess of formaldehyde. The conversion of novolacs into insoluble and infusible networks requires catalysts, whereas resoles require only heating over a certain time period [121–123]. The phenolic networks are highly aromatic and resistant to thermal oxidation. However, some of the inherent qualities derived from their special chemical structures hamper their acceptance as universal polymeric materials in many engineering areas. These resins cure at moderately high temperature by a condensation mechanism with the evolution of volatiles. The need for a catalyst for curing and the limited shelf-life of resins at ambient conditions are major shortcomings of these systems. When compared with many known thermally stable polymers, their thermo-oxidative stability is low. The rigid aromatic units tightly held by short ethylene linkages make the matrix brittle. The degradation of phenolic resins is depicted in Scheme 1.10. In view of this, a new chemistry is needed to modify the cure of phenolic resins. Addition cure phenolic resins can obviate the shortcomings associated with condensation-type phenolics. The major strategies in designing addition-cure phenolics are: (i) incorporation of thermally stable addition-curable groups onto a novolac backbone; (ii) structural modification (transformation) involving phenolic hydroxyl groups; (iii) curing of novolac by suitable curatives through addition reactions of OH groups; and (iv) reactive blending of structurally modified phenolic resin with a functional reactant [124–127]. A brief account of addition-cure phenolic resin chemistry is discussed in the following section.

17

Polybenzoxazines: Chemistry and Properties

HO

CH 2 OH O2

OOH

HO

O

HO

CH

CH

OH

HO

OH

OH CH

O

HO

C

OH

OH

OH

OH +

CO OH

OH O C.

. +

O C O

OH .

+

C O

Scheme 1.10 Thermal degradation of phenolic resins

18

OH

Polybenzoxazines and State-of-the-Art High-Temperature Polymers

1.5.1 Allyl-functional Phenolics Allyl phenol–formaldehyde novolac, synthesised by the allylation of novolac, can cure thermally at 180 °C without the evolution of volatiles. The allyl derivatives of phenols have been used for the manufacture of glass fibre-reinforced plastics and mouldings, as well as casting or impregnating compositions with high heat resistance, mechanical strength and chemical stability [126]. However, the cured matrix is not thermally stable due to the thermal fragility of the crosslinks arising from the polymerisation of allyl groups [127]. The thermal stability of the allyl phenolic novolac resins could be further improved by reactive blending with BMI compounds. Studies by Enoki and co-workers [128] showed that the reaction between the two components proceeds via the Ene reaction. The unsaturated Ene adduct intermediate undergoes a further Diels–Alder type reaction with BMI to give the -bis and -tris adducts. The intermediate step (Diels–Alder) is sometimes referred to as the Wagner–Jauregg reaction [129]. Ideally, a ratio 1:3 (allyl:maleimide) provides maximum crosslinking and enhanced thermal stability, but this could lead to brittle matrices. In most cases, a compromise of various properties was achieved at an allyl:BMI ratio of 1:2. Commercial addition-cure formulations based on the co-reaction of diallyl phenols and BMI are available such as Matrimide-5292. Matrimide-5292 is used as a matrix resin in carbon fibre composites for advanced aerospace applications. Suitably formulated, the resin system made up of 4,4′-bismaleimido diphenyl methane, DABA and desirable catalysts can give a high Tg (315 °C) matrix that is stable up to 450 °C [130]. The Alder–ene polymers can be conferred good ablative properties by introducing boron into the molecular backbone of allyl compounds [131]. Allyl naphthols can replace allyl phenols in Alder–ene adducts [132].

1.5.2 Bisoxazoline–phenolics The unusual addition co-reaction of novolac phenolic resins with phenylene bisoxazoline has been explored to derive a new class of non-conventional phenolic thermosetting resins by Culbertson and co-workers [133]. The polymerisation involves a tertiary phosphine-catalysed reaction of bisoxazoline with a phenol-free novolac resin, leading to an ether–amide copolymer. The reactions and cure chemistry are shown in Scheme 1.11. The systems are suitable for high-performance composite applications [134]. These materials have low cure shrinkage, high neat resin modulus, no volatiles during cure, low coefficient of thermal expansion, and excellent toughness. The fibre-reinforced copolymers possess the low smoke and heat-release requirements of materials for aircraft interior applications [135]. Based on this chemistry, several compositions with many interesting properties have been patented. Electrical, physical and mechanical properties of the neat resin suggest that these new thermosets could be

19

Polybenzoxazines: Chemistry and Properties useful in various electrical applications. Incorporation of siloxane and montmorillonite clay is very effective in enhancing the flame resistance of the bisoxazoline–phenolic system [136]. A Tg of 220 °C was observed for the 8% polysiloxane-modified material, compared with the Tg of 248 °C for the unmodified system.

O

N

N

C

C

OH

OH

O

1,3-Phenylene bisoxazoline (PBOX)

PHEN 175-225°C

O C

O NH

CH 2 CH 2 O

O

CH 2 CH 2 NH

C

PBOX-PHEN

Scheme 1.11 Addition polymerisation of the bisoxazoline-phenolic system

1.5.3 Phenolic Resin–Epoxy Systems Curing of epoxy with novolac by making use of the OH–epoxy reaction appears to be the simplest way to design an addition-cure phenol system. Although less preferred, polyphenols are used as curative for epoxies because the addition-curing results in void-free products that are comparatively tougher due to the formation of flexible ether networks [137–140]. Interest in these systems has been revived further by the need for void-free, low moisture-absorbing matrices with low dielectric properties for various electronic applications. Their cure kinetics have been studied extensively [141–155].

20

Polybenzoxazines and State-of-the-Art High-Temperature Polymers Novel phenolic novolac resins bearing maleimide groups (PMF resin) can undergo cure mainly through addition polymerisation of these groups. They were synthesised by polymerising a mixture of phenol and HPM with formaldehyde in the presence of an acid catalyst [156] (Scheme 1.12). The thermal curing of the PMF system through polymerisation of the maleimide group resulted in comparatively brittle matrices [157].

O O

N

O

HCHO

+

OH

H+

H2 C

N

CH 2

OH

OH

O CH2OH CH2

CH2 OH

OH



HPM

Soluble PMF resin

Scheme 1.12 Synthesis of PMF resin from N-(4-hydroxy phenyl) maleimide and phenol

1.5.4 Propargyl Ether and Ethynyl Novolacs Although less commercially exploited, propargyl ether-functional phenolic resins (PN resins) were developed as a potential hydrophobic substitute for epoxies in advanced composites, electronics, adhesives and coatings. Most thermosets such as epoxy and BMI absorb moisture up to 5%, resulting in low hot/wet physico-chemical properties. This problem could be avoided by using propargyl phenolics. The structural similarity of propargyl ether to epoxy resins is useful for the preparation, processing and development of thermally stable polymers [158]. The curing of propargyl ether resins proceeds by a Claisen rearrangement followed by addition polymerisation of the resultant chromene [159]. A Tg of ~300 °C and moisture absorption of ~0.3–0.4% have been observed. Addition-curable phenolic resins bearing terminal ethynyl groups anchored to a benzene ring through a phenyl azo linkage such as ethynyl phenyl azo novolac were realised by a simple synthetic strategy involving the coupling reaction between novolac and 3-ethynyl phenyl diazonium sulfate [160] (Scheme 1.13). These resins showed a broad cure exotherm in DSC in the 140–240 °C range due to the curing of acetylene groups. The cured resin retained a char yield of 70%.

21

Polybenzoxazines: Chemistry and Properties

PN polymer

Scheme 1.13 Synthesis and polymerisation of propargyl ether-functional novolac resins (PN resins)

Phenyl ethynyl functional phenol–formaldehyde (novolac-type) addition-curable resins were synthesised by reacting a mixture of phenol and 3-(phenylethynyl)phenol with formaldehyde in the presence of an acid catalyst [161]. The polymerisation reaction was done at 75 °C. The resin underwent thermal curing at ~250–275 °C. The cure mechanism was proposed (Scheme 1.14) as a combination of acetylene addition [162] and by addition of phenol to the triple bond as implied in a model study [163]. These addition-cure phenolics provided an overall char yield of ~70%.

22

Polybenzoxazines and State-of-the-Art High-Temperature Polymers OH

OH

OH H 2C

HCHO

H+

C

CH2

HO

C

OH CH2

OH CH 2 C

C

C

C

CH2

Phenylethynylphenol (PEP) PEPFN

CH 275ºC

-

OH H2 C Further addition and crosslinking

OH CH2

CH2

C

OH CH 2

Heat

HO

C C

CH

CH2

C OH

CH 2

O

CH 2 CH2 CH 2

Scheme 1.14 Crosslinking of phenyl ethynyl functional phenolics phenyl ethynyl functional novolac resins (PEPFN)

1.5.5 Recent Developments in Phenolics By incorporation of propargyl and methylol groups onto a novolac backbone, a series of addition-curable phenolic resins and condensation-addition dual-cure-type phenolic resins (novolac modified by propargyl groups is referred to as PN; novolac modified by propargyl and methylol groups simultaneously is referred to as MPN) were synthesised and PN and MPN resins exhibited excellent processing properties [164, 165]. It was recognised that PN resin is an ideal candidate for advanced composite matrices in thermo-structural and ablative applications [164]. Modified novolac resins with BZ rings were prepared and copolymerised with a glycidyl phosphinate. The materials exhibited a high Tg and retardation on thermal degradation rates [166]. Novel molybdenum-phenolic resins were prepared. When the mixing ratio of the molybdenum-phenolic resin (with 12% molybdenum) to the curing agent was

23

Polybenzoxazines: Chemistry and Properties 100/10 (w/w), the curing temperature and activation energy were at a minimum, the thermal degradation stability of the cured product was optimal, and the temperature corresponding to the maximum extent of curing was 200 °C. The curing mechanism was similar to that of conventional phenolic systems [167]. A series of MWNT were obtained as products from catalytic pyrolysis of the crosslinked phenol-formaldehyde resins with different ferrocene content under an inert atmosphere. The amount of nanotubes increased with iron content released from the ferrocene catalyst during pyrolysis [168]. A composite prepared from 1% alkali-treated glass fibre and 55% resin showed the highest tensile strength, whereas a 5% alkali-treated glass fibre and 55% resin composites showed maximum flexural properties [169]. The MWNT and carbon fibres (CF) were added to the phenolic resin to fabricate MWNT/phenolic, MWNT/CF/phenolic nanocomposites and CF/phenolic composites by the hot press method. The MWNT/phenolic nanocomposites had the lowest Tg among the three types of composites, which indicated the better thermal conductivity of MWNT [170]. The foregoing discussion has presented a consolidated view of the recent developments in non-conventional, addition-curable phenolic resins. PBZ is another interesting addition-cure phenolic which is the focus of this book.

1.6 Polybenzoxazines The preceding sections highlighted the significance of high-performance thermosets such as BMI, phenolic resins, advanced epoxy resins, CE and others that have been engineered to meet the demands of industry. Phenolic resins are widely used as a high-technology material in the aerospace arena. Although these resins have improved thermal stability, thermo-oxidative resistance, chemical resistance, and high mechanical strength, they face numerous processability issues due to their high viscosity, short shelf-life, brittleness, high processing temperature, and low solubility in organic solvents. Most importantly, they release byproducts during processing which adversely affects the material performance due to formation of microvoids. From the family of phenolic resins, the novel polymer PBZ has been the focus of attention because it overcomes many of the problems associated with existing stateof-the-art polymers. PBZ is a newly developed addition-polymerised phenolic system having a wide range of interesting features. It has the capability to overcome several shortcomings of conventional novolac- and resole-type phenolic resins while retaining their beneficial properties. It is an addition-cure phenolic system based on oxazine-modified phenolic resin that undergoes a ring-opening polymerisation. PBZ exhibit: (i) near-zero volumetric change upon curing; (ii) low water absorption; (iii) high char yield; (iv) no

24

Polybenzoxazines and State-of-the-Art High-Temperature Polymers strong acid catalysts required for curing; and (v) release of no toxic byproducts during curing. PBZ was developed to combine the thermal properties and flame retardant properties of phenolics and the mechanical performance and the flexibility of molecular design of advanced epoxy systems. From the synthesis viewpoint, PBZ resins offer enormous design flexibility, allowing tailoring the properties of the cured materials for a wide range of applications. PBZ resins are expected to replace traditional phenolics, polyesters, vinyl esters, epoxies, BMI, CE and polyimides in many respects. They have tremendous advantages over other state-of-the-art thermosetting resins. However, some drawbacks of these materials must be addressed. Hence, blends, compsoites, and alloys have been derived from PBZ. The precursors (BZ monomers) of PBZ are formed from phenols and formaldehyde in the presence of aliphatic or aromatic amines [171]. The choice of phenol and amine permits design flexibility and tailoring of polymer properties. The most investigated PBZ is derived from bisphenol A and aniline (BA-a). This BZ is treated as a ‘benchmark’ among PBZ, and the properties of new PBZ are compared with it. The as-synthesised mixture consists of monomer, and oligomers that contain phenolic groups. For practical applications, the mixture is sufficient but, for controlled structure and properties, the monomer is freed of the oligomers. The synthesis and polymerisation of BZ monomers is depicted in Scheme 1.15.

OH

O

N

R OH

OH N R

4 HCHO

2R-NH2

N OH

N

O

OH

R

R

Scheme 1.15 General protocol for the synthesis and polymerisation of benzoxazine based on bisphenol A (when R= phenyl group, the monomer is denoted as BA-a)

25

Polybenzoxazines: Chemistry and Properties A property correlation of state-of-the-art matrices and PBZ is given in Table 1.3. This helps us to understand the properties of PBZ and its significance as a hightemperature material [172]. The relative advantages of PBZ are obvious. They are usually cured in the temperature window of 160–220 °C. The ring-opening polymerisation of these new materials occurs with near-zero shrinkage or even with a slight expansion upon cure. It is proposed that the volumetric expansion of the BZ resin is mostly due to the consequence of molecular packing influenced by inter- and intramolecular hydrogen bonding. The polymers exhibit a Tg in the 160–340 °C range dependent upon the structure. This new family of phenolic resins features a wide range of mechanical and physical properties that can be tailored to various needs. Dynamic mechanical analysis (DMA) reveals that these are candidate resins for composite applications and that they possess high moduli and Tg at low crosslink densities. Long-term immersion studies indicate that these materials have a lower rate of water absorption and low saturation content. Impact, tensile, and flexural properties are also good. Dielectric analyses on these polymers demonstrate their suitability for electrical applications. Char yield of ≤82% has been claimed. Their composites are comparable with polyimides and other highperformance polymers, and they are readily processable [173, 174]. In comparison with the other known expanding monomers and spiro ortho compounds, PBZ resins have been shown to have good potential for structural/engineering applications. PBZ have the lowest heat release during combustion and are, therefore, more flameresistant, surpassing that of phenolics and polyether imides (the current aerospace matrices of choice).

26

Table 1.3 Comparative properties of various high-performance polymers (neat resins) [172] Property

Phenolics

Toughened BMI

Bisoxazolinephenolics (40:60)

Cyanate ester

PT resin

PBZ

1.2–1.25

1.24–1.32

1.2–1.3

1.3

1.1–1.35

1.25

1.19

180

200

~200

250

150–200

300

130–280

Tensile strength (MPa)

90–120

24–45

50–90

91

70–130

42

100–125

Tensile modulus (GPa)

3.1–3.8

3–5

3.5–4.5

4.6–5.1

3.1–3.4

4.1

3.8–4.5

3–4.3

0.3

3

1.8

2–4

2

2.3–2.9

Dielectric constant (1 MHz)

3.8–4.5

4–10

3.4–3.7

–

2.7–3.0

3.1

3–3.5

Cure temperature (°C)

RT–180

150–190

220–300

175–225

180–250

177–316

160–220

>3

0.002

0.007

33 were also observed with the cured PBZ, reflecting its good flame-retardant characteristics.

56

Structure–Property Relationships Another series of maleimide-containing BZ were prepared from hydroxyphenylmaleimide and various amines (e.g., aniline, allylamine, and amino phenyl propargyl ether) [4]. The structures and synthesis are given in Scheme 2.5. The cure of BZ and maleimide were shifted to lower temperatures (probably because of the phenolic OH that remained in the monomer). The cure temperatures for a BZ derived from maleimidophenol and allyl amine (Mal-BZ-Al) and hydroxyphenylmaleimide and phenyl propargyl ether-based BZ (Mal-BZ-Pg) (see Scheme 2.5) were also shifted to lower temperatures. The ring-opening polymerisation of BZ occurred at a faster rate than the polymerisation of maleimide. However, in the presence of allyl functionality, the cure of maleimide and allyl groups occurred faster than that of BZ.

R O

O OH

R NH 2

N

HCHO

O

O

O

Mal BZ R

Mal BZ Al O

CH 2

C

CH

Mal BZ Pg

Scheme 2.5 Preparation and chemical structure of maleimido benzoxazine monomers. Reproduced with permission from T. Agag and T. Takeichi, Journal of Polymer Science, Part A: Polymer Chemistry Edition, 2006, 44, 4, 1424. ©2006, John Wiley & Sons [4] The cured Mal-BZ-Al showed a Tg of 299 °C and the cured Mal-BZ-Pg exhibited a Tg of 355 °C. High values of Tg were due to participation of allyl and propargyl groups in crosslinking. Overall, the thermal cure of the monomers produced novel thermosets with high Tg values ranging from 241 °C to 335 °C with excellent thermal stability. Monofunctional BZ containing norbornene functionalities have also been reported [5] (Scheme 2.6). Maleimido benzoxazine (MIB) and norbornene benzoxazine (NOB) showed BZ polymerisation exotherms at 213 °C and 261 °C, respectively. MIB has a maleimide group, so it could be further polymerised by a free-radical mechanism by thermal activation or with an initiator. Char yields >55% and Tg >250 °C have been observed for maleimide and norbornene functional BZ structures.

57

Polybenzoxazines: Chemistry and Properties O

O N

N

O

O

O

O

N

MIB

N

NOB

Scheme 2.6 Maleimido and norbornene benzoxazines. Reproduced with permission from Y.L. Liu and J.M. Yu, Journal of Polymer Science, Part A: Polymer Chemistry Edition, 2006, 44, 6, 1890. ©2006, John Wiley & Sons [6] The polymerisation features of the mixtures of BZ and N-phenyl maleimides have been investigated [6]. Three N-phenyl maleimides, which have carboxylic acid, hydroxyl, and hydrogen moieties, respectively, attaching on the phenyl group, were employed in the studies (Scheme 2.7).

O N

N

N

O

O

P-a O

O

HPM-Ba O

O

N O

OH

N

N O

O

MI-OH

COOH

MI-COOH

MI-H

Scheme 2.7 Chemical structures of benzoxazine and maleimide compounds. Reproduced with permission from Y.L. Liu and J.M. Yu, Journal of Polymer Science, Part A: Polymer Chemistry Edition, 2006, 44, 6, 1890. ©2006, John Wiley & Sons [6] 58

Structure–Property Relationships The peak curing temperature of phenol- and aniline-based BZ (P-a) was observed at ~220 °C and that of MI-OH at ~316 °C. However, a relatively low polymerisation temperature was observed with the mixture of P-a/MI-OH (1:1 molar) compared with the polymerisation temperature of individual components. The ring-opening polymerisation of BZ was an acid-catalytic reaction; therefore the acidic MI-OH promoted the polymerisation of P-a. Conversely, the presence of BZ compounds may also catalyse the reaction of the maleimide groups of MI-OH. P-a/MI-H showed only one exothermic peak at 186 °C, corresponding to the polymerisation of P-a. This indicated that MI-H did not promote the polymerisation of BZ. Conversely, polymerisation of the maleimide group in MI-H was catalysed in the presence of P-a because the polymerisation temperature of MI-H in the blend was shifted from 280 °C (corresponding to pure MI-H) to the low-temperature region, and overlapped with the polymerisation of P-a. Therefore, the amine group of P-a was presumed to help catalyse the maleimide polymerisation. Further, P-a and MI-H polymerise simultaneously in the P-a/MI-H co-curing system. However, a shoulder of the exothermic peak indicated that the polymerisations of P-a and MI-OH occurred independently. To investigate this issue, another maleimide possessing a carboxylicacid moiety (MI-COOH) was used in the co-curing composition. The carboxylic acid group in MI-COOH strongly catalysed the ring-opening reaction of BZ groups. Interestingly, the reaction of maleimide groups in the co-curing compositions occurred at very different temperatures. The reaction temperatures were in the order MICOOH < MI-OH < MI-H. However, the mechanism of catalysis of maleimide polymerisation by BZ has not been proved.

2.2.3 BZ-Containing Propargyl Groups Propargyl functional groups are interesting because they can polymerise independently and impart thermal stability to the PBZ structure. Hence, monofunctional (phenol- and propargylamine-based, P-appe) and difunctional (bisphenol A and propargylamine, B-appe) propargyl BZ monomers were synthesised and their polymerisation investigated [7] (Scheme 2.8). P-appe showed an exotherm at 191 °C with a maximum at 235 °C, whereas the exotherm of B-appe was observed at 223 °C with a maximum at 249 °C. The polymerisation of propargyl and BZ occurred simultaneously and, as a result, only one exotherm was observed. However, from infrared (IR) studies, it was substantiated that the propargyl group underwent polymerisation earlier than oxazine ring-opening. The polymers P-appe and B-appe showed Tg values of 251 °C and 318 °C, respectively. They also showed better thermal stability, and P-appe showed a char yield of 66% whereas the value for B-appe was 61%.

59

O

CH

O

O

N

N

N CH

HC C

P-appe

C O

B-appe

O

Scheme 2.8 Chemical structures of propargyl functionalised benzoxazine monomers. Reproduced with permission from T. Agag and T. Takeichi, Macromolecules, 2001, 34, 21, 7257. ©2006, ACS [7]

Polybenzoxazines: Chemistry and Properties

60 OCH2C

Structure–Property Relationships Propargyl BZ have been used to produce polyacetylenes (in which PBZ served as side groups) which undergo irreversible cis–trans isomerisation. These polymers have a higher char yield than propargyl PBZ [8].

2.2.4 BZ-Containing Furan Groups The furan-containing BZ monomers 3-furfuryl-3, 4-dihydro-2H-1, 3-benzoxazine (P-af) and bis(3-furfuryl-3,4-dihydro-2H-1,3-benzoxazinyl)isopropane (BA-af) were prepared using furfurylamine as a raw material (Scheme 2.9) [9]. P-af exhibited an exothermic peak centred at 241 °C, and a relatively broad peak centred at 247 °C was observed for BA-af. The polymerisation temperature of P-af and BA-af was 233 °C. After initial polymerisation, difunctional BA-af experienced steric hindrance during further polymerisation, resulting in a shift of the exothermic peak to a higher temperature and broadening of the exothermic peak. The activation energies of polymerisation were 96 kJ/mol and 98 kJ/mol for P-af and BA-af, respectively, by the Kissinger method. This implied that functionality does not have a bearing on reactivity. The polymerisation is given in Scheme 2.10.

O

O

N

O

N

N O

O

O

P-af

BA-af

Scheme 2.9 Furan-containing benzoxazine monomers. Reproduced with permission from Y.L. Liu and C.I. Chou, Journal of Polymer Science, Part A: Polymer Chemistry Edition, 2005, 43, 21, 5267. ©2006, John Wiley & Sons [9]

61

Polybenzoxazines: Chemistry and Properties

O O

O N O

N

N OH

O

N

O O

O O N

N

O OH

O

N N

HO

O OH N

N O

Scheme 2.10 Polymerisation of furan-containing benzoxazines. Reproduced with permission from Y.L. Liu and C.I. Chou, Journal of Polymer Science, Part A: Polymer Chemistry Edition, 2005, 43, 21, 5267. ©2006, John Wiley & Sons [9]

62

Structure–Property Relationships The Tg of phenol- and aminofuran-based BZ (PP-af) and bis(3-furfuryl-3,4-dihydro2H-1,3-benzoxazinyl)isopropane (PBA-af) was 315 °C and 308 °C, respectively. Introduction of furan groups into the crosslinked polymers can also increase their Tg, with formation of hydrogen bonding between furan and hydroxyl groups. The coefficients of thermal expansion (CTE) were 45 ppm and 37 ppm for PP-af and PBA-af, respectively. These values were lower than the typical phenol-aniline-based PBZs (PP-a) because the latter exhibited CTE values of 56 ppm at 161 °C (Tg of PP-a). The thermal stability of the PBZ is in the order PP-af >PBA-af. Their char yields are 53% and 47%; T5% values are 336 °C and 347 °C; T10% values are 382 °C and 391 °C; and the LOI values are 30 and 31, respectively.

2.2.5 BZ-Containing Acetylene Groups High char-yielding PBZ were obtained from acetylene-functional BZ monomers. Polymerisation of acetylene functional groups (in addition to oxazine polymerisation) contributes to thermal stability. The reaction exotherm of acetylene polymerisation broadly overlaps with the reaction exotherm of BZ ring-opening polymerisation [10]. A significant change in char yield was found for these PBZ in air and an inert atmosphere [11]. These BZ polymerised in air, resulting in a higher char yield and thermal stability than those polymerised under an inert atmosphere due to the different concentration and structure of newly formed polyene chains by acetylene-group polymerisation, and due to the different extent of ring-opening polymerisation. Much effort has been made to investigate thermally curable acetylene-containing materials (acetylene-terminated pre-polymers) because they can be polymerised under moderate conditions without the evolution of volatiles [12]. The high char yield achieved for this class of materials is 71–81% by weight at 800 °C in a nitrogen atmosphere and 30% by weight at 700 °C in air; T10% is 520–600 °C. These polymers provide many desirable properties, such as resistance to solvents and moisture, and good physical properties as well as high thermal stability. These characteristics make them good candidates for matrix resins for advanced composite materials. The acetylenic group can also react under cationic, coordination, free-radical, photolytic, and thermal inducement. Dynamic mechanical analysis (DMA) was carried out on these PBZ, bisphenol A- and 3-aminophenylacetylene-based BZ (BA-apa), BAF-apa, as well as phenol- and 3-aminophenylacetylene-based BZ (Ph-apa) monomers (Scheme 2.11). The storage modulus (G′) of these polymers was in 1.9–2.2 GPa at room temperature. The resulting Tg values were 330 °C to 368 °C, which were significantly higher than those of the analogous PBZ and much higher than their polymerisation temperature (190 °C). Their higher Tg values were attributed to acetylene functional 63

64

BA-apa

Ph-apa

N C CH

CH

CH

N C

BPFP-apa

CF3

O

C

N

O

Scheme 2.11 Acetylene functional benzoxazine monomers. Reproduced with permission from H.J. Kim, Z. Brinovska and H. Ishida, Polymer, 1999, 40, 23, 6565. ©1999, Elsevier [11]

CH

CH

O

C

N

N

C

O

O

CF3

Polybenzoxazines: Chemistry and Properties

groups that provided an additional crosslinking site. These monomers polymerise at moderate temperatures without added catalysts.

Structure–Property Relationships

2.2.6 BZ-Containing Nitrile Groups Phthalonitrile PBZ (Scheme 2.12) showed a lower cure temperature (250 °C) and high char yield (68%) at 800 °C as a result of polymerisation of nitrile groups at a higher temperature [13]. The reactive phthalonitrile terminal group contributed to the formation of highly thermally stable crosslinked structures. A polymerisation temperature of 250 °C was sufficient to achieve material of high thermal stability and a Tg of 275–300 °C. Similarly, the phenylnitrile crosslinking sites containing polymers showed excellent thermal properties and improved mechanical properties [14] (Scheme 2.13).

N

C

N

N

N

O

C

N

CH3 N

C

C

I

C

N C

O

N

N CH3

O C

N C

N

N O

CH3

II C

O

O

C

O

CH3 N

N

C C

C

N C

N

III

N

N

IV

Scheme 2.12 Phthalonitrile benzoxazines. Reproduced with permission from Z. Brunovska, R. Lyon and H. Ishida, Thermochimica Acta, 2000, 357/358, 195. ©2000, Elsevier [13]

65

Polybenzoxazines: Chemistry and Properties

O

N O

O N CN

PN-1

NC

O

PN-2

N

CN

Scheme 2.13 Phenylnitrile benzoxazines. Reproduced with permission from H. Qi, H. Ren, G. Pan, Y. Zhuang, R. Huang and L. Du, Polymers for Advanced Technologies, 2009, 20, 3, 268. ©2009, John Wiley & Sons [14]

2.2.7 Main-chain or High-molecular-weight PBZ Several new synthetic techniques have been introduced for preparing diverse BZ, including polymers with BZ units in the main chain, which enables BZ technology to be much more versatile to tailor desired properties in the final product. These polymers can be processed into self-supporting flexible films. During this phase of processing, they behave as thermoplastics. A linear PBZ molecule with oxazine rings in the main chain was synthesised with a molecular weight of ~10 kDa. The resultant polymer had a moderately broad polydispersity index [15]. However, insolubility of the products due to extreme rigidity resulted in a low molecular weight and broad polydispersity. An attempt to overcome this difficulty using a flexible and thus more soluble segment (i.e., an aliphatic amine) was made (Scheme 2.14).

Scheme 2.14 A main-chain benzoxazine polymer. Reproduced with permission from A. Chernykh, J. Liu and H. Ishida, Polymer, 2006, 47, 22, 7664. ©2006, Elsevier [15] 66

Structure–Property Relationships Two distinctive exothermic peaks (161 °C and 242 °C) appeared in the DSC analysis. The first peak could be assigned to the crosslinking reaction due to methylol end groups. The higher temperature peak was due to conventional BZ polymerisation. The derivative weight-loss curve showed that the polymer degraded in a three-stage weight-loss process. The crosslinked polymer seemed to be thermally more stable (T1% was 270 °C and T5% was 302 °C). High-molecular-weight polyether esters containing BZ units showed thermal stability more or less close to the stability of BA-a, but they exhibited better film properties. The resultant polymer showed better toughness induced by the soft ether-ester group, but the aliphatic ether ester group modules had some drawbacks such as high water absorption and low thermal stability [16]. Linear main-chain PBZ with molecular weight in the range 20–40 kDa range were also prepared by the ‘click chemistry approach’ (Scheme 2.15). Azide-containing BZ monomers produce PBZ with a triazole unit in the chain, enhancing the Tg up to 278 °C with low moisture absorption [17].

67

CH3

O

CH3

N

O

O

N3

CH3 CH3

O

N O

N N N

CuI/pyridine/DMF

CH3

O

CH3

n

N O H3C N O

N3

O

N N N

O

N

N

O

CH3

O

N

CuI/pyridine/DMF N

CH3 O

N

O

CH3

O

N

O

O

N N N

O

N

O

O

N

O

n

Scheme 2.15 Polybenzoxazines via the click chemistry approach (dimethylformamide (DMF)). Reproduced with permission from A. Chernykh, T.Agag and H. Ishida, Polymer, 2009, 50, 2, 382. ©2009, Elsevier [17]

Polybenzoxazines: Chemistry and Properties

68 O

N N N

Structure–Property Relationships A polycaprolactone-naphthoxazine (PCL-NZ)-containing polycaprolactone (PCL) has been reported (Scheme 2.16). This naphthoxazine macromonomer does not exhibit the exotherms usually observed with low-molecular-weight BZ due to the polymeric nature of the macromonomers [18]. However, 1H-nuclear magnetic resonance (NMR) spectroscopy and Fourier-transform infrared (FTIR) investigations confirmed the ring opening of the naphthoxazine groups. The cured products contain chemically incorporated PCL segments, which may significantly influence physical and mechanical properties.

Scheme 2.16 Chemical structure of PCL-NZ. Reproduced with permission from B. Kiskan and Y. Yagci, Polymer, 2005, 46, 25, 11690. ©2005, Elsevier [18]

Thermally curable naphthoxazine-functionalised polymers were synthesised by the reaction of linear (diamines) and branched (triamines) polypropyleneoxides (PPOA) of various molecular weights with paraformaldehyde and 2-naphthol [19] to prepare thin films (Scheme 2.17). The DSC results showed that the maximum cure temperature increased with increase in the molecular weight of PPOA, and that it reduced the exothermicity. The films of polybenzoxazine had a water contact angle of 67°. The water contact angles of the BA-a incorporated system increased with the increase in the amount of monomer in the film and by curing.

69

N

N

O n

O

or

R1 O

N

R2

O

O

O

x

O y

N

O

R1= H, CH 2 CH3

R2=

CH2

z

OCH2 CH CH3

O

g

CH 2 CH

NH2

CH 3

Scheme 2.17 Naphthoxazine-functionalised polymers. Reproduced with permission from A. Yildirim, B. Kiskan, A.L. Demirel and Y. Yagci, European Polymer Journal, 2006, 42, 11, 3006. ©2006, Elsevier [19]

Polybenzoxazines: Chemistry and Properties

70 O

Structure–Property Relationships The controlled synthesis of a series of PBZ model oligomers was also reported [20]. A synthetic strategy was developed in which bromine was used as an ortho positionblocking group, allowing a stepwise synthesis of structurally uniform compounds. The report opened a path to prepare many different structurally uniform compounds to aid the characterisation and deeper understanding of this new class of resin. Table 2.1 provides an overall idea of the performance of various functional PBZ.

Table 2.1 Thermal properties of various polybenzoxazines Structure of benzoxazine monomer Tg T5% T10% Char (°C) (°C) (°C) yield (%)

O

N

107 288 356

45

297 348 374

44

322 343 367

28

        P-alp O

N

                 P-ala O

O

N

N

      B-ala

71

Polybenzoxazines: Chemistry and Properties

O

O

N

N

298 395 425

42

278 349 376

62

299 394 412

59

355 390 421

70

                     BA-allyl

N

O

N

O

O

O

        Mal-BZ

N

N

O

O

       Mal-BZ-Al O

O

CH 2

C

CH

N N

O

O

                  

72

Mal-BZ-Pg

Structure–Property Relationships

O O N

N O

158 330 366

50

252 375 392

56

-

365 383

58

251 362 400

61

318 352 388

66

      HPM-Ba

O O

N O

N

          MIB O O

N O

N

       NOB OCH2 C

O

CH

N

       P-appe O

O

N

N CH

HC C

C O

O

                      B-appe

73

Polybenzoxazines: Chemistry and Properties

O N O

315 336 382

53

308 347 391

47

-

415 513

75

-

470 575

78

       (BMO-apa) -

478 547

80

362 494 539

71

           P-af O

O

N

N

O

O

    BA-af X

O

O

N

N

C

when

CH

C

CH

X = -O-              (BO-apa)

         =

CH2

     (BM-apa)

O

         =

C

CF3

=         

74

(BPFP-apa) CF3

    

Structure–Property Relationships

         =

S

       (BS-apa)

-

489 592

79

(BSO-apa) -

440 540

78

(BA-apa) 347 458 524

74

O

         =

S O

       

CH 3

         = CH 3

        

         = Nil           (BP-apa) -

O

N

C

462 492

73

380 428

76

319 450 560

76

CH

N

O

C N

N

C N

CH

  O

C

      I

75

Polybenzoxazines: Chemistry and Properties

N

C N

N

O CH 3

C CH 3

C

73

-

544 596

80

330 423 468

68

N C

O

414 505

       II C

O

-

N

N O

N

C C

N

N

    III C

N C

CH3 C

O

N O

CH3

N

C C

N

N

N

    IV

BO-apa = 4,4′-oxydiphenol- and 3-aminophenylacetylene-based benzoxazine MAL-BZ = 4-Maleimidophenol- and aniline-based benzoxazine BM-apa = 4,4′-Methylenediphenol- and 3-aminophenylacetylene-based benzoxazine BMO-apa = Bis(4-hydroxy phenyl)methanone- and 3-aminophenylacetylenebased benzoxazine BPFP-apa = 2,2 Bis(4-hydroxyphenyl)perfluoropropane- and 3-aminophenylacetylene-based benzoxazine BS-apa = 4,4′-Thiodiphenol- and 3-aminophenylacetylene-based benzoxazine BSO-apa = Bis(4-hydroxyphenyl)sulfone- and 3-aminophenylacetylene-based benzoxazine BP-apa = Biphenyl-4,4′-diol- and 3-aminophenylacetylene-based benzoxazine 76

Structure–Property Relationships

2.3 BZ Derived from Various Precursors (Non-functionalised BZ) Dihydrobenzoxazines were also synthesised from 4,4′-biphenol (BIP), and dicyclopentadienephenol adduct (DCPD) (Scheme 2.18) [21]. The DCPD BZ resins exhibited properties such as low dielectric constants and low dissipation factors for high-frequency application, whereas the PBZ with a rigid biphenyl structure provided high values of Tg and mechanical properties. The BZ polymer resulting from DCPD (DCPDBZ) had a dielectric constant of 2.94, which was better than that of polymers derived from BA-a (3.31), BIP (3.45), and traditional phenolic resin (3.9–4.0). The nonplanar structure of DCPD led to more spacing between polymer molecules, resulting in less efficient chain packing and an increase in the free volume of the polymer.

O

O

N

N

BIPBZ

N

O

O

N

DCPDBZ

Scheme 2.18 Structure of some dihydrobenzoxazines (BIPBZ = 4,4′-Biphenol- and aniline-based benzoxazine). Reproduced with permission from J.Y. Shieh, C.Y. Lin, C.L. Huang and C.S. Wang, Journal of Applied Polymer Science, 2006, 101, 1, 342. ©2006, John Wiley & Sons [21]

The key factors affecting the viscosity of a difunctional BZ resin are the bulky diphenol part between two oxazine rings and the pendant amine groups. Hence, aromatic amine-based PBZ have better properties than their aliphatic amine counterparts. Monofunctional BZ resins based on non-substituted phenol and primary aromatic amines that were liquid at room temperature were synthesised. With non-substituted phenol, formaldehyde, and primary amines as starting materials, a series of monofunctional BZ resins with low viscosities at room temperature were developed [22]. They showed highly improved thermal stability and a high Tg. A series of linear aliphatic diamine-based BZ monomers has been successfully polymerised into transparent, crosslinked materials that are free of voids and with good mechanical integrity (Scheme 2.19) [23]. The density of these PBZ was shown to decrease as a function of the chain length of the aliphatic diamine. In DMA, the linear aliphatic diamine-based PBZ exhibited two fairly strong, aliphatic chain length77

Polybenzoxazines: Chemistry and Properties dependent, low-temperature relaxation processes. The room temperature modulus was also shown to be a strong function of diamine length, decreasing from a rather stiff 2.1 GPa for P-ad2 to 0.87 GPa for the polymer with the longest aliphatic chain (P-ad12). The Tg of P-ad2 and P-ad6 surpassed the 170 °C of bisphenol A-based PBZ (Table 2.2). Thus, linear aliphatic diamine-based polymers possess inherently flexible PBZ network structures.

Pad-8 Pad-4 O N

(CH2)

N O

n

n

2, 4, 6, 8, 12 Pad-2 Pad-6 Pad-12

Scheme 2.19 Aliphatic diamine-based benzoxazine monomers. Reproduced with permission from D.J. Allen and H. Ishida, Journal of Applied Polymer Science, 2006, 101, 5, 2798. ©2006, John Wiley & Sons [23]

Table 2.2 Tg of the diamine-based series of polybenzoxazine as a function of the length of the aliphatic chain

78

Polymer

Tg (°C)

Pad-2

184

Pad-4

160

Pad-6

169

Pad-8

151

Pad-12

118

Structure–Property Relationships The polyfunctional BZ 8,8′-bis(3,4-dihydro-3-phenyl-2H-1,3-benzoxazine) (22P-a) and 6,6′-bis(2,3-dihydro-3-phenyl-4H-1,3-benzoxazinyl)ketone (44O-a) were successfully cured to produce void-free resins in an autoclave with a pressure of 1.33 MPa (Scheme 2.20) [24]. The maximum Tg achieved for the 22P-a and 44O-a PBZ was 200 °C and 365 °C, respectively. The highly crosslinked 44O-a PBZ exhibited a Tg that was higher than the Tcure. The polymer was processed at 290 °C and showed a Tg of 365 °C. The maximum service temperatures were found to be 177 °C and 290 °C, respectively.

O O

O

N

N

Scheme 2.20 Structure of 44O-a

A series of BZ resins having different amine moieties have been synthesised [25] that, upon polymerisation, produce varying amounts of phenolic Mannich bridges, arylamine Mannich bridges, and methylene linkages (Scheme 2.21).

79

O

N

N

BA-a O

O

N

N

CH 3

H3C

BA-ot

H 3C

O

O

N

N

O

N

N

BA-mt

H 3C

H 3C CH 3 H 3C

BA-pt

O

CH 3

O

O

N

N

BA-35X

CH 3

CH 3

Scheme 2.21 Benzoxazine monomers possessing methyl substituents (BA-ot = bisphenol A- and o-methyl aniline-based benzoxazine). Reproduced with permission from H. Ishida and D.P. Sanders, Journal of Polymer Science, Part B: Polymers Physics Edition, 2000, 38, 24, 3289. ©2000, John Wiley & Sons [25]

Polybenzoxazines: Chemistry and Properties

80 O

Structure–Property Relationships BA-ot is the least thermally stable compound, with an onset degradation temperature of 45% epoxy) showed poor mechanical properties because phenolic groups generated by the oxazine ring-opening reaction not only serve to catalyse the copolymerisation, but also participate as reactants and are therefore consumed by the reaction. Thus, as the stoichiometric ratio of components is approached, non-reacted or small-molecularweight epoxy molecules may remain and interfere with network formation or act as plasticisers.

95

Polybenzoxazines: Chemistry and Properties Increasing molecular weight between epoxy groups by chain extension has afforded copolymers with reduced crosslink density, improved storage modulus, and reduced Tg [1]. Copolymerisation with epoxy reduced char yields compared with pure PBZ, but chain-extended epoxy slightly increased the char yield. The addition of epoxy as a reactive diluent in the BZ matrix reduced the viscosity of the resin, but the curing was shifted to a higher temperature [2].

OH R O

N

O

H 2C

HC

CH2 O

OH O HC R

N

O

O

CH2

CH2

O

OH R

O

H 2C

HC CH2 OH

Scheme 3.1 Copolymerisation of benzoxazine with epoxy. Reproduced with permission from B.S. Rao, K.R. Reddy, S.K. Pathak and A.R. Pasala, Polymer International, 2005, 54, 10, 1371. © 2005, John Wiley & Sons [1] On the contrary, addition of phenolic novolac into the BZ resin resulted in a mixture that could be cured at a lower temperature. Phenolic novolac resin acts mainly as an initiator for these ternary systems, whereas the low melt viscosity, flexibility and improved crosslink density of the materials are attributed to the epoxy fraction. PBZ imparts thermal stability and mechanical properties, as well as low water-uptake, to the ternary systems [3]. The properties of a high-performance thermosetting resin can be improved by tightening its network structure. Hence, the modified novolac resins with BZ rings were prepared and cured with isobutyl bis(glycidylpropylether) phosphine oxide as the crosslinking agent [4]. The maleimide-functionalised BZ was copolymerised with diglycidyl ether bisphenol A (DGEBA) in various compositions in which three polymerisation reactions involving 96

Blends and Composites of Polybenzoxazines an epoxy ring, a BZ ring, and a vinyl group were observed [5]. For copolymers with a high content of epoxy, the polymerisation of the maleimide group was incomplete, whereas polymerisations of epoxy and BZ were less affected. The resulting polymer achieved a high Tg of 278 °C at 10 mol% DGEBA, and was higher than that of the homopolymer (Tg = 253 °C). Meanwhile, flexural strain at breakage increased with the increase in epoxy content. The flexural modulus was ~5.0 GPa for 10 mol% DGEBA and decreased to 4.2 GPa with 50 mol% DGEBA. The toughness of PBZ can effectively be improved by alloying with isophorone diisocyanate-based urethane prepolymers (PU) or with flexible epoxy [6].

3.3 PBZ–Poly(ε-Caprolactone) Blends Hydrogen bonding is adequate to induce rigidity and constrain the mobility in the glassy state for thermosetting resins. Polymer blends of bisphenol A and an anilinebased benzoxazine (BA-a), and poly(e-caprolactone) (PCL) indicated that hydrogenbonding interactions occur between the carbonyl groups of PCL and the hydroxyl groups of PBA-a upon curing [7]. The most pronounced effect of the addition of PCL was to broaden the glass-transition region, and a decrease in the value of the loss tangent (tan δ) in the transition region upon increasing the PCL content. Before curing, the benzoxazine BA-a/PCL blends were miscible, but the phase separation induced by polymerisation was observed after curing at elevated temperature [8]. The Tg of the PCL/BZ monomer blend exhibited a continuous increase as the PCL composition increased from 0 wt% to 33 wt% [9]. The intermolecular hydrogen bonding between PBZ and polycarbonate (PC) is illustrated in Scheme 3.2 [10].

CH3 O

C CH3

CH3 O C O

C

O

O C O

CH3

O H

O

OH N

O H N

N

Scheme 3.2 Intermolecular hydrogen bonding between poly(ε-caprolactone) and polybenzoxazine. Reproduced with permission from H. Ishida and Y.H. Lee, Journal of Polymer Science, Part B: Polymers Physics Edition, 2001, 39, 7, 736. © 2001, John Wiley & Sons [9]

97

Polybenzoxazines: Chemistry and Properties

3.4 PBZ–Polyimide Blends Benzoxazine monomer (BA-a) was blended with soluble polyimide-siloxanes (PDMS) with and without pendant phenolic groups [11]. The onset and maximum of the exotherm due to the ring-opening polymerisation for the pristine BA-a appeared on differential scanning calorimetry (DSC) curves at ~200 °C and 240 °C, respectively. In the presence of poly(imide-siloxane)s, the exothermic temperatures were lowered: the onset to 130–140 °C and the maximum to 210–220 °C. The cured blends containing poly(imide-siloxane) with -OH functionality showed two Tg values, at a low temperature ~255 °C and at a high temperature ~250–300 °C, displaying phase separation between PDMS and the combined phase consisting of polyimide and PBZ (PBA-a) components due to the formation of AB-crosslinked polymers. For the blends containing poly(imide-siloxane) without OH functionalities, in addition to the Tg due to PDMS, two Tg values were observed in high-temperature ranges at 230–260 °C and 300–350 °C. This indicated further phase separation between the polyimide and PBZ components due to the formation of semi-interpenetrating networks (IPN). In both cases, the Tg increased with increasing poly(imide-siloxane) content. Tensile measurements showed that the toughness of PBA-a was enhanced by the addition of poly(imide-siloxane). Thermogravimetric analysis (TGA) revealed that the thermal stability of PBA-a was also enhanced by the addition of poly(imidesiloxane). Polymer alloys of PBZ (BA-a) and soluble polyimide (PI) or its precursor, poly(amide acid) (PAA) were prepared and characterised [12]. The BA-a/PI formed a IPN structure. The ring-opening polymerisation of BA-a occurred in situ with the imidisation of PAA, accordingly causing a complicated reaction. It was proposed that, in addition to the imidisation of PAA, the reaction between the phenolic OH of polyBA-a and the carboxylic acid of PAA could occur, affording an AB-crosslinked structure (Scheme 3.3). The polymer alloy films from PI/BA-a and PAA/BA-a showed only one Tg. The Tg values and thermal stabilities were increased remarkably as the content of PI increased. As the content of BA-a increased, the modulus of the polymer alloy films was also enhanced.

98

Blends and Composites of Polybenzoxazines

Scheme 3.3 A possible copolymerisation between polyBA-a and PAA. Reproduced with permission from T. Takeichi, Y. Guo and S. Rimdusit, Polymer, 2005, 46, 13, 4909. ©2005, Elsevier [12] Polysiloxane-block-polyimide (SPI) has several attractive characteristics such as low moisture absorption and excellent thermal stability. Hence, this flexible material was incorporated into a PBZ matrix (PBA-a). The Tg of blends with different SPI content showed a slight increase from the neat matrix, i.e., 160 °C to 169 °C, and the system exhibited partial miscibility as evidenced from an opaque appearance of cured blends. A noted advantage of blending is that the flexibility of PBZ is improved by SPI. In addition, the temperature at 5% weight loss (T10%) increased from 360 °C to 450 °C (75% weight of SPI) and the char yield increased from 30% (PBA-a) to 45% at 5% weight of SPI [13].

99

Polybenzoxazines: Chemistry and Properties

3.5 Other Blend Systems Any chemical modification or additive to enhance toughness (with a minimum sacrifice of the original mechanical and physical properties of the PBZ) is attractive. Sequential IPN, based on PU and PBZ, was synthesised. The kinetics indicated the presence of physical bonding only in the resulting IPN. Morphological investigations revealed slight phase separation behaviour in all of the IPN studied [14]. In the blend of PBZ/ PC, the ring-opening reaction and subsequent polymerisation reaction of the BZ were significantly inhibited by the presence of PC [15]. The hydrogen-bonding interaction in the blends occurs between the hydroxyl groups of the PBZ and the carbonyl groups of the PC. This is the ‘driving force’ that results in the miscibility of the PC/ BZ blend in the entire composition range along with possible copolymer formation. The Tg of the resulting blends decreased as the concentration of PC increased and deviated markedly from the Fox equation. In addition, an earlier degradation event appeared in the blend with 11 wt% and 33 wt% of PC. The possibility of the exchange reaction (which can occur in the blends containing PBZ and PC) was also confirmed [16]. The rubber-modified system of this PBZ-PCL matrix showed poorer thermal properties [17]. Study of thermosetting blends composed of BA-a and polyethylene oxide indicated that the phenolic hydroxyl groups could not form favourable intermolecular hydrogenbonding interactions at elevated temperatures (e.g., the curing temperatures), i.e., the phenolic hydroxyl groups existed mainly in the non-associated form in the system [18]. Therefore, the phase separation is ascribed to the decrease of the entropic contribution to mixing energy due to the increase in molecular weight. The occurrence of the trans esterification replaced the original hydroxyl groups from the BZ main chain to the phenolic chain ends of the PC, and ‘scissored’ the long chain of PC into short segments. The result of the former can facilitate the ring-opening polymerisation, whereas the latter sacrificed the thermal properties of the blends. Polybenzoxazine/ poly(N-vinyl-2-pyrrolidone) exhibited strong hydrogen-bonding interactions between PBA-a and polyvinyl pyrrolidone segments [19]. Ortho-, meta-, and para-phenylnitrile functional BZ were polymerised at different compositions with phthalonitrile-functional monomers (structures given in Scheme 2.12 in Chapter 2) and which produced copolybenzoxazines of high thermal stability and easy processability [20]. The Tg also dramatically increased from 180 °C for neat ortho-phenylnitrile polymer to 294 °C for the copolymer with 30 mol% of phthalonitrile-functional monomer. Additionally, the high melt viscosity of phthalonitrile-functional BZ decreases upon blending with phenylnitrile-functional monomer. It has been demonstrated that only 30 mol% of phthalonitrile-functional BZ added to the ortho-phenylnitrile-substituted monomer significantly improves the

100

Blends and Composites of Polybenzoxazines char yield from 59 wt% to 77 wt%, which is the value of the neat phthalonitrilebased PBZ. Amine-terminated butadiene–acrylonitrile copolymer (ATBN) and carboxyl-terminated butadiene acrylonitrile rubber (CTBN) were introduced to PBZ. On a comparative scale, ATBN was more effective than CTBN in improving the fracture toughness of PBA-a. This was attributed to the better distribution of rubber particles in an ATBNmodified matrix than for the CTBN-modified matrix. Dynamic mechanical analysis (DMA) showed the existence of two networks in the ATBN-modified matrix [21]. Poly(urethane-benzoxazine) films as novel PU/phenolic resin composites were prepared by blending a BZ monomer (BA-a) and PU prepolymer [22]. All the films had only one Tg from viscoelastic measurements, indicating no phase separation in poly(urethanebenzoxazine) due to in-situ polymerisation. The films containing 20% of BA-a had plastic characteristics. Polymer alloys composed of epoxy-terminated polyurethane and high-molecularweight PBZ (derived from bisphenol A and methylenedianiline) showed good electrical and mechanical properties. These blends have excellent solvent resistance and a moisture uptake of 1.21–1.61%. However, the blends exhibited two alpha transitions in dynamic mechanical thermal analysis (DMTA) related to each of the components, which showed the phase-separated nature of the blends. The tensile strength of blends was of 24.0–30.5 MPa, which was lower than that of PBZ (30–40 MPa) [23]. A blend of bisphenol A-based BZ (BA-a) and a bismaleimide (2,2-bis[4(4maleimidophenoxy) phenyl] propane (BMI) was thermally polymerised in varying proportions, and the cure and thermal characteristics investigated. The DSC analysis, supplemented by rheology, confirmed lowering of the cure temperature of BMI in the blend, implying catalysis of the maleimide polymerisation by BZ. The peak cure temperature (Tp) of the blend decreased to 211 °C in comparison with those of BMI (270 °C) and BA-a (218 °C). Moreover, the final cure temperature (Tf) of the blend was 284 °C, compared with 339 °C for BMI. Hence, from the viewpoint of BMI, the processing characteristics of the blend are improved. A wide cure regimen between 142 °C and 284 °C was observed for the blend (Table 3.1). Hence, by realising a blend of these resins, the initial and peak curing temperatures are lowered in comparison with those of the component resins, i.e., the processing is facilitated [24].

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Polybenzoxazines: Chemistry and Properties

Table 3.1 DSC results of BA-a/BMI blends Samples

Ti (°C)

Tp (°C)

Tf (°C)

CCW (°C)

Cure window (°C)

Heat of reaction (J/g) Experimental

Theoretical

BA-a

156

218

270

62

114

257

–

BMI

195

270

339

75

144

130

–

BA-a/BMI (1/0.5)

128

212

293

81

165

225

209

BA-a/BMI (1/1)

142

211

284

69

142

180

191

BA-a/BMI (1/3)

130

224

342

118

212

174

157

Ti: Initial curing temperature Tp: Peak curing temperature Tf: Final curing temperature CCW: Cure-controllable window (Tp–Ti) Cure window: Tf–Ti

Fourier-transform infrared (FTIR) studies provided evidence for the hydrogen bonding between the carbonyl group of BMI and the –OH group of PBZ in the cured matrix. In the BA-a/BMI blend, there are two areas of interest in the FTIR spectra of the blends. One is the carbonyl region (1800–1650 cm–1) and the other is the hydroxyl region (3600–3000 cm–1). In the cured blend, the characteristic carbonyl band was observed at 1714 cm–1. In addition, two relatively weak bands were observed at 1702 cm–1 and 1687 cm–1. These two bands are assigned to the hydrogen-bonded carbonyl groups generated in different environments in the blend. The two possibilities are depicted in Scheme 3.4. The hydroxyl group may be hydrogen-bonded with the carbonyl moiety of the bismaleimides, and hydroxyl groups which are already hydrogen-bonded (intermolecular hydrogen bonding) may be bonded to the carbonyl moiety of another bismaleimide group. Hydrogen bonding is expected to facilitate a homogeneous phase in the IPN or co-reacted matrices.

102

Blends and Composites of Polybenzoxazines Hydrogen bonding of carbonyl moiety with free hydroxyl group

H O

O

N

O

O O

H O

N

O H

H

H-bonding of carbonyl moiety with intermolecular H-bonded hydroxyl grou

p

O

Scheme 3.4 Hydrogen bonding in BA-a/BMI (1/1) blend

The cured matrix manifested dual-phase behaviour in scanning electron microscopy (SEM) and DMTA with the minor phase constituted by PBZ dispersed in an IPN of PBZ and cured BMI. The DMTA of the monomers and their blends implied a mutual catalysis of the two monomers. The maleimide groups are electron deficient whereas BZ groups possess electron-rich centres. The likely nπ–pπ interaction between the amine group of the BZ and the unsaturated group of maleimide may render the O-CH2-N bond weaker and facilitate its cleavage. The π bond in maleimide becomes electron-rich and susceptible to addition polymerisation (Scheme 3.5) [25]. Similar situations have been encountered for maleimide mixed with electron-rich monomers (e.g., copolymerisation with styrene).

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Polybenzoxazines: Chemistry and Properties

O

Weakening of -C-O bond N

N

O

O

Scheme 3.5 Weakening of the –C-O bond of the oxazine group by nπ–pπ interaction in the BA-a/BMI blend The DMTA of the cured resins is shown in Figure 3.1. Two distinct peaks in tan δ at 145 °C and 267 °C are seen for the BA-a/BMI system. This is due to some microlevel phase separation as also seen in SEM analysis. The BA-a/BMI IPN resulted in a higher Tg. The lower Tg is attributed to the phase-separated PBZ segments.

Cured BA-a/BMI

1.0 0.8 0.6 tan δ

Cured BA-a

0.4 Cured BMI

0.2 0.0 0

100

200 T(°C)

300

400

Figure 3.1 Tan-δ behaviour of cured BA-a/BMI (1/1) in comparison with BA-a and BMI

104

Blends and Composites of Polybenzoxazines The curing of allyl containing BZ–bismaleimide blends (BA-allyl/BMI) was also studied. In the 1/0.5 (BA-allyl/BMI) blend, two exotherms were observed, at 228 °C and 269 °C, respectively. The first exotherm is the co-curing of allyl-bismaleimide and the second exotherm corresponds to the ring-opening of BZ. Further addition of bismaleimide to BZ resulted in a decrease in the overall curing temperature. As the BMI concentration was enhanced from a molar ratio of 0.5 to 1, a broad exotherm was observed, predominantly due to the reaction between the allyl groups in BZ and the maleimide groups. The Tp was lowered to 229 °C from 273 °C. Beyond this composition, bismaleimide did not significantly influence the cure because the Tp and heat of reaction remained constant. The heat of reaction for independent curing of BA-allyl is 40 J/g and is 130 J/g for BMI. The heat of reaction in the 1/1 blend was found to be 190 J/g, indicating a change in the mechanism of cure. A proposed mechanism is shown in Scheme 3.6, involving the Alder-Ene reaction and ring opening of BA-allyl.

105

Polybenzoxazines: Chemistry and Properties O

N Ph

N R N

O

Ph N

O

O

O

Ph N

O

N

O

O

N R N

O O

O

O Ph

O

O

Ph

Wagner-Jauregg

R

N

N

O

N R N O

O

N

Ene adduct

O

O

O O

O

O

R N

O

N R N N

OH N

HO

O Ph

HO

O Ph

OH

N

Further crosslinking

R=

O

O

Scheme 3.6 Cure reaction in BA-allyl and BMI blends

106

N

O O Ph

R

Blends and Composites of Polybenzoxazines BA-allyl/BMI blend formed predominantly a co-reacted network, so a homogeneous morphology was observed in SEM analysis. The Tg of cured BMI and cured BAallyl are 307 °C and 298 °C, respectively, (Figure 3.2). The BA-allyl/BMI system demonstrated a single Tg at 274 °C owing to the formation of co-reacted networks. The thermal stability of BA-a/BMI and BA-allyl/BMI blends was improved compared with PBZ (Table 3.2).

Cured BA-allyl/BMI

0.3

tan δ

0.2 Cured BA-allyl

0.1 Cured BMI 0.0 0

100

200 T(°C)

300

400

Figure 3.2 DMTA of BA-allyl/BMI, BA-allyl and BMI

However, polymer alloys were reported by blending 1,1′-(methylenedi-4,1-phenylene)bismaleimide (BMI) and BA-a. The obtained alloy films had improved toughness which increased with increase in BMI content. The authors proposed a thermal reaction between the double bond of BMI and the hydroxyl groups of BA-a. They suggested

107

Polybenzoxazines: Chemistry and Properties that the reactions of the double bond and ring opening of BA-a occurs simultaneously, leading to a AB-co-crosslinked structure. The Tg of alloys also increased from 154 °C to 268 °C at 76% of BMI content [26].

Table 3.2 Thermal stability of BA-allyl/BMI blends Cured samples

T5% (°C)

T10% (°C)

Char yield (%) at 800 °C

BA-allyl

395

422

44.0

BA-allyl/BMI (1/0.5)

350

397

36.0

BA-allyl/BMI (1/1)

387

418

39.0

BA-allyl/BMI (1/1.75)

373

412

38.5

BA-allyl/BMI (1/2.5)

385

418

39.0

BMI

410

430

43.4

BA-a

275

335

35.6

BA-a/BMI (1/1)

335

385

43.2

As described in Chapter 1, cyanate ester (CE) resins have good thermal and hygrothermal properties for use in the electronics, aerospace and adhesive arenas. To incorporate the properties of CE into PBZ, thermosetting polymer blends composed of benzoxazine (BA-a) and bisphenol A cyanate ester (BACY; structure is shown in the inset of Figure 3.3) was investigated [27]. The blend was prepared by powder mixing of two monomers in a mortar and by using a solvent method (AR acetone). The DSC of the powder-blended system was conducted immediately after mixing. Four distinct exotherms were observed in the DSC analysis. The exothermic heat for curing of BA-a and BACY are 262 J/g and 760 J/g, respectively, but the exothermicity of the (1/1) blend was reduced by as much as 210 J/g (powder blend).

108

Blends and Composites of Polybenzoxazines

CH3

BACY

NCO

Exo Heat flow

h

f b

CH3

BACY

g e

OCN

solution blended (1/1)

c

a

d

powder blended (1/1) BA-a 0

100

200

T (°C)

300

Figure 3.3 DSC traces of BA-a/BACY blend (1:1) (inset shows the structure of BACY)

The first exotherm below 100 °C (marked as a) was due to a reaction between the two components before the polymerisation. The second exotherm (marked as b) and the third (marked as c) correspond (in all probability) to a certain co-reaction between the two components, leading to polymers. The fourth (marked as d) can be ascribed to the hompolymerisation of BACY. The exotherm corresponding to the exotherm a in the thermogram of the powder-blended system is also observed in the solution-blended system but with a relatively weak intensity (e). This indicates that the reaction corresponding to the first exotherm (corresponding to a) may have occurred during the solution blending of the two resins because the solvent was evaporated by keeping it at 60 °C for 24 h under vacuum. Apparently, the four exotherms observed in the powder-blended system are also seen in the solution-blended system but with weak intensities and a shift towards lower temperature regimens. The exothermicity of solution-blended system was lower than that of the powder-mixed system. This

109

Polybenzoxazines: Chemistry and Properties implies that the reaction corresponding to exotherm b in the powder-mixed blend may have partly occurred during evaporation of the solvent for the solution-blended system. The reaction corresponding to exotherm a in the powder-blended system was completed in a solution-blended system at 60 °C under vacuum. In this case, the investigations show that the cyanate trimerisation and ring opening of BZ take place almost simultaneously (in a mutually catalysed manner). During polymerisation, it is likely that some of the phenolic hydroxyl groups resulting from the ring opening of the oxazine moiety may react with the cyanate moiety via an iminocarbonate intermediate that ultimately forms the triazine network. It is probable that part of BACY may be trapped in the PBZ and triazine matrix that could cure independently (accounting for the third exotherm in DSC analysis; marked as d in Figure 3.3). A triazine ring as a part of a PBZ network (Scheme 3.7) could be the major structure. Based on these evidences, a reaction between oxazine and cyanate groups (i.e., a cycloaddition reaction) followed by a crosslinked structure comprising triazine as a part of a PBZ structure (formed via an iminocarbonate intermediate) can be postulated (Scheme 3.7).

N

RO N

OR RO N

N

O N

OR

N N O

N

N

Scheme 3.7 Postulated co-reacted network in a BA-a and BACY blend

PBZ decomposes at 275 °C and polycyanurate at ~400 °C. The blends start decomposing at a temperature closer to that of PBZ (Figure 3.4). The degradation pattern of the blend at a higher temperature matches that of the pure polycyanurate. The initial decomposition could be related to the degradation of PBZ units, and the degradation at higher temperature to polycyanurate. This observation again confirms the presence of these two matrix segments in the network, whereby the initial degradation is decided by the PBZ groups. Interestingly, the char residue for the blend is closer to that of polycyanurate. If the matrix was a blend of the two

110

Blends and Composites of Polybenzoxazines individual components, the decomposition could have occurred in two stages, and the residue would have been the average of the two homopolymers. The TGA behaviour also confirms that the matrix in the co-cured blend is different from the mixture of two independent hompolymers.

Residual Weight (%)

100

0/1

80

60 1/1

1/0

40

0

200

400

600 T(°C)

800

1000

Figure 3.4 Thermal stability of homopolymers and BA-a/BACY blends under N2 (heating rate, 10 °C/min)

The Tg and crosslink density of the BA-a/CE blends were calculated from DMTA data. The crosslink density of the cured BA-a is 2900 mol/m3 and that of polycyanurate is 4380 mol/m3. The BA-a/BACY blend exhibited a higher crosslink density over neat PBZ and polycyanurate. The Tg of the blend is in-between those of the component polymers, implying a near homogeneous matrix. The Tg values and crosslink density of the blend are shown in Table 3.3.

111

Polybenzoxazines: Chemistry and Properties

Table 3.3 DMTA analysis of BA-a/BACY blends Samples (molar ratio) BA-a/BACY

Tg (°C)

Crosslink density (mol/m3)

1/0

215

2900

1/1

230

6270

0/1

252

4380

The incorporation of phenol aniline-type into bisphenol A aniline-type (BA-a) BZ improved the processability, but the viscosity of the latter was significantly decreased [28]. PBZ is also copolymerised with the siloxane-containing difunctional BZ monomer 1,3-Bis(3-aminopropyl)tetramethyldisiloxane) (BATMS-BZ), which resulted in toughening of the polymer [29]. In the phenol and aniline based benzoxazine (P-a) and BATMS-BZ copolymers, the chains formed strong intermolecular hydrogen bonding which resulted in a positive deviation of Tg. However, in phenol and aminofuran based benzoxazine, this behaviour as well as toughening were not observed.

3.6 Nanocomposites of PBZ Recently, PBZ-based nanomaterials have been the centre of attention for researchers because they offer tremendous improvement on many properties, such as tensile strength and modulus, thermal stability, solvent resistance, gas permeability, and flammability, with a small content of nanoparticles in the polymer matrix. The enhancement in overall properties is due to the nanoscale dispersion of nanoparticles through the pristine polymer matrix. A substantial number of nanomaterials have been reported, with a strong bias towards nanocomposites [30–33]. Dependent upon the nanoparticles employed for the modification of the PBZ matrix, studies are categorised and described in the next sections.

3.6.1 PBZ-Clay Systems Clay-based PBZ nanocomposites are the most widely investigated nanomaterials of PBZ. Polymer-layered silicate nanocomposites are a class of materials that have attracted considerable interest because of their potential technological applications. They exhibited superior properties over pristine BA-a matrices using organically

112

Blends and Composites of Polybenzoxazines modified montmorillonite (OMMT) as a type of layered silicate [30]. The ringopening polymerisation of BA-a in the presence of OMMT was observed at 170–190 °C compared with the onset of pristine BA-a at 223 °C. This decrease in the onset of the ring-opening polymerisation was due to the catalytic effect of acidic onium protons present on the montmorillonite (MMT) surface on BZ ring opening. Studies on BA-a/modified MMT (using stearyl and dodecyl ammonium chloride) showed that there is no sharp difference in the onset of the exotherms with change in the type of surfactant or amount of OMMT. The thermal stability and Tg of the nanocomposites were also enhanced compared with the pristine PBZ matrix. MMT was also modified using the amines tyramine, phenyl ethylamine, aminolauric acid and dodecylamine. DSC analyses showed that inclusion of all OMMT increased the thermal stability and marginally increased Tg from pristine resin PBA-a [31]. A report on immiscible polymer-clay nanocomposites using organically modified clay stated that the aggregation of silicate layers could be due to entrapment of clay in the pristine matrix [32]. The results indicated that the BZ monomer became intercalated into the galleries of the clay; the nanocomposite possessed an exfoliated structure at 3% clay content. The curing reaction in the synthesis of PBZ–MMT nanocomposites showed autocatalytic characteristics [33, 34]. X-ray diffraction (XRD) analysis showed that the chains of PBZ were successfully integrated into the interlayers of the OMMT to form the intercalated nanocomposite because of the good compatibility between the PBZ and OMMT [35]. Also, aminolauric acid-modified mica has been incorporated into BA-a resin. The polymerisation occurred rapidly in the presence of modified mica, and the storage modulus increased due to the nano-reinforcement. As expected, it showed a wide glass-transition window because of the restriction of segmental motion by mica. The authors reported that the increase in the glassy modulus at ambient temperature is an indication of better adhesion, but it is quite likely that it implies polymer brittleness [36]. A series of polyurethane-PBZ/clay hybrid nanocomposites (PU/P-a–OMMT) films were successfully prepared using a solvent method by in-situ copolymerisation of PU prepolymer and P-a (phenol- and aniline-based monomer) in the presence of OMMT [37]. An exfoliated structure was achieved for 7% of OMMT loading. The peak curing temperature of PU/P-a was lowered by hybridisation with OMMT due to the catalytic effect of the clay surface on the ring-opening polymerisation of P-a, which enabled complete cure of P-a without thermally decomposing the PU component. The tensile strength and modulus of the PU/P-a films increased remarkably, whereas the elongation decreased with the increase of OMMT loading. The effects of 2,2′(1,3-phenylene)-bis (4,5-dihydro-oxazoles) (PBO) content on the cure of pristine

113

Polybenzoxazines: Chemistry and Properties BA-a monomer (molar ratio of PBO to BA-a, 0.8:1) and their nanocomposites were studied [38]. On the introduction of OMMT, the onset curing temperature of the copolymerisation of BA-a and PBO decreased, and dispersion of OMMT in the PBA-a/ PBO matrix resulted in the exfoliated structure of OMMT. The nanocomposites exhibited much higher values of Tg than the PBA-a/PBO resin and pristine PBZ.

3.6.2 PBZ-Polyhedral Oligomeric Silsesquioxane (POSS) Systems Another interesting nanomaterial of PBZ is based on POSS. The octa(propylglycidyl ether) polyhedral oligomeric silsesquioxane (OpePOSS) was incorporated into PBA-a to prepare PBA-a nanocomposites with POSS concentrations up to 40 wt% [39]. An inter-component reaction between the phenolic hydroxyls of PBA-a and the epoxide groups of OpePOSS, i.e., the phenolic hydroxyls are converted into the secondary alcohol hydroxyls in hydroxy ether structural units (-O-CH2-CH (OH)- CH2-O-), was noted. Polymer properties were enhanced due to nano-reinforcement. Octa-aminophenyl polyhedral oligomeric silsesquioxane (OAPS) was also used to modify BZ in the presence of PBO [40]. The nanocomposites exhibited higher Tg values than the pristine PBZ and PBZ-PBO resin, and the storage modulus of the nanocomposites was maintained at higher temperatures (although only a small amount of OAPS was incorporated into the systems). The thermal stability of the hybrid was also improved by the inclusion of OAPS. To improve the thermal stability of PBZ, a hydrosilane-functionalised polyhedral oligomeric silsesquioxane was incorporated into the vinyl-terminated benzoxazine monomer (VB-a) [41]. Hybrids from a non-reactive POSS and VB-a were also studied. These POSS-containing composite materials displayed significant improvements in their thermal stability relative to the typical PBZ formed in the absence of POSS. The Tg increased from 307 °C for the polyvinyl-terminated BZ monomer to 333 °C for the copolymer hybrid incorporating 5 wt% of POSS. The degradation temperature and char yield under nitrogen increased with the increase in POSS content. A similar class of PBZ/POSS nanocomposites with a network structure was prepared by reacting multifunctional benzoxazine POSS (MBZ-POSS) with BZ monomers (P-a and BA-a) at various compositional ratios [42]. Octafunctional cubic silsesquioxane (MBZPOSS) was used as a curing agent. The Tg and decomposition temperature of these nanocomposites was improved by incorporating the MBZ-POSS on the BA-a- or P-atype PBZ. However, the Tg values of POSS/PBZ hybrids with 10 wt% POSS content were lower than that of 5 wt% POSS, probably due to hindrance of BZ crosslinking caused by hard POSS-rich particles resulting from phase separation.

114

Blends and Composites of Polybenzoxazines

3.6.3 Miscellaneous Nanocomposites PBZ (PBA-a) nanocomposites containing multi-walled carbon nanotubes (MWNT) with hydroxyl, carboxyl, and isocyanate groups on the surface of carbon nanotubes have been reported [43]. The carboxyl groups on the surface catalysed the ring-opening reaction of BZ and decreased the curing temperature of the system. The isocyanate groups reacted with the phenolic hydroxyl groups generated by the ring opening of BZ, resulting in a significant improvement in the adhesion between PBA-a and MWNT. The Tg and storage modulus were increased by the addition of MWNT into PBA-a. MMT organoclay has been modified with allyl-dimethylstearylammonium and propyldimethylstearylammonium groups, and they were used for the preparation of PBZ nanocomposites [44]. The results showed no significant improvement in properties. The Tg and thermal stability of the nanomaterials were lower than those of the pristine PBZ matrix (derived from allylamine and bisphenol A: B-ala). In addition, in the DSC analysis of B-ala, the broad peak attributed to the allyl and oxazine polymerisation became separated in the nanocomposites, and allyl polymerisation became more distinct compared with oxazine polymerisation. Clay could loosen the PBZ matrix, which would be reflected in the Tg value and thermal properties. The porous materials of BZ (BA-a) containing various molecular weights of PCL as the labile constituent showed low dielectric constants (1.95 × 105 Hz at 25 °C) relative to that of the virgin polymer (3.56 × 105 Hz at 25 °C) [45]. When the P-a containing PCL (P-a-PCL) was eliminated by hydrolysis of the P-a-PCL/PBA-a-copolymer, pores were generated. The slight degree of hydrogen bonding that exists between the two polymers results in micro-phase separation without an excess degree of aggregation. Organic–inorganic hybrids were prepared from PBZ and titania using a sol–gel process by blending titanium isopropoxide as a precursor for titania with a typical BZ monomer (BA-a) [46]. The acidity of the formed Ti–OH group in the titania network could initiate ring opening of the oxazine ring and lower the curing temperature. The Tg of the neat PBZ was shifted slightly to 179 °C from 151 °C with the inclusion of titania. The increase in thermal stability was attributed to the thermal insulation effect of a metal-oxide network that protects the underlying polymer matrix. Thermally curable BZ ring-containing polystyrene macromonomers were synthesised and characterised [47]. The macromonomers could undergo thermal curing and formed thermoset networks comprising polystyrene segments.

3.7 Fibre Composites and Microcomposites Various PBZ micro composites and fibre composites have been reported. The PBZ matrix has been modified by using fibres or other fillers. In PBZ-glass composites, the untreated PBZ composite suffers a 53% loss in interlaminar shear strength

115

Polybenzoxazines: Chemistry and Properties (ILSS). However, if a benzoxazine silane-coupling agent is used to treat the glass fibre, the composite retains 100% of its strength even after treatment in boiling water. Surprisingly high wet strength retention of 100% (after boiling for 12 h in distilled water) was obtained with this silane treatment on the PBZ composite [48]. A thermal conductivity of 32.5 W/mK was achieved for a boron nitride-filled PBZ at its maximum filler loading of 78.5% by volume [49]. The specific heat capacity of boron nitride-filled PBZ showed a value of 1098 J/K/kg for 50% filler by weight and 860 J/K/kg for 90% filler content [50]. The stiff boron nitride filler could highly restrict the mobility of the polymer matrix, which adhered on the filler surface and could lead to the large increase in the Tg values of their composites. Interestingly, water absorption of the filled systems at 24 h was 700 °C, so route 1 is a favoured degradation pattern. Route 2 produces aniline and an unsaturated hydrocarbon, both of which were detected from FTIR and GC-MS analyses of the degradation products at higher temperatures. Therefore, degradation route 2 is also possible, but most likely at higher temperatures. A substantial amount of CO2 was detected from the degradation under nitrogen and air environments. The evolution of CO2 and NH3 during the thermal degradation is beneficial from a flammability viewpoint because these are non-flammable volatiles. The degradation of acetylene containing PBZ is shown in Scheme 4.3.

138

OH

OH

CH 3

CH 2

1 R

N

C CH

te

R

N

NH 2

H abstarctio n

R C CH

C

CH

C CH

OH H 2C

NH 2

Ro

ute

2

Substituted benzenes HC CH

OH H 3C R R

Scheme 4.3 Degradation of acetylene-functional polybenzoxazines. Reproduced with permission from H.Y. Low and H. Ishida, Journal of Polymer Science, Part B: Polymer Physics Edition, 1999, 37, 7, 647. ©1999, John Wiley & Sons [6] 139

Stability, Degradation Chemistry and Applications

R

NH3

OH

Ro u

OH

Polybenzoxazines: Chemistry and Properties

4.4 UV Stability and Photochemical Degradation The UV stability of PBZ has also been investigated [18, 19]. It has been determined that a photo-oxidation reaction occurred in BA-m PBZ upon irradiation in air at room temperature under the experimental conditions employed. Model compound studies demonstrated that the isopropylidene linkage was the site at which oxidation and cleavage occurred to form a 2,6-disubstituted benzoquinone. Model compound studies also indicated that the backbone structure of the Mannich bridge did not oxidise in any situation, nor did it cleave by non-oxidatively means, resulting in the formation of a detectable amount of double-bond-containing Schiff base. By the detection of substituted benzoquinone, it is evident that intramolecular hydrogen bonding decreased and intermolecular hydrogen bonding increased as the hydroxyl groups of the phenolic linkages were converted into quinone carbonyls. However, the depth of penetration of UV radiation into the polymer (to produce active radicals), coupled with the distance of diffused oxygen into the polymer to react with the radical species formed, dictated the extent of photo-oxidation in BA-m PBZ. The photo-oxidation of BA-m is proposed as given in Scheme 4.4.

O

N

CH 3

O

N

CH 3

O

ROO hu

O O OR

H H 2C

N

H 3C

N

O

H 3C

N

O

H 3C

N

O

O

O

Scheme 4.4 Proposed mechanism of photo-oxidation of BA-m monomer. Reproduced with permission from J.A. Macko and H. Ishida, Journal of Polymer Science, Part B: Polymer Physics Edition, 2000, 38, 20, 2687. ©2000, John Wiley & Sons [18]

BA-a is shown to have the highest degree of substituted benzoquinone formation followed by those polymers derived from hydroquinone, i.e., 4,4′-(hexafluoroisopropylidene)

140

Stability, Degradation Chemistry and Applications diphenol, 4,4′-thiodiphenol, 4,4′-dihydroxybenzophenone, p-cresol and phenol. The nature of the para-position in phenolic substituents was found to have an impact on the oxidation process and affect the degrees of substituted benzoquinone formation. For the irradiated BA-m monomer, the benzoquinone was the primary photooxidation product. Some secondary reactions were also found to occur as a result of photo-oxidation. The photo-oxidative behaviour of several PBZ based upon various phenols with different phenolic substitution but with the same amine functionality (methylamine) was examined upon exposure to UV radiation (290 nm). Each of the PBZ samples investigated had a substituted benzoquinone photo-oxidation product developed upon irradiation [20]. The thermal characteristics of major thermosetting polymers along with phenolic resin and PBZ were illustrated in Chapter 1. However, for comparison, the degradation mechanism and decomposition products of PBZ are summarised along with those of phenolic resin, cyanate ester (CE) and phenolic-triazine (PT) resins.

4.5 Degradation Mechanism of PBZ: A Comparison with other Thermosets The predecessors of BZ are phenolic resins, so their degradation is significant. Several studies have been carried out to explore the degradation of phenol formaldehyde resins [21–25]. The degradation of phenolic resin under an inert atmosphere involves three stages and finally forms char. The volatiles in the first step of degradation (400 °C. However, the degradation pattern and products vary with structures. In phenolic resins, the major decomposition products are water, H2 gas and finally the fusion of aromatic units to form char, which bring excellent thermal resistance. In PT resin, it combines the degradation products of phenolic resin and CE. In CE, the ratio of CO2/HOCN to substituted phenols is, in general, 21/29% whereas, in PT resins, it is almost equal (~39/40%). Comparative thermograms of these resins are shown in Figure 4.4. The thermal stability indicators (values) of PBZ with these thermosets are compiled in Table 4.2. The onset temperature of the thermal decomposition of PBZ is higher than those of the other two notwithstanding the fact their thermal stability is a slightly inferior compared with others.

Table 4.2 Thermal stabilities of PBZ and other major thermosets Property

PBZ

Phenolic resin

CE resin

PT resin

Tg (°C)

170–340

170

250–270

300

T5 (%)

275–350

335

375–425

420

T10 (%)

325–450

410

440–490

435

Char yield at 700 °C (%)

30–80

58

40–60

63

144

Stability, Degradation Chemistry and Applications 110 100

BA-a Phenolic resisn Cyanate ester P-T resin

90 80

Weight loss (%)

70 60 50 40 30 20 10 0

0

100

200

300

400

500

600

700

800

900

1000

Temperature (° C)

Figure 4.4 Comparative thermogravimetric profiles of BA-a, phenolic resin, cyanate ester and PT resin

4.6 Applications Though a new entrant, PBZ appear to have a lot of applications, as evident from several patents having been registered in this regard. This section is a review of potential patents which go directly to the applications; their properties are also discussed. PBZ find major uses in microelectronics, the aerospace industry, and also in fuel cells. A BZ monomer which was cured with a dicarboxylic acid as a catalyst led to the formation of PBZ surfaces which found use as coating materials in electronic devices such as circuit boards and semiconductors [31]. The processed PBZ have high Tg, low flammability, good electrical properties (e.g., dielectric constant), expansion upon demoulding, post-curing, and cooling and near-zero percentage shrinkage. In addition, this curing technique required only 1–5 min for completion at 150–250 °C. In the fabrication of microchips, semiconductor wafers are processed and sliced into individual chips. These separated chips are then protected by means of package forms.

145

Polybenzoxazines: Chemistry and Properties The protective packages prevent damage to the chip and provide an electrical path to the circuitry of the chip. In general, an epoxy adhesive in liquid form is used as an underfill material. The PBZ-based materials were developed as wafer-level underfill composition as an alternative underfill compound to epoxy resins which have better properties than present epoxy systems [32]. Another application of PBZ is in the form of same underfill material but instead one utilises its lower coefficient of thermal expansion (CTE) [33]. Differences in CTE values between the semiconductor die and the package substrate cause relative shrinkage or expansion, which deteriorates the final performance in the microelectronics industry. The underfill material serves to reduce stresses on the conductive members due to such relative expansion or contraction. It has recently found that PBZ preferably comprising a filler material can be used as an electrical insulation system, for example, for bushings, instruments and distribution transformers [34]. PBZ is anhydride-free and easy to process. It has surprisingly good electrical properties and near-zero shrinkage upon cure. PBZ further offers new manufacturing possibilities which allow a decrease in the production cycle time and also offers the possibility of manufacturing bulky parts with limited residual stresses. The polymers from BZ are useful as precursors for char-yielding material (e.g., precursors to aircraft brake pads). They are also useful as temperature- and flameresistant polymers for electrical components, planes, cars, and buses. PBZ material has the following characteristics that make it desirable as a microelectronic or optoelectronic underfill material: (i) low moisture uptake; (ii) low CTE (iii) good film-forming properties (i.e., thickness control and tackiness at room temperature); and (iv) low volume shrinkage during curing. Table 4.3 compares the properties of a typical PBZ with a typical epoxy resin. Most PBZ-based monomers are solids at room temperature, but they usually have low melt viscosity at slightly elevated temperatures (e.g., 70–80° C). Cured pure PBZ-based film materials also have a lower CTE than a typical epoxy material, which allows lower filler loading to reach the same CTE value as a typical epoxy. The low melt viscosity and lower filler loading facilitate material flow and wetness on contact surfaces, and therefore provide a wider window for processing. The problem of heat dissipation in microelectronics is becoming increasingly important as the demands for denser and faster circuits intensify. To date, the maximum thermal conductivity for commercially available polymer materials remains substantially below 5 W/mK independent of the filler material and/or epoxy resin formulation. Most commercially available moulding compounds presently used in plastic microelectronic packaging typically have thermal conductivity values of ~0.7 W/mK. A composition of PBZ with high thermal conductivity has been introduced. The composition comprises BZ resin and boron nitride as a filler material, which confers thermal conductivity

146

Stability, Degradation Chemistry and Applications between ~3 W/mK and 37 W/mK [35]. The minimum concentration of boron nitride filler is 60 wt%. It was also discovered that very high thermal conductivity is attainable independent of the BZ precursors (in several cases). Polymer compounds with high thermal conductivity are also useful for other products such as computer cases, battery cases, electronic controller housings, and for other encasements where heat removal is an important consideration.

Table 4.3 Properties of a typical PBZ and a typical epoxy resin Property

Typical PBZ

Typical epoxy

Tg (°C)

170

150

Tensile strength (MPa)

130

120

Curing shrinkage (%)

0/expansion

3–4

Moisture uptake (%)

1.5

3

Modulus (GPa)

4

2–3

CTE (ppm/°C)

55

65

Impact strength (J/m)

30

30

Another invention is related to the preparation of novel epoxy moulding resins containing benzoxazines for encapsulation purposes. Known epoxy moulding encapsulants are, in general, prepared from blend epoxy resins, phenol hardeners, silica fillers, catalysts, flame-retardant materials, processing aids and colourants. The addition of PBZ as a co-reactant with one or several epoxy resins provided a product with reduced moisture adsorption while maintaining a high Tg [36]. To minimise the stresses that the package encounters at elevated temperatures in microelectronics, it is desirable that the encapsulant has low moisture adsorption and a Tg as high as possible. Table 4.4 lists the properties of resultant encapsualant material from epoxy-containing BZ. The advantages of these PBZ/epoxy compounds, therefore, include a combination of low moisture adsorption, a high Tg, low viscosity, and good processability. This unique set of properties has not been matched by any other epoxy moulding compound.

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Polybenzoxazines: Chemistry and Properties

Table 4.4 Properties of epoxy-containing benzoxazine for encapsulation use Epoxy moulding compounds containing PBZ Property

Value

Flexural strength (MPa)

109–120

Flexural modulus (MPa)

17,900–20,700

Tg (°C)

130–197

Water absorption at 85% RH/85 °C for 7 days (%)

0.3–0.6

CTE (ppm/°C)

14.4–16.8

Ash content (%)

76.4–79.4

RH = Relative humidity

Polymer electrolyte membrane fuel cells (PEMFC) have been developed as highly efficient cells. A PEMFC comprises a membrane electrode assembly (MEA) including an anode layer, a cathode layer and a polymer electrolyte membrane which is interposed between the two electrode layers. A voltage generated between the anode and the cathode of one fuel cell is ~0.7 V. Therefore, to obtain an appropriate available voltage (10 V to 100 V), several fuel cells must be laminated to form a stack, and adjacent fuel cells separated by bipolar plates are preferable. Hence, the MEA is laminated using a bipolar plate with fluid flow channels formed thereof. The bipolar plate provides an electric connection between the cathode and the anode. It also provides the cathode with a gas flow channel and has strong corrosion resistance and gas impermeability. A composite material for a bipolar plate for application of fuel cells was developed from a PBZ matrix comprising dispersed conductive carbon [37]. These polymers have good workability because there is little volume change during polymerisation, good mechanical and chemical properties, and they can be manufactured at a low cost. Partially polymerised PBZ was introduced into a mould for a bipolar plate, and further polymerisation carried out at 300 °C for 30 min to obtain a bipolar plate. The crosslinked form of PBZ has been developed for fuel-cell applications which have a strong acid-trapping capacity with respect to the BZ monomer and good mechanical properties due to crosslinking. Also, the insolubility of the crosslinked object in

148

Stability, Degradation Chemistry and Applications polyphosphoric acid is desirable, so the crosslinked object is also chemically stable [38]. The electrolyte membrane including the crosslinked object has excellent phosphoric acid-supplementing capacity at a high temperature, as well as good mechanical and chemical stability. Specifically, even if the impregnated amount of a proton carrier (e.g., phosphoric acid) is increased to enhance proton conductivity, the electrolyte membrane has excellent mechanical and chemical stability. Accordingly, the electrolyte membrane can be used in a fuel cell at high temperature and without humidification. A naphthoxazine benzoxazine-based monomer has also been developed with improved performance for use in fuel cells [39]. Conventionally, prepregs are prepared from a matrix resin that is based on one or more epoxy resins. Recently, epoxy resins blended with BZ were found to be potentially useful prepregs [40] because the epoxy resins could reduce the melt viscosity of BZ, facilitating the possibility of higher filler loading while maintaining a processable viscosity. Ceramic–polymer composites were also fabricated from PBZ and barium strontium titanate [41]. It was found that, the composite dielectric constant increases with increasing ceramic content. By adding 80 wt% (48 vol%) of ceramic fillers, the dielectric constant was ≤28. The dielectric constants of the composites were nearly stable, with a frequency range of 1 kHz to 10 MHz and temperature range of 20–130 °C.

4.7 Conclusion This chapter summarised the chemical, thermal and photochemical stability of various PBZ and their degradation features. The ultimate performance of PBZ in harsh environments and outdoor applications is dictated by their thermo-oxidative stability. The degradation of PBZ is non-oxidative. In the thermal degradation of PBZ, the amine fragment is the first decomposition product, which is liberated from the polymer moiety. The thermal stability can be substantially improved by using reactive amines such as acetylene- and nitrile-containing amines. Various decomposition products have been identified by GC-MS analyses. Amines are the major product at temperatures 400 °C. The chemical stability of PBZ in acids is structure-dependent. BA-a is stable against acids, but BA-m decomposes. UV irradiation has a profound effect on PBZ. In the case of BA-a, the isopropylidine linkage is the site for oxidation. In all polymers studied under UV radiation, benzoquinone was the major oxidation product. The applications of PBZ have been found use in microelectronics, laminate prepregs, and in fuel cells.

149

Polybenzoxazines: Chemistry and Properties

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D.W. van Krevelen, Polymer, 1975, 16, 8, 615.

2.

R.A. Dine-Hart and W.W. Wright, Die Makromolekulare Chemie, 1972, 153, 1, 237.

3.

Plastics Additives Handbook: Stabilisers, Processing Aids, Plasticisers, Fillers, Reinforcements, Colorants for Thermoplastics, 4th Edition, Eds., R. Gächter and H. Müller, Hanser/Gardner Publishing, Cincinnati, OH, USA, 1993.

4.

R. Antony and C.K.S. Pillai, Journal of Applied Polymer Science, 1994, 54, 4, 429.

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H.D. Kim and H. Ishida, Journal of Applied Polymer Science, 2001, 79, 7, 1207.

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H.Y. Low and H. Ishida, Journal of Polymer Science, Part B: Polymer Physics Edition, 1999, 37, 7, 647.

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H.Y. Low and H. Ishida, Polymer, 1999, 40, 15, 4365.

8.

K.S. Santhosh Kumar, Investigations on Polybenzoxazines, Their Blends and Composites, Kerala University, India, 2008. [PhD Thesis]

9.

Z. Brunovska, R. Lyon and H. Ishida, Thermochimica Acta, 2000, 357/358, 195.

10. Z. Brunovska and H. Ishida, Journal of Applied Polymer Science, 1999, 73, 14, 2937. 11. S.B. Shen and H. Ishida, Journal of Applied Polymer Science, 1996, 61, 9, 1595. 12. W. Men and Z. Lu, Journal of Applied Polymer Science, 2007, 106, 4, 2769. 13. K. Hemvichian and H. Ishida, Polymer, 2002, 43, 16, 4391. 14. K. Hemvichian, A. Laobuthee, S. Chirachanchai and H. Ishida, Polymer Degradation and Stability, 2002, 76, 1, 1. 15. H.Y. Low and H. Ishida, Journal of Polymer Science, Part B: Polymer Physics Edition, 1998, 36, 11, 1935.

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Stability, Degradation Chemistry and Applications 16. K. Hemvichian, H.D. Kim and H. Ishida, Polymer Degradation and Stability, 2005, 87, 2, 213. 17. S-W. Choi, S. Ohba, Z. Brunovska, K. Hemvichian and H. Ishida, Polymer Degradation and Stability, 2006, 91, 5, 1166. 18. J.A. Macko and H. Ishida, Journal of Polymer Science, Part B: Polymer Physics Edition, 2000, 38, 20, 2687. 19. J.A. Macko and H. Ishida, Polymer, 2001, 42, 1, 227. 20. J.A. Macko and H. Ishida, Macromolecular Chemistry and Physics, 2001, 202, 11, 2351. 21. K.A. Trick and T.E. Saliba, Carbon, 1995, 33, 11, 1509. 22. J. Hetper and M. Sobera, Journal of Chromatography A, 1999, 833, 2, 277. 23. M.C. Roman-Martinez, D. Cazorla-Amoros, A. Linares-Solano, C. SalinasMartinez de Lecea and F. Atamy, Carbon, 1996, 34, 6, 719. 24. L.B. Manfredi, O. Osa, N.G. Fernandez and A. Vazquez, Polymer, 1999, 40, 13, 3867. 25. C. Morterra and M.J.D. Low, Carbon, 1985, 23, 5, 525. 26. M.L. Ramirez, R. Walters, R.E. Lyon and E.P. Savitski, Polymer Degradation and Stability, 2002, 78, 1, 73. 27. I. Hamerton, Chemistry and Technology of Cyanate Ester Resins, Blackie Academic and Professional, London, UK, 1994. 28. D.A. Shimp, S.J. Ising and J.R. Christenson in the Proceedings of the 34th International SAMPE Symposium, Reno, NV, USA, 1989, p.222. 29. D.A. Shimp and S.J. Ising in the Proceedings of the American Chemical Society National Meetings PMSE Division, San Francisco, CA, USA, 1992. 30. M.L. Ramirez, R. Walters, E.P. Savitsky and R.E. Lyon, Thermal Decomposition of Cyanate Ester Resins, Report No. DOT/FAA/AR-01/32, US Department of Transportation (Federal Aviation Administration), Washington, DC, USA, 2001. 31. G.A. Anthony, inventor; Loctite Corporation, assignee; US 6,376,080, 2002.

151

Polybenzoxazines: Chemistry and Properties 32. L. Wang, S. Song-Hua and C. Tian-An, inventors; Intel Corporation, assignee; US 6,730,542, 2004. 33. S. Song-Hua, L. Wang and C. Tian-An, inventors; Intel Corporation, assignee; US 6,899,960, 2005. 34. K. Xavier, K.Kurt, R. Jens and W. Reto, inventors; ABB Research Limited, assignee; EP 1,901,312, 2008. 35. H. Ishida, inventor; Edison Polymer Innovation Corporation, assignee; US 5,900,447, 1999. 36. G. D. William, inventor; Cookson Singapore PTE Limited, assignee; US 6,437,026, 2002. 37. K. Hyoung-Juhn, E. Yeong-Chan, C. Sung-Yong, K. Ho-Jin, M. Jin-Kyoung, L. Dong-Hun, K. Ju-Yong and A. Seong-Jin, inventors; Samsung SDI Co Limited, assignee; US 7,510,678, 2009. 38. C. Seong-Woo, S. Hee-Young, L. Myung-Jin and J. Woo-Sung, inventors; Samsung SDI Co Limited, assignee; EP 1,760,110, 2007. 39. C. Seongwoo and P. Jungock, inventors; Samsung Electronics Co Limited, assignee; EP 2,055,706, 2009. 40. L.L. Stanley, L. W. Helen and W. S. Raymond, inventors; Henkel Corporation, assignee; US 7,537,827, 2009. 41. G. Panomsuwan, H. Ishida and H. Manuspiya in the Materials Research Society Symposium Proceedings, Ed., E. Charson, Volume 993E, 2007, 0993E03-08. [Electronic papers only]

152

A

bbreviations

22BP-a

2,2′-Biphenol- and aniline-based benzoxazine

22P-a

8,8′-Bis(3,4-dihydro-3-phenyl-2H-1,3-benzoxazine)

24DMP-a

2,4-Dimethyl phenol- and aniline-based benzoxazine

35x

3,5-Xylidine

44O-a

6,6′-Bis(2,3-dihydro-3-phenyl-4H-1,3-benzoxazinyl) ketone

4TBUPH-a

4-t-Butyl phenol- and aniline-based benzoxazines

AR

Analar reagent

ATBN

Amine-terminated butadiene–acrylonitrile copolymer

BA-35x

Bisphenol A- and 3,5-xylidine-based benzoxazine

BA-a

Bisphenol A- and aniline-based benzoxazine

BA-a-co-F-1

Copolymer of BA-a and F-1

BA-af

Bis(3-furfuryl-3,4-dihydro-2H-1,3-benzoxazinyl) isopropane

BA-allyl

Diallyl bisphenol A- and aniline-based benzoxazine

BA-apa

Bisphenol A- and 3-aminophenylacetylene-based benzoxazine

BACY

Bisphenol A cyanate [2,2-bis (4-cyanatophenyl) propane]

BAF-apa

Bisphenol F- and aminophenylacetylene-based benzoxazine

B-ala Bisphenol A- and allylamine-based benzoxazine. IUPAC name: [Bis

153

Polybenzoxazines: Chemistry and Properties (3-allyl-3,4-dihydro-2H-1,3-benzoxazinyl) isopropane] BA-m

Bisphenol A- and methyl amine-based benzoxazine

BA-mt

Bisphenol A- and m-methyl aniline-based benzoxazine

BA-ot

Bisphenol A- and o-methyl aniline-based benzoxazine

B-appe

Bisphenol A- and propargylamine-based benzoxazine

BA-pt

Bisphenol A- and p-methyl aniline-based benzoxazine

BATMS-BZ 1,3-Bis(3-aminopropyl)tetramethyldisiloxane) containing a difunctional benzoxazine monomer BDM

4,4′-Bismaleimide diphenyl methane

BIP

4,4′-Biphenol

BIPBZ

4,4′-Biphenol- and aniline-based benzoxazine

BISE 1,3-Bis[3-(4,5-epoxy-1,2,3,6-tetrahydrophalimido) propyl] tetramethyl disiloxane BM-apa 4,4′-Methylenediphenol- and 3-aminophenylacetylene-based benzoxazine BMI

Bismaleimide(s)

BMM

4,4′ Bismaleimido diphenyl methane

BMO-apa Bis(4-hydroxy phenyl) methanone- and 3-aminophenylacetylenebased benzoxazine BMP

2,2-Bis[4-(4-maleimido phenoxy)phenyl]propane

BO-apa 4,4′-Oxydiphenol- and 3-aminophenylacetylene-based benzoxazine BPA-FBZ

Bis(3-furfuryl-3,4-dihydro-2H-1,3-benzoxazinyl) isopropane

BP-apa Biphenyl-4,4′-diol- and 3-aminophenylacetylene-based benzoxazine

154

Abbreviations BPFP-apa 2,2 Bis(4-hydroxyphenyl) perfluoropropane- and 3-aminophenylacetylene-based benzoxazine BS

Benzoxazine-silica fibre composites

BS2

Benzoxazines/silica fibre

BS-apa 4,4′-Thiodiphenol- and 3-aminophenylacetylene-based benzoxazine BSM

Benzoxazine-silica-microballon composites

BSM3

Benzoxazines/silica fibre/microballoon

BSO-apa Bis(4-hydroxyphenyl)sulfone- and 3-aminophenylacetylene-based benzoxazine BZ

Benzoxazine(s)

BZ-CH 1,1′ Bis(4-hydroxyphenyl) cyclohexane- and aniline-based benzoxazine BZ-PD-CH 1,1′ Bis(4-hydroxyphenyl) pentadecyl cyclohexane- and anilinebased benzoxazine BZ-PHC-CH 1,1′ Bis(4-hydroxyphenyl) perhydrocumene cyclohexane- and aniline-based benzoxazines CCW

Cure-controllable window

CD

Cyclodextrin

CE

Cyanate ester(s)

CF

Carbon fibre(s)

CTBN

Carboxyl-terminated butadiene acrylonitrile rubber

CTE

Coefficient(s) of thermal expansion

DABA

Diallyl bisphenol A

DCPD

Dicyclopentadienephenol

155

Polybenzoxazines: Chemistry and Properties DCPDBZ

DCPD-based benzoxazine monomer

DDM

4,4′-Diaminodiphenyl methane

DGEBA

Diglycidyl ether bisphenol A

DMA

Dynamic mechanical analysis

DMF

Dimethylformamide

DMTA

Dynamic mechanical thermal analysis

DOPO

9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide

Dopot-m

Dopotriol- and methylamine-based benzoxazine

DSC

Differential scanning calorimetry

EPN

Epoxy phenol novolac resins

F-1

Fluorinated benzoxazine

F-a

6′,6-Bis(3-phenyl-3, 4-dihydro-2H-1, 3-benzoxazineyl) methane

FTIR

Fourier-transform infrared

G′

Storage modulus

G′′

Loss modulus

GC-MS

Gas chromatography–mass spectroscopy

HHPA

Hexahydrophthalic anhydride

HMPA

Hexahydro-4-methylphthalic anhydride

HPM

N-(4-Hydroxy phenyl) maleimide

HPM-Ba

[N-(4-Hydroxyphenyl) maleimide]-benzoxazine

ILSS

Interlaminar shear strength

IPN

Interpenetrating polymer network(s)

IR

Infrared

156

Abbreviations LOI

Limiting oxygen index

LSS

Lap shear strength

Mal-BZ

4-Maleimidophenol- and aniline-based benzoxazine

Mal-BZ-Al

Hydroxyphenylmaleimide- and allyl amine-based benzoxazine

Mal-BZ-Pg Hydroxyphenylmaleimide- and phenyl propargyl ether-based benzoxazine MAS

Magic angle spinning

MBZ-POSS

Octafunctional cubic silsesquioxane

MEA

Membrane electrode assembly(ies)

MIB

Maleimido benzoxazine

MI-COOH

4-Maleimido benzoic acid

MI-H

4-Maleimido benzene

MI-OH

4-Maleimido phenol

MMT

Montmorillonite

MPN Novolac modified by propargyl and methylol groups simultaneously mt

m-Toluidine

MWNT

Multi-walled carbon nanotube(s)

NMR

Nuclear magnetic resonance

NOB

Norbornene benzoxazine

OAPS

Octaaminophenyl polyhedral oligomeric silsesquioxane

OF

Octakis (dimethylsiloxyhexafluoropropyl ether) silsesquioxane

OMMT

Organically modified montmorillonite(s)

157

Polybenzoxazines: Chemistry and Properties OpePOSS

Octa(propylglycidyl ether) polyhedral oligomeric silsesquioxane

ot

o-Toluidine

P-a

Phenol- and aniline-based benzoxazine

PAA

Poly(amide acid)

Pad-12 Phenol- and dodecamethyl diamine-based benzoxazine with 12 methylene groups Pad-2 Phenol- and dimethyl diamine-based benzoxazine with 2 methylene groups Pad-4 Phenol- and tetramethyl diamine-based benzoxazine with 4 methylene groups Pad-6 Phenol- and hexamethyl diamine-based benzoxazine with 6 methylene groups Pad-8 Phenol- and octamethyl diamine-based benzoxazine with 8 methylene groups P-af

3-Furfuryl-3,4-dihydro-2H-1,3-benzoxazine

P-ala Phenol- and allylamine-based benzoxazines (3-allyl-3,4-dihydro2H-1,3-benzoxazine) P-alp 2-Allylphenol- and aniline-based benzoxazines (3-phenyl-3,4dihydro-8-allyl-2H-1,3-benzoxazine) P-a-PCL

P-a containing PCL

P-appe

Phenol- and propargylamine-based benzoxazine

PBA-a

Bisphenol A- and anline-based polybenzoxazine

PBA-af

Bis(3-furfuryl-3,4-dihydro-2H-1,3-benzoxazinyl) isopropane

PBA-allyl

Diallyl bisphenol A- and aniline-based polybenzoxazine

PB-hda

Bisphenol A- and hexamethyl diamine-based polybenzoxazine

PB-mda

Bisphenol A- and dimethyl diamine-based polybenzoxazine

158

Abbreviations PBO

2,2′-(1,3-Phenylene)-bis (4,5-dihydro-oxazoles)

PBPA

Bis propargyl ether bisphenol A

PBZ

Polybenzoxazine(s)

PC

Polycarbonate

PCL

Polycaprolactone

PCL-NZ

Polycaprolactone-naphthoxazine

PDMS

Poly(dimethylsiloxane)

PEMFC

Polymer electrolyte membrane fuel cells

PEP

Phenylethynylphenol

PEPFN

Phenyl ethynyl functional novolac resins

Ph-apa

Phenol- and 3-aminophenylacetylene-based benzoxazine

PHPM-Ba-I N-(4-Hydroxyphenyl) maleimide]-benzoxazine in which only maleimide groups are polymerised PHPM-Ba-II N-(4-Hydroxyphenyl) maleimide]-benzoxazine in which both maleimide and benzoxazines groups are polymerised PI

Polyimide

PMF resin

Phenolic novolac resins bearing maleimide groups

PN resins Propargyl ether-functional phenolic resins POSS

Polyhedral oligomeric silsesquioxane

PP-a

Polymer of phenol and aniline based benzoxazine

PP-af

Polymer of phenol and aminofuran based benzoxazine

ppm

Parts per million

PPOA

Branched (triamines) poly(propyleneoxide)(s)

159

Polybenzoxazines: Chemistry and Properties pt

p-Toluidine

PT

Phenolic-triazine

PU

Polyurethane

PU/P-a–OMMT Polyurethane-benzoxazine/clay hybrid nanocomposites RH

Relative humidity

SEM

Scanning electron microscopy

SPI

Polysiloxane-block-polyimide

T10%

Temperature at which 10% weight loss occurs

T5%

Temperature at which 5% weight loss occurs

TBA

Torsional braid analysis

Tf

Final cure temperature

Tg

Glass transition temperature

TGA

Thermogravimetric analysis

Ti

Initial cure temperature

Tid

Initial decomposition temperature(s)

Tp

Peak cure temperature

UV

Ultraviolet

VB-a

Vinyl-terminated benzoxazine monomer

Xdensity

Crosslink density

160

I

ndex

A Acetylene, polymerisation of 63 Acrylates 1 3-Allyl-3,4-dihydro-2H-1,3-benzoxazine 52 Allylation, degree of 9 Allyl benzoxazine 91 Allyl, curing of 4 Allyl naphthols 19 Allyl phenoxy phosphazene 15 Allyl phenoxy triazine 15 Allyl phenyl oligomers 6 Aniline dimmers, asymmetric 33 Arylamine Mannich bridge network 29, 30-31, 79

B β-Cyclodextrin 84 Benzoquinone 141 Benzoxazine 25, 29-30, 51, 55-61, 64, 66, 71, 80-84, 95, 101, 105, 115, 141 allyl based 53-55, 72, 106-107 2-allylphenol and aniline based 52 3-aminophenylacetylene based 64 aniline-based/bismaleimide system 104 2,2-bis(4-hydroxyphenol)perfluoropropane and 3-aminophenylacetylene based 64 bisphenol A and allylamine based 71 bisphenol A and aniline based 25, 28, 34-36, 53, 55, 61, 63, 80-81, 87, 89-90, 98, 100, 108, 112, 114-115, 132, 145 bisphenol A and m-methyl aniline based 80 bisphenol A and o-methyl aniline based 80-81 bisphenol A and p-methyl aniline based 80-81 bisphenol A and propargylamine based 59-60, 73 bispenol A and 3,5-xylidine based 80-81 curing 35

161

Polybenzoxazines: Chemistry and Properties epoxy copolymers 95 functionalised 51-52 hydroxyphenylmaleimide- and allyl amine based 72 hydroxyphenylmaleimide- and phenyl propargyl ether based 72 maleimido 57, 73 4-maleimidophenol- and aniline based 72 methyl amine based 132, 141, 149 model dimer 31 norbornene 57-58, 73 oligomers 34 polymerisation 67 ring 96-97 ring-opening polymerisation 63 phenol and allylamine based 52 phenol-and aminofuran based 63 phenol- and 3-aminophenylacetylene based 64 phenol and propargylamine based 59-60, 73 phthalonitrile 65-66 propargyl 61 silica composites 123-125 silica-microballon 123-125 silane-coupling agent 116 4,4'-Biphenol 77 Bis(3-allyl-3, 4-dihydro-2H-1,3-benzoxazinyl) and isopropane 52 6',6-Bis(3-phenyl-3,4-dihydro-2H-1,3-benzoxazineyl) methane 82 Bis propargyl ether bisphenol A 10 Bis(3-furfuryl-3,4-dihydro-2H-1,3-benzoxazinyl) isopropane 63 Biscitraconimides 10 4,4'-Bismaleimide diphenyl methane resins 10 Bismaleimide resins, aromatic 6 Bismaleimides 1, 5-6, 10, 12-15, 19, 21, 24-25, 101, 103, 107 4,4'-Bismaleimido diphenyl methane 8 Bisoxazoline-phenolic system 19-20 Bisphenol 86 Bisphenol A 25 Bisphenol A and dimethyl diamine-based polybenzoxazines 85 Bisphenol A cyanate ester (BACY) 12-13, 143 Bisphenol F epoxy resins 3 Bisphenolic methylene bridge network 30 Bismaleimide-modified polyurethane-epoxy systems 8 Bismaleimide-triazine resins 10 Bisphenol A, brominated 3

162

Index Butadiene-acrylonitrile copolymer, amine-terminated 101, 120 Butadiene acrylonitrile rubber, carboxyl-terminated 101, 120

C Carbon fibre 24, 116, 118-120 loading 119 Carbon nanotube composites, epoxy-coiled 4 Carboxylic acid treatment 132 Casting 19 Ceramic-polymer composites 149 Click chemistry 67-68 Coefficient of thermal expansion 63, 124, 146 Composites, fibre 115 Composites, silica fibre-filled 122 Composites, void-free 12 Copolybenzoxazines 83 Copolymerisation 96 Cracking 141 Cure window 102 Curing 1, 3, 20, 24, 36, 53. 69, 82, 96 temperature 79 initial 102 thermal 6, 12 Cyanate trimerisation 110 Cyclodextrin 84 Cyclohexyl-benzoxazine monomers 87 Cyclotrimerisation 12 thermal 15, 143 Cynate ester resins 1, 9, 12-15, 25, 108 141, 142-144

D Decomposition temperature, initial 134-135 Decyclisation, thermal 142 Degradation, oxidative 142 Dehydration 141 Dehydrogenation 141 Dehydrohalogenation 2 Diallyl bisphenol A 4, 8 Diamine-terminated amide resins 8 4,4'-Diaminodiphenyl methane 86 Dicyclopentadiene phenol adduct 77 Dicyclopentadiene phenol – epoxy phenol novolac resins 4

163

Polybenzoxazines: Chemistry and Properties Dielectric constant 149 Diels-Alder polymerisation 7 Diels-Alder reaction 6-8, 19 Differential scanning calorimetry 5, 21, 35, 53, 67, 69, 87, 98, 108-110, 113 Diglycidyl ether bisphenol A 2-3, 96-97 Dihydrobenzoxazines 77 9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide 82 Dimers, asymmetric model 32 Dopotriol 82 Dopton-m 81 Double quantum magic angle spinning spectroscopy 34 Dynamic mechanical analysis 26, 63, 77, 124 Dynamic mechanical thermal analysis 89, 101, 103, 111

E Ene-Alder reaction 8 Epichlorohydrin 2 Epoxies 13-14, 25 Epoxy moulding compounds 148 Epoxy moulding resins 147 Epoxy, phenolic-based 4 Epoxy ring 97 Ether resins 21

F Fibre clustering 117 Fibre loading 120 Final cure temperature 101-102 Formaldehyde 11 Fourier-transform infrared spectroscopy 31, 35, 69, 102, 138, 143 Free-radical mechanism 57 (3-Furfuryl-3,4-dihydro-2H-1,3-benzoxazinyl) isopropane 61 3-Furfuryl-3, 4-dihydro-2H-1, 3-benzoxazine 61

G Gas chromatography-mass spectroscopy 135, 138, 143, 149 Glass fibre-reinforced plastics 19 Glass transition temperature 5-6, 8-10, 13-14, 19, 23-24, 26, 28, 31, 34-35, 55, 63, 65, 78-79, 82, 85, 88-89, 91, 95, 98, 100, 104, 108, 111-112, 114-115, 124, 135, 147 Glycidyl-based resins 2

164

Index

H H-nuclear magnetic resonance spectroscopy 69 Hompolymerisation 109 [N-(4-Hydroxyphenyl)maleimide]-benzoxazine 56, 73 Hydrogen bond, chelated 34 Hydrogen bonding 31-32, 34, 36, 102-103, 141 Hydrolysis 13, 115 Hydroxyphenylmaleimide 11, 21 [N-4(4-Hydroxyphenyl) maleimide]-benzoxazine 55 1

I Interlaminar shear strength 116 Interpenetrating polymer networks 8-9, 100 Isocyanate-derived polymers 1 Isomerisation, thermal 142

K Kapton films 5 Kenaf fibre 126 Kissinger method 61

L Lap shear strength 11 Limiting oxygen index 56

M Macroscopic stress cracking 132 Maleimide end-capping agents 6 Maleimide polymerisation 101 Maleimide-terminated oligomers 6 Mannich base bridge 28, 32, 34, 132, 136 Membrane electrode assembly 148 Microballoons 121-123 Molecular degradation, ultraviolet-induced 131 Molybdenum-phenolic resins 23 Montmorillonite 113, 115 organically modified 113 Moulding, property-oriented 95 Mouldings 19 Multi-component polymeric material 95 Multi-walled carbon nanotubes 3, 24, 115

165

Polybenzoxazines: Chemistry and Properties

N Nanocomposites, epoxy-clay 4 Nanocomposites, polymer-clay 113 Nanocomposites, polymer-layered silicate 112 Naphthoxazine benzoxazine based monomer 149 Novolac resin, ethynyl 21 Novolac resins, epoxy cresol 3 Novolac resins, epoxy phenol 1-4, 11, 21, 24, 146, 149 Novolac resins modified by propargyl and methylol groups 23 Novolac resins, phenolic 96 Nuclear magnetic resonance 28, 34, 69, 85 solid-state 13C-cross polarisation/magic angle spinning 85

O Octa-aminophenyl polyhedral oligomeric silsesquioxane 114 Octakis 4

P Peak cure temperature 87, 101-102 Phenol-(4-hydroxy) phenylmaleimide-formaldehyde resins 10, 11, 21 Phenolic Mannich bridges 79 network 30 Phenolic resin 1, 17-18, 24, 77, 131-133, 141-142 epoxy systems 20 Phenolic system, addition-cure 24 Phenolic-triazine resins 13, 15, 141, 143-144 Phenolics, allyl-functional 19 Phenolics, condensation-type 17 Phenols, alkenyl 8 3-Phenyl-3-4,-dihydro-8-allyl-2H-1,3-benzoxazine 52 Phenyl ethynyl functional novolac resins 23 N-phenyl maleimides 58 Phosphazene-triazine network polymers 15-16 Phenol novolac resin with maleimide groups – epoxy novolac resin 11 Polyacetylenes 61 Polyacids 3 Polyamines 3 Polybenzimidazole 8 Polybenzoxazine acetylene-functional 140 bisphenol A and aniline based 99 composites 126

166

Index curing 35 cyclohexyl-based 88 hexamethyl diamine-based 85 microballoon-embedded 123 micro composites 115 model 137 model oligomers 71 nanocomposites, clay-based 112 nanomaterials 112 syntactic foams 121-122, 126 phthalonitrile 65 poly(ε-caprolactone) 97, 115 polyhedral oligomeric silsesquioxane systems 114 polyimide blends 98 wood composites 126 Polycaprolactone-naphthoxazine 69 Polycyanurate 12-14 Polycyclotrimerisation 12 Polyesters 14, 25 unsaturated 1 Polyether imides 14 Polyether ketones 14 Polyethersulfone 14 Polyimides 5, 25 Polymer electrolyte membrane fuel cells 148 Polymer matrix 116 Polymers, Alder-ene 19 Polymers, high-performance 1, 27 Polymercaptans 3 Polymerisation 12, 21-22, 25, 28-31, 59, 61, 79, 83, 96, 100, 103, 109, 113, 135, 148 allyl 52-53 oxazine 52, 115 ring-opening 24, 26, 31, 35, 59, 63, 113 Polyoxyalkyleneamines 8 Polyphenols 3 Polypropyleneoxides 69 Polysiloxane-block-polyimide 99 Polythiourethanes, mercaptan-terminated 3 Powder-blended system 109 Powder-mixed blend 110 Powder-mixed system 109

167

Polybenzoxazines: Chemistry and Properties Pre-polymers, acetylene-terminated 63 Propargyl ether 21 Propargyl ether-functional novolac resins 21-23 Pyrolysis 24 Pyrolysis gas chromatography 54

R Regioselectivity 28 Relaxation processes, low-temperature 78 Resin transfer moulding technique 126 Rheokinetic measurements 35 Rubber toughening 7

S Scanning electron microscopy 103-104, 107, 116 Schiff base 138, 140 Semi-interprenetrating networks 98 Solution-blended system 109-110 Solvent method 108 Solvent-less method 51 Storage modulus 126 Symmetric aniline dimers 33 Syntactic foams 121-124, 126

T Tetraglycidyl-4,4-diaminodiphenylmethane 3 Thermogravimetric analysis 90, 111, 133, 135 dynamic 135 isothermal 135 Thermogravimetric analysis - Fourier-transform infrared spectroscopy 135 Thermal oxidation 17 Thermo-oxidative degradation 13 Thermoplastic resins 14 Thermoset polymers 1 Thermoset resins 13 Thermosetting resins 25 Torsional braid analysis 35 Toughening 14, 112

V Vinyl esters 25

168

Index

W Wagner-Jauregg reaction 19 Water contact angle 90 Weight-loss curve, derivative 67

X X-ray diffraction analysis 85, 113

169

Polybenzoxazines: Chemistry and Properties

170