Reactive Polymers Fundamentals and Applications || Bismaleimide Resins

11

Bismaleimide Resins

Bismaleimide resin systems are noted for their high-strength, high-temper-ature performance, particularly as matrix resins in fiber-reinforced prepregsand composites. They are bridging the gap between the relatively low tem-perature-resistant epoxy systems and the very high temperature-resistantpolyimides. Unfortunately, bismaleimides are somewhat brittle, and thussubject to impact induced damage.

11.1 MONOMERS

Monomers for bismaleimide resins are summarized in Table 11.1 and areshown in Figure 11.1.

11.1.1 4,4′-Bis(maleimido)diphenylmethane

The most important monomer is 4,4′-bis(maleimido)diphenylmethane(BMI). BMI has a melting temperature of 155 to 156°C and it polymerizesradically above the melting point. Networks resulting from BMI are verybrittle.

11.1.2 2,2′-Diallyl bisphenol A

BMI can be used together with 2,2′-Diallyl bisphenol A (DBA). DBA co-polymerizes with BMI. The reaction is an ene reaction that leads to a chain

397

398 Reactive Polymers Fundamentals and Applications

Table 11.1: Monomers for Bismaleimide ResinsCompound Reference

4,4′-Bis(maleimido)diphenylmethane (BMIM)a

Bisphenol A bismaleimide (BMIP)b

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

2,2′-Diallyl bisphenol A (DBA)1,3-Bis(maleimidomethyl)cyclohexane 1

Multi Ring Maleimides 2

N,N-4,4-Diphenylmethanebismaleimide 3

N,N-4,4-Diphenyl ether bismaleimide (BMIE) 3

N,N-4,4-dibenzylbismaleimide 3

Bis(4-maleimidophenyl)sulfone (BMIS)1,6-Hexane bismaleimide 4

Divalent metal bismaleimides 5

4-(N-maleimidophenyl)glycidyl ether (MPGE) 6

4,4′-Bismaleimidophenylphosphonate 7

Bismaleimide bisimides 8

Imides with pendant naphthalene 9

Ester-containing bismaleimides 10, 11

Cardo ester bismaleimides 12

Poly(aminoaspartimide)s 12

a also BMI, BMDPM and BDM, however BMI is used in gen-eral for bismaleimides

b BMIP is not uniquely used in the literature. BMIP stands foreither Bisphenol A bismaleimide or 2,2-Bis[4-(4-maleimidophenoxy)phenyl]propane

Bismaleimide Resins 399

O NN

O

O

O

O

BMIE

C

CH3

CH3

O O

O

O

N

O

O

N

BMIP

NS

O

O

O

O

-ON S

O

O

O

O

O- M2+

Divalent metal bismaleimide

S NN

O

O

O

O

O

O

BMIS

CH2 NN

O

O

O

O

BMIM

Figure 11.1: Bis(4-maleimidophenyl)methane (BMIM), Bis(4-maleimidophen-yl)ether (BMIE), Bis(4-maleimidophenyl)sulfone (BMIS), 2,2-Bis[4-(4-maleim-ido phenoxy)phenyl]propane (BMIP), Divalent metal bismaleimide


extension reaction. Subsequently a Diels-Alder reaction follows. Such co-polymers exhibit less brittleness, because the crosslinking density is lessthan that of pure BMI resins. Mixtures of 2,2′-Diallyl bisphenol A etherand 1,4-diallyl phenyl ether also have been used. These compounds arereactive diluents for BMI because they reduce the apparent viscosity of theBMI.13 N,N′-Diallyl p-phenyl diamine (DPD) is a reactive diluent in thissense. Reducing the viscosity is important for the preparation of the ad-vanced composites by techniques such as resin transfer molding (RTM).14

For example, instead using diallyl bisphenol A, a novolak resin canbe obtained from diallyl bisphenol A and formaldehyde using p-toluene-sulfonic acid as catalyst. The resin is then reactively blended with bis-phenol A bismaleimide (BMIP) and cured through an Alder-ene reactionat high temperatures. The materials are useful as adhesives.15 The lapshear strength properties are not significantly affected by the structure ofthe particular BMI used. It has been demonstrated that using 2,2′-Diallylbisphenol A gives products with better adhesion at elevated temperature.16

11.1.3 Poly(ethylene glycol) End-capped with Maleimide

The addition of maleimido end-capped poly(ethylene glycol) (PEG) toa bismaleimide resin (4,4′-bis(maleimido)diphenylmethane) (BDM) en-hances the processability of the BDM resin significantly. The processingtemperatures of the BDM resin increase from approximately 20 to 80°C.However, the modified resins show a decreased thermal stability of theblended BDM resin, and the coefficient of thermal expansion increases.The curing behavior and the thermal and mechanical properties are inde-pendent of the molecular weight of the PEG segment.17

11.1.4 Bismaleimide Bisimides

The monomers for bisimide resins are prepared by reacting N,N ′-(4-am-inophenyl)-p-benzoquinone diimine (QA) with maleic anhydride or 5-nor-bornene-2,3-dicarboxylic anhydride (also called nadic anhydride) in glacialacetic acid as shown in Figure 11.2. The cured resins exhibit a char residueat 800°C in nitrogen atmosphere greater than 55%. Chain-extended typeswith flexible ether linkages, i.e., 1,3-bis(4-maleimido phenoxy)benzene or1,4-bis(4-maleimido phenoxy)benzene, show a lower thermal stability thanthe neat resins.8


NN NN

O

O

O

O

NN NN

O

O

O

O

O

O

O

NN NH2H2N

O

O

O

Figure 11.2: Bismaleimide Adducts of N,N ′-(4-Aminophenyl)-p-benzoquinonediimine with Maleic anhydride and Nadic anhydride8


11.1.5 Maleimide Epoxy Monomers

The use of 4-(N-maleimidophenyl)glycidyl ether (MPGE) is a convenientapproach for synthesizing BMIs with epoxy linkage backbones.6 MPGE issynthesized from N-(4-hydroxyphenyl)maleimide and epichlorohydrin byusing benzyltrimethylammonium chloride as a catalyst.18

In a similar manner, maleimide-modified epoxy compounds can beprepared from N-(4-hydroxyphenyl)maleimide (HPM) with the diglycidylether of bisphenol A.19 The reaction scheme is shown in Figure 11.3. Tri-phenylphosphine and methylethylketone were utilized as catalyst and sol-vent, respectively. The resulting compounds bear both the oxirane ring andthe maleimide group.

Curing can be achieved by amine curing agents, such as 4,4′-diam-inodiphenylmethane (DDM) and dicyandiamide (DICY). The incorpora-tion of maleimide groups into epoxy resins provides a cyclic imide struc-ture and high crosslinking density. The cured resins show high char yieldsand high LOI values up to 30.

Further, specific chemical groups can be introduced into the BMIbridging linkages, such as silicon groups and phosphorus groups. The di-merization is shown in Figure 11.4. The cured resin with silicone exhibitsa limiting oxygen index of greater than 50.

11.1.6 Phosphorous-containing Monomers

A phosphorus-containing bismaleimide (BMI) monomer, bis(3-maleim-idophenyl)phenylphosphine oxide (BMIPO), can be accessed by theimidization of bis(3-aminophenyl)phenylphosphine oxide. This bismale-imide exhibits a good solubility in common organic solvents and a wideprocessing window.20, 21 It is an excellent flame retardant with a high glasstransition temperature, high onset decomposition temperature, and highlimiting oxygen index. Copolymers with BMIPO, BMI, and epoxy based4,4′-methylenedianiline (DDM) are homogeneous products without phaseseparation.22

Epoxy resins can be modified by 3,3′-bis(maleimidophenyl)phenyl-phosphine oxide. The cured resins have good thermal properties.23 Fur-ther, phenyl-(4,4′-bismaleimidophenyl)phosphonate and ethyl-(4,4′-bis-maleimidophenyl)phosphonate were tested as flame retardants in epoxysystems. The flame retardancy of phosphonate-containing epoxy systemswas improved significantly with BMI.24 An increase of the BMI com-


C CH3

O

O

H3C

CH2

CH2

CH

CH

CH2

CH2

O

O

NO O

OH

C CH3

O

O

H3C

CH2

CH2

CH

CH

CH2

CH2

O

OH

O

O

O

N

+

N

O

O

OHOHH2NO

O

O

+

Figure 11.3: Synthesis of Epoxy-modified Maleimide Monomers19


N

O

O

O CH2 CH

OH

CH2

O

R

O

CH2

OH

CH CH2 O

O

O

N

OHRHO

N

O

O

O CH2 CH CH2

O

MPGE

R= one of the following groups

C

CH3

CH3

O P OSi

Figure 11.4: Dimerization of 4-(N-maleimidophenyl)glycidyl ether (MPGE) withFunctional Diols


Table 11.2: Reactions of MaleimidesReaction Type Reference

Radical polymerizationDiels-Alder reaction with a pentamathylcyclopenta-diene derivative

25

Diels-Alder reaction with furans 26

pounds also increased the storage modulus and glass transition temperaturebut reduced the mechanical strength of the epoxy blends.

More bulky phosphorous-containing bismaleimides have been ob-tained by the reaction of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and 4,4′-bis(maleimido)diphenylmethane.27, 28 Theglass transition temperatures of the cured resins decrease with phosphoruscontent. The limiting oxygen index (LOI) is improved by the incorporationof DOPO.

11.1.7 Multiring Monomers with Pendant Chains

The synthesis of multiring monomers with long pendant chains is shownin Figure 11.5. The synthesis runs via a two-fold Friedel-Crafts reaction,followed by a reduction of the dinitro compounds. The diamines are thenreacted with maleic anhydride into bismaleimides. The properties of thecrosslinked poly(benzylimide) are not strongly affected by the presenceof the long alkyl chains. Therefore, linear thermoplastic polyimides withgood thermal stability can be obtained.2

11.1.8 Reactions of Maleimides

We summarize some of the reactions of maleimides, all of them suitablefor obtaining polymers. The reactions are given in Table 11.2.

11.1.8.1 Radical Polymerization

The double bond in the maleic group undergoes an ordinary radical poly-merization.


CH3

CH3

H3C

R

CH2H2CN N

O

O

O

O

O

O

O

CH3

CH3

H3C

R

CH2H2C NO2O2N

CH3

CH3

H3C

R

CH2H2C NH2H2N

+CH3

CH3

H3C

R

R=C16H33;C8H17;C6H13

CH2Cl NO2

Figure 11.5: Multiring Monomers with Flexible Side Chains2


11.1.8.2 Michael Addition

The Michael addition is an addition of resonance-stabilized carbanions toactivated double bonds. The Michael addition is thermodynamically con-trolled. It was first described in 1887.29

α,ω-Polyaminoglycols. Amino-terminated oligomers based on propyleneglycol, ethylene glycol, and dimethylsiloxane, have been chain-extendedvia Michael additions with bismaleimides. The polymers have a degreeof polymerization up to 15. The polymers are either linear or crosslinked,depending on the starting materials and the conditions of preparation.30, 31

Maleimide-urethanes. The reaction of 4-maleimidophenyl isocyanateand oligoether diols or oligoester diols results in bismaleimide-containingurethane groups. The bismaleimides can be chain-extended by means of aMichael reaction into linear polymers.32 The reaction scheme is shownin Figure 11.6. Chain extenders are 4,4′-diaminodiphenylmethane and4,4′-oxydianiline. Elastic films are obtained that show good mechanicalproperties and a better thermal stability than the traditional polyurethaneelastomers.

11.1.8.3 Diels-Alder Reaction

Chain Extension. Bismaleimide oligomers can be synthesized by chainextension reaction utilizing a Diels-Alder reaction as shown in Figure 11.7.In the first step, a bisfuranylmethylcarbamate is formed from toluene di-isocyanate (TDI), or hexamethylene diisocyanate with two mol of fur-furyl alcohol. The furan (via its double bonds) then reacts with a bis-maleimide, such as 4,4′-bis(maleimido)diphenylmethane using a Diels-Alder reaction.33 These bismaleimide oligomers can be used as a tough-ness modification agent for other BMI resins. Finally, the ether link in theoriginal furan moiety is eliminated by acetic anhydride, and replaced by anaromatic group.

Furan-containing Adducts. Furan-terminated compounds react withBMI at 70°C to an oxygen-containing cycloadduct. The simple adducts areobtained from the monofunctional dienophiles. Crosslinked products areobtained from the coupling of furanic polymers with the bisdienophiles.26


NHN

O

O

C

O

O O

O

C

HN

O

O

N

O NH2H2N

4,4’-Oxydianiline

4,4’-Diaminodiphenylmethane

CH2 NH2H2N

+ +NCON

O

O

N

O

O

OCNHO OH

Figure 11.6: Dimaleimide urethanes and Michael Reaction with aromatic Diam-ines


OCH2 OH ROCN NCO

OCH2HO

OCH2O

RHN NH

C C

O O

OCH2O

N

O

O

OCH2O

RHN NH

C C

O O

OCH2O

N NOO O O

CH3 C

O

O C

O

CH3

CH2O

RHN NH

C C

O O

OCH2

N NOOOO

+

Figure 11.7: Chain Extension Reaction


On heating the polymerized materials in various solvents with high boilingpoints, no soluble products were obtained. This indicates the absence of aretro Diels-Alder reaction.

It was concluded that aromatization of the imino heterocycles arisingfrom the cycloaddition took place, resulting in irreversible crosslinks. Forexample, 1,1′-(1-methylethylidene)bis(4-(1-(2-furanylmethoxy)-2-propan-olyloxy))benzene reacts with several bismaleimides, such as N,N ′-hexa-methylenebismaleimide and N,N′-p-phenylenedimaleimide. In a subse-quent polymerization in the presence of acetic anhydride the aromatizationof the tetrahydrophthalimide intermediates occurs.34

Networks from the linear copolymer Poly(styrene-co-furfuryl meth-acrylate) can be prepared by Diels-Alder reaction at 25°C by adding bis-maleimide.35 In such a crosslinked copolymer, an endothermic peak with-out a glass transition is observed. On reheating the sample, a glass tran-sition is found. This is attributed to the formation of a linear copolymerproduced by the retro Diels-Alder reaction in the course of the first heattreatment.36

Bisdienes. Phenylated poly(dihydrophthalimide)s have been synthes-ized from 3,3′-(oxydi-p-phenylene)bis(2,4,5-triphenylcyclopentadienone),3,3′-(p-phenylene)bis(2,4,5-triphenylcyclopentadienone), N,N ′-o-phenyl-enedimaleimide, N,N′-m-phenylenedimaleimide, and N,N ′-p-phenylenedi-maleimide.37 Ketonic adducts are formed as intermediates, but the carbonmonoxide evolution proceeds spontaneously. Difunctional cyclohexadi-enes with dihydrophthalimide as central units can act as bisdienes in Di-els-Alder polymerization polyadditions with bis(4-(1,2,4-triazoline-3,5-di-one-4-yl)phenyl)methane as the difunctional dienophile. The introductionof phenyl side groups increases the solubility.38

Pyrones. Pyrones also behave as diene and react with bismaleimides,thus forming a bis-cycloadduct.39

Diabietylketone. Another bisdiene is the dehydrodecarboxylation prod-uct of abietic acid, also addressed as diabietylketone.40 The dehydro-decarboxylation reaction is shown in Figure 11.8. A Diels-Alder poly-merization of diabietylketone with 4,4′-diphenylmethanedimaleimide (bis-maleimide) is possible. The resulting polymer is expected to be a poly-(ketoimide) with hydrophenanthrene moieties in the backbone. However,


COOH

C O

Figure 11.8: Dimerization of Abietic Acid by Dehydrodecarboxylation

it was found that the repeating units are bismaleimide and diabietylket-one units not in a molar ratio of 1:1, but in a ratio of 5:1 to 6:1. Thisobservation was explained by the difference between the rates of the twoconcomitant reactions, i.e., the homopolymerization of bismaleimide andthe Diels-Alder polymerization.

On the other hand, a polymer of the two monomer units in a ratioof 1:1 can be obtained by the dehydrodecarboxylation of the diacid result-ing from the Diels-Alder reaction between abietic acid and 4,4′-diphen-ylmethanedimaleimide and also by the polycondensation of the ketone ofmaleated abietic acid with 4,4′-diaminodiphenylmethane. The polymersare stable in air up to 360°C.

Photochemical Generation of Dienes. Certain dienes, such as o-quinodi-methanes can be generated by photochemical reactions.41 When the pho-tochemical generation occurs in the presence of bismaleimides, the dienesmay react immediately with the bismaleimide in a Diels-Alder reaction,thus forming a polymer.

Naphthols. Several 2-naphthols undergo a Diels-Alder addition reactionwith maleimides. This reaction can be utilized in curing bismaleimides.For example, 7-allyloxy-2-naphthol satisfactorily cures bismaleimides.42


Urethane-imides. Poly(ester-urethane-imides) can be prepared by theDiels-Alder polyaddition of 1,6-hexamethylene-bis(2-furanylmethylcarb-amate) with various bismaleimides that contain ester groups in the back-bone.43

Triol Extenders. Poly(bismaleimide-ether) polymers with functionalpendant groups can be obtained from a Michael polyaddition of flexi-ble bismaleimides, such as N,N-4,4-diphenylmethanebismaleimide, N,N-4,4-diphenyl ether bismaleimide and N,N-4,4-dibenzylbismaleimide to tri-functional monomers, such as glycerol and phenolphthalein. Additional-ly, the hydroxyl functional poly(bismaleimide-ether) can be modified withcinnamoyl moieties.3

Hyperbranched Polyamides. Monomers that contain the diphenylquin-oxaline group are 2,3-bis(4-aminophenyl)quinoxaline-6-carboxylic acid(BAQ) and 2,3-bis(4-(4-aminophenoxy)phenyl)quinoxaline-6-carboxylicacid (BAPQ), c.f. Figure 11.9.These compounds form hyperbranched aro-matic polyamides on polycondensation.

Although the monomers are structurally similar, the properties ofboth monomers and the respective hyperbranched polymers are different.BAQ reacts normally with BMI in a Michael addition fashion, followed byhomopolymerization of the excess BMI. However, BAPQ seems to initiatea free radical polymerization of BMI at room temperature. This unexpectedproperty of BAPQ suggests it can be used as a prototype for the develop-ment of low-temperature, thermally curable thermosetting resin systemsfor high-temperature applications.44

11.1.9 Specialities

11.1.9.1 1,3-Bis(maleimidomethyl)cyclohexane

Imides are often substantially insoluble in ordinary organic solvents andare soluble only in high boiling aprotic polar solvents, such as N-methyl-2-pyrrolidone, N,N-dimethylacetamide, etc. This is a drawback when im-pregated varnishes are prepared by dissolving the imides in these solvents.High temperature is required for removing the solvents and the solventsare liable to remain in the prepregs formed from the varnishes, causingfoaming in the laminates and considerably lowering the quality of flexibleprinted circuits (FPC).


N

N

COOHNH2

NH2

2,3-Bis(4-aminophenyl)quinoxaline-6-carboxylic acid

N

N

COOHO

O

NH2

NH2

2,3-Bis[4-(4-aminophenoxy)phenylquinoxaline-6-carboxylic acid

Figure 11.9: Monomers for Hyperbranched Oligoamides44


1,3-Bis(maleimidomethyl)cyclohexane is a bismaleimide compoundthat is readily soluble in a variety of ordinary low boiling point organic sol-vents.1 For instance, it is soluble in acetone, methylethylketone, tetrahy-drofuran, chloroform, and N,N-dimethylformamide. Despite its aliphaticstructure, the monomer can provide good heat-resistant bismaleimide re-sins by thermal polymerization.

11.1.9.2 Siliconized Bismaleimides

Siliconized epoxy-1,3-bis(maleimido)benzene has been synthesized fromsiloxanes.45, 46 In the first step, epoxy based on the diglycidyl ether ofbisphenol A, and 4,4′-diaminodiphenylmethane (DDM) was extended with(3-aminopropyl)triethoxysilane.

The pendent ethoxysilane groups were further reacted with a hy-droxy-terminated poly(dimethylsiloxane) (HTPDMS) with dibutyltin di-laurate as catalyst. The scheme is shown in Figure 11.10. 1,3-Bis(maleim-ido)benzene is prepared from m-phenylene diamine and maleic anhydride.Finally, the 1,3-bis(maleimido)benzene is dissolved in the siliconized ep-oxy system at 125°C. To this mixture, a stoichiometric amount of DDMis added homogenized. This mixture is cured at 120°C for 1 hour andpostcured at 205°C.

The curing is a comparatively complex process. It is proposed thatthe curing is due to the following reactions:46

1. Oxirane ring opening reaction with active amine hydrogens,2. Autocatalytic reaction of the oxirane ring with pendent hydroxyl

groups of epoxy resin,3. Addition reaction of the amine groups of DDM with double bonds

of BMI (Michael addition), and4. Homopolymerization reaction of BMI.

Bismaleimides with silicone linkages can also be prepared via theDiels-Alder reaction of bismaleimide-containing silicone and bisfuran con-taining silicone. The bismaleimides are soluble in low boiling point sol-vents, and the cured resins are stable up to 350 to 385°C.47

Still another reaction path to prepare bismaleimides with siliconegroups is the reaction of N-(4-hydroxyphenyl)maleimide with dichlorodi-methylsilane. In a second step, the adduct is reacted with a polysiloxanethat is terminated with hydroxyl groups.48


NHH

CH2

CH2

CH2

SiO O

O

CH2

CH2

CH3

CH3CH2CH3

Si

OH

Si

OH

Si

OH

CHCH2

O

CH2 CH2

O

CH CH2

CHCH2 CH2

O

OO Si

CH2

CH2

CH2

NCH2CHCH2

OH OH

Si Si

Si

+ +

+ ++

Figure 11.10: Formation of a Silane-modified Epoxy Resin


11.1.9.3 Maleimide Phenolic Resins

Phenolic novolak resins with pendant maleimide groups are accessible bythe polymerization of a mixture of phenol and N-(4-hydroxyphenyl)male-imide (HPM) with formaldehyde in the presence of an acid catalyst.49

HPM is less reactive than phenol toward formaldehyde. In fact, N-phenyl-maleimide is also reactive towards phenol and formaldehyde.

Curing is done by both possible typical reaction mechanisms forthese groups. Around 150 to 170°C, there is a condensation reaction of themethylol groups formed in minor quantities on the phenyl ring of HPM.The curing at around 275°C is associated with the addition polymerizationreaction of the maleimide groups.

Polymerization studies of non-hydroxy-functional N-phenyl male-imides indicate that the phenyl groups of these molecules are activatedtoward an electrophilic substitution reaction by the protonated methylolintermediates formed by the acid-catalyzed reaction of phenol and formal-dehyde.

Allyl-functional Novolak. The maleimide-functional phenolic resin canbe reactively blended with an allyl-functional novolak. This system under-goes a multistep curing process over a temperature range of 110 to 270°C.The presence of maleimide reduces the isothermal gel time of the blend.

Increasing the allylphenol content decreases the crosslinking in thecured matrix, leading to an enhanced toughness and to improved mechan-ical properties of the resultant composites. Increasing the maleimide con-tent results in an enhanced thermal stability.50

Epoxy-functional Novolak. Epoxy-novolak (EPN) resins have beencured together with a 1,1′-(methylene di-4,1-phenylene)bismaleimide. Asuitably blended EPN and BMI with 30% bismaleimide shows higher Tg

than the neat resin.

With an increase of bismaleimide, the thermal stability is increased.A single exothermic reaction is observed on curing. The morphology ofthe cured samples indicates the formation of a homogeneous network inthe blends.51, 52


Table 11.3: ModifiersCompound Reference

2,2′-Diallyl bisphenol A (DBA)Reactive rubbersPolysulfonePolyetherimide 53

Poly(hydantoin)4,4′-Bis(o-propenylphenoxy)benzophenoneN-Phenylmaleimide-styrene copolymerAcetylene-terminated polymers2,4-Di(2-allylphenoxy)-6-(2-naphthyloxy)-1,3,5-triazine (DAPNPT) 54

2,4-Di(2-allylphenoxy)-6-N,N-dimethylamino-1,3,5-triazine 55

2,6-Di(4-aminophenoxy)benzonitrile (DAPB) 56

Poly(propylene phthalate) 57

11.2 SPECIAL ADDITIVES

11.2.1 Tougheners and Modifiers

The toughness of bismaleimide resins is a major problem that is limitingthe field of application. The toughness can be improved by adding reactivecomponents that reduce the crosslinking density. Modifiers are summa-rized in Table 11.3.

11.2.1.1 Reactive Rubbers

Blending with reactive liquid rubbers such as carboxyl-terminated butadi-ene acrylonitrile rubbers increases the toughness.

11.2.1.2 Polyetherimide

Polyetherimide (PEI) is highly effective as a toughness improver for a bis-maleimide resin. Increasing the modifier content increases the miscibilityof the two phases. At a content of 20% PEI, the morphological structureof the modified resin changes from a dispersed system to a particle co-continuous structure and eventually with still more PEI to a phase invertedsystem.53 Polyetherimide is also used in bismaleimide resins composed of4,4′-bis(maleimido)diphenylmethane and 2,2′-Diallyl bisphenol A.58, 59


11.2.1.3 Polyesterimide

Polyesterimides can be used to improve the toughness of bismaleim-ide resins, composed of 4,4′-bis(maleimido)diphenylmethane (BDM) and2,2′-Diallyl bisphenol A (DBA). The fracture energy of the cured sam-ples increases with the increase of polyesterimide content in the modifiedbismaleimide system.60

11.2.1.4 Polysiloxanes

The addition of alkenylphenols such as 2-allylphenol, 2-propenylphenol,and 2,2′-diallyl bisphenol A increases the toughness of bismaleimide resinsystems, but the degree of toughness obtained is less than that ultimatelydesirable. Polysiloxanes that are capped with alkenylphenols are compat-ible with bismaleimide resins and can be used in appreciable amounts totoughen such resins. The toughened systems maintain a high degree ofthermal stability.

A 2-allylphenoxy-terminated diphenyldimethylpolysiloxane can beprepared from an epoxy-terminated siloxane, 2-allylphenol and triphenyl-phosphine as catalyst.61

11.2.1.5 Poly(ether ketone ketone)

Poly(ether ether ketones) (PEEK) to improve the brittleness of the bismale-imide resin include poly(phthaloyl diphenyl ether) (PPDE), poly(phthaloyldiphenyl ether-co-isophthaloyl diphenyl ether) (PPIDE), and phthaloyl di-phenyl ether-co-terephthaloyl diphenyl ether (PPTDE). The bismaleimideresin is a mixture of 4,4′-bis(maleimido)diphenylmethane and 2,2′-Diallylbisphenol A. It was shown that PPIDE with 50 mol-% isophthaloyl unitis more effective as a modifier for the bismaleimide resin than the otherpoly(ether ketone ketone)s. The most effective modification for the curedresins could be achieved with a co-continuous phase or a phase-invertedstructure of the modified resins.62

Similarly, in a three-component bismaleimide resin composedof 4,4′-bis(maleimido)diphenylmethane (BDM), 2,2′-diallyl bisphenol A(DBA), and o,o′-dimethallyl bisphenol A, PPIDE and PPTDE are moreeffective as modifiers than PPDE.63


11.2.1.6 Triazines

2,4-di(2-allylphenoxy)-6-(2-naphthyloxy)-1,3,5-triazine (DAPNPT) canbe prepared by the reaction of cyanuric chloride with 2-allylphenol fol-lowed by a treatment with 2-naphthol.54 The procedure is shown inFigure 11.11.

Copolymers of DAPNPT with 4,4′-bis(maleimido)diphenylmeth-ane (BMDPM) show improved mechanical properties compared to pureBMDPM. The copolymer shows up to 10 times higher impact strength and3 times higher shear strength. However, the impact strength and the shearstrength dramatically decrease when the molar ratio of DAPNPT/BMDPMin the copolymer exceeds 1:2.

Completely analogous, as shown in Figure 11.11, 2,4-di(2-allyl-phenoxy)-6-N,N-dimethylamino-1,3,5-triazine can be prepared by the re-action of 2-allylphenol with cyanuric chloride and then by dimethylam-ine.55 This monomer is a modifier for bismaleimide resins. It effectivelyimproves the mechanical properties of the resin without greatly decreasingheat resistance of the resin.

11.2.1.7 Others

Polyamide-imide (PAI), poly(phenylene sulfide) cannot be used in BMIallyl systems. These compounds have poor miscibilities with allyl com-pounds.

11.2.1.8 Boric Esters

Boron can be incorporated into allylic compounds by esterification of allyl-phenol and boric acid. Such compounds are suitable as comonomers in thepolymerization of bismaleimide resins. The cured resins show an excellentthermal stability.

No weight loss was observed when the copolymer was heated up to465°C in nitrogen atmosphere. The char yields at 800°C in nitrogen aremore than 50%.64 Allyl boron compounds improve the ablative propertiesof bismaleimide resins.65


N

N

N

O O

O

HO

N

N

N

Cl

O O

+ +N

N

N

Cl

ClCl

HOOH

Figure 11.11: Preparation of2,4-Di(2-allylphenoxy)-6-(2-naphthyloxy)-1,3,5-triazine


11.2.2 Fillers

11.2.2.1 Aluminum nitride Ceramic Powders

To prevent the failure of integrated circuites (IC) during processing andoperation, materials with a low dielectric constant, and a silicon compati-ble coefficient of thermal expansion (CTE) ca. 4.0×10−6 K−1 are needed.A low dielectric constant reduces the delay time of signal transmission.Further, a high glass transition temperature and a high conductivity is sub-stantial, especially in high powered ICs.

Silica has a high thermal conductivity, but it has a high dielectricconstant of around 40. Aluminum nitride66 has a melting point of 2230°Cand is highly chemically inert. It is used in refractory materials, also inconjunction with silica nitride and boron nitride. Aluminum nitride (AlN)ceramic is superior to silica, since it not only has a high thermal conduc-tivity of up to 320 W/K and a compatible CTE with silicon, but it also hasa relatively low dielectric constant (ca. 8.9).

AlN ceramic powders, used as fillers in a modified bismaleimide re-sin, change the curing performance. The addition of AlN increases theactivation energy of curing of the BMI. Also, the glass transition tempera-ture is raised slightly.67

11.2.2.2 Silsesquioxane Nanofillers

Silsesquioxane nanofillers in a bismaleimide modified novolak resin ex-hibit improvements in the glass transition temperature and the heat resis-tance of the material. The modulus at high temperatures is also improved.The particle size of the dispersed phase was about 100 nm, and particleaggregates were observed.68

11.2.3 Titanium dioxide

Ternary hybrids of bismaleimide-polyetherimide-titanium dioxide weresynthesized by sol-gel reaction. A 10% solution of BMI prepolymer inN-methyl-2-pyrrolidone was mixed with 30 phr of polyetherimide.

Dibutoxybis(acetylacetonato)titanium(IV) was obtained from tetra-butyltitanate and acetylacetone. This compound was added, and after stir-ring again tetrabutyltitanate and acid were added. After drying, the re-sulting film was thermally cured. The titanium dioxide particles weredispersed uniformly in both the PEI-rich phase and the BMI-rich phase,


Table 11.4: Flame Retardant BismaleimidesCompound Reference

Bis(3-maleimidophenyl)phenylphosphineoxide (BMIPO)

20–22

3,3′-Bis(maleimidophenyl)phenylphosphine oxide 23

Phenyl-(4,4′-bismaleimidophenyl)phosphonate 24

Ethyl-(4,4′-bismaleimidophenyl)phosphonate 24

9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide(DOPO)

27, 28

having a mean diameter of around 50 nm.69 Increasing titanium dioxidecontent improves the mechanical properties. However, the thermal decom-position temperatures of the hybrids decrease from 374°C of the unfilledresin to 294°C of a resin with a titanium dioxide content of 20 phr. It isbelieved that titanium dioxide exerts a catalytic effect in this aspect.

11.2.4 Reinforcing Materials

11.2.4.1 Silica Coatings

The usual way to reinforce is to add the reinforcing material to polymericmaterials. For medical applications, ceramic coatings have been appliedto a bismaleimide. Non-reinforced BMI specimens are coated with a thin,protective layer of a dense silicate ceramic material. Testing of the Vickershardness on the coated and uncoated BMI specimens indicates that thecoatings adhere well to the substrate.70

11.2.5 Flame Retardants

Bismaleimide resins are flame retardant, because they are comprised ofaromatic groups and nitrogen. Therefore, for many applications, flameretardancy is not a major problem. Phosphorous-containing monomershave been described as flame retardants. They are used not only for bis-maleimides, but also for epoxy systems. Flame retardants are shown inTable 11.4.


11.3 CURING

11.3.1 Monitoring Curing Reactions

11.3.1.1 DSC

Experimental data for a kinetic model of a modified bismaleimide resinwere obtained by isothermal DSC. A curing mechanism involving multiplereactions was established. The reaction is dominated by different mecha-nisms at different stages of curing. At the beginning of curing, an auto-catalytic reaction was observed.71 A reaction model was set up, and theactivation energy and the frequency factor were calculated.72

11.3.1.2 Dielectric Method

A dielectric sensor for the cure monitoring of high temperature compositeshas been developed. The on-line cure monitoring of a bismaleimide resinwas performed using a Wheatstone bridge type circuit and a high tempera-ture dielectric sensor.73

11.3.1.3 Infrared Spectroscopy

An in-situ technique for studying the polymerization kinetics has been de-veloped. Fourier self-deconvolution of the spectra was used to enhance thepeak separations and the calculation of the peak areas needed for quan-titative monitoring of the curing process. During curing of 1,1′-(methyl-ene di-4,1-phenylene)bismaleimide (MDP-BMI) with 4,4′-diaminodiphen-ylmethane (DDM), a substantial difference in the reactivity between pri-mary and secondary amine was observed.74

11.3.2 Polymerization

11.3.2.1 Gel Point

When a monomer containing two double bonds is incorporated into a radi-cally growing chain, it is first incorporated with one double bond only. Thepolymer chain then will bear pendant double bonds, but initially no cross-links. There are special cases where, after incorporation of the first doublebond, the second, now pendant double bond will be consumed by the samegrowing radical. This behavior is termed backbiting, or if it occurs morerandomly, intramolecular cyclization.


The pendant double bonds may react in a further stage of the poly-merization with another growing chain. Accordingly a complete polymerchain becomes part of another growing chain by the reaction of a singlependant double bond. The molecular weight of the polymer grows rapidlyuntil a certain stage of conversion is reached and a gel is formed.

The formation of networks during the copolymerization of styrenewith various maleimide compounds was investigated.75 In particular, p-maleimidobenzoic anhydride, or mixtures of p-maleimidobenzoic anhy-dride, methyl-p-maleimidobenzoate, and styrene were studied.

In resin systems containing bismaleimides, during radical polymer-ization, the concentrations of pendent double bonds in copolymers, cal-culated from the consumption of monomers and copolymer composition,follow the general trend typical for vinyl-divinyl copolymerization.

At the end of polymerization, a substantial fraction of pendent male-imide bonds remains in the system. The conversions at the gel point aremuch higher than for ring-free copolymerization due to cyclization and thesteric hindrance of the pendent double bonds.

11.3.2.2 Thermal Polymerization

In stoichiometric formulations of 1,1′-(methylene di-4,1-phenylene)bis-maleimide, modified with 2,2′-Diallyl bisphenol A, during the thermal cur-ing, copolymerization and homopolymerization do not overlap with eachother.76 The reactions progress sequentially and homopolymerization oc-curs only when the copolymerization is completed.

This conclusion is based on the Tg–conversion relationship that wasmodelled by the DiBenedetto equation.77 The DiBenedetto equation, Eq.11.1 is based on the corresponding states.

Tg,α=0

Tg

= 1+C1α+C1α2 (11.1)

Tg Glass transition temperatureα ConversionC1 Constant, characteristic for the systemC2 Constant, characteristic for the mobility of the repeating units

In a modified diallyl bisphenol A/diaminodiphenylsulfone/bismale-imide resin, the different temperature regimes were characterized by IRspectroscopy. The major crosslinking occurs below 150°C. At 190°C themaleimide moiety is converted into succinimide.78


Cure Reaction Pathways. In a homopolymerized bismaleimide resinsystem, the maleimide ring addition is the only observable reaction withconventional methods. When the maleimide is cured in the presence ofan amine, the Michael addition of the amine to the maleimide ring can beobserved.

In solution, using special reagents and conditions, a ring-openingaminolysis reaction has been observed. Such a reaction has been postulatedas a curing mechanism for bismaleimides.

It has been verified that such an aminolysis reaction, accompaniedby ring opening, occurs to a significant extent during the cure of a neat BMIresin. This partial structure can remain in the network even after a high-temperature postcure treatment. The existence of the amide product hasbeen demonstrated in bismaleimide resin formulations selectively labelledwith 13C atoms and 15N atoms.79

Cure Kinetics and Mechanism. Maleimide reacts with allylphenols inan ene reaction via an intermediate Wagner-Jauregg reaction, followed bya Diels-Alder reaction.80, 81 The Wagner-Jauregg reaction is essentially aDiels-Alder addition of BMI to the ene adduct of BMI and the allylphenol.The reaction shows a strong dependency on the electron density of theBMI. The Diels-Alder reaction is facilitated by an increased electrophilic-ity of the dienophile. However, a reverse trend is observed for the Wag-ner-Jauregg reaction. Therefore, it was concluded that this reaction couldfollow a mechanism different from the conventional Diels-Alder reaction,although the final product looks the same as in the Diels-Alder reaction.82

In a mixture of 4,4′-bis(maleimido)diphenylmethane and 2,2′-diall-yl-bisphenol A (BMDM/DABPA) and other models, it was established thatthe cure mechanism consists of a combination of step-wise and chain poly-merization and polycondensation reactions:83

1. Step-wise ene addition reaction of allyl group to maleimide,(shown in Figure 11.12).

2. Chain polymerization of the maleimide and the propenyl groupsgenerated by first reaction.

The chain polymerization is the main crosslinking reaction. Themechanism of the reaction involving monofunctional model compoundsdiffers from the curing of the actual system because of steric hindrances in2,2′-diallyl bisphenol A, which retard reversible Diels-Alder reactions, and


N

O

O

N

O

O

OH

N

O

OO

O

N

N

O

O

OH

N

O

O

N

O

O

N

O

O

OH

N

O

O

OH

+

Figure 11.12: Ene Reaction of Allylphenol and Maleimide, Followed byWagner-Jauregg Reaction and a Diels-Alder Reaction


different reactivity of maleimide groups.84 Another mechanism of cross-linking is the dehydration reaction of phenol groups. The dehydration ofphenolic groups necessarily involves the 1:1 adduct of maleimide and allylfunction as a reactant.85

The homopolymerization of maleimide groups proceeds autocatalyt-ically under the action of free radicals generated by thermal decomposi-tion of maleimide propenyl groups donor-acceptor pairs. The steric hin-drance in 2,2′-diallyl-bisphenol A prevents the reversible Diels-Alder re-action. The methylated analog of 2,2′-diallyl bisphenol A shows a higherreactivity in thermal free-radical polymerization.86 The curing kinetics ofbismaleimide modified with diallyl bisphenol A has been modelled by anautocatalytic and nth-order model.87

Microwave Curing. A comparative study between thermal and micro-wave curing of bismaleimide resin was done. The degree of cure wasdetermined with differential scanning calorimetry. No difference in thechemical reactions taking place during the microwave cure and the ther-mal cure was detected. Samples that were cured with a conventional ovenshowed slightly higher glass transition temperatures than the microwave-cured samples at higher conversions.88

11.3.2.3 Photo Curing

N-alkylmaleimides homopolymerize in the absence of a photoinitiatorwhen exposed to UV light in solvents bearing a labile hydrogen.89 Sincethe maleimide is a chromophore, it is considered a photoinitiator togetherwith a co-initiator. A co-initiator may be methyldiethanolamine, trimeth-ylolpropane trismercaptopropionate, or poly(ethylene glycol).

Maleimide/vinyl ether systems belong to electron donor/electron ac-ceptor monomers. Maleimide acts as an electron acceptor and the vinylether acts as an electron donor. With stoichiometric maleimide-vinyl ethermixtures, the reaction proceeds within seconds upon UV exposure.90 Theinitiation reaction is shown in Figure 11.13.

The initiator radicals are formed by hydrogen abstraction from theexcited maleimide molecules. Highly crosslinked polymer networks canbe obtained.

The molecular structure of bismaleimides is quite rigid because ofthe presence of aromatic rings. The presence of the aromatic rings, as well


hν

C

CC

N

C

R

O

OH

H

CH2 CH O CH2 R’

C*

CN R

O

O

H

H

H

CH2 CH O CH* R’

Figure 11.13: Photoinitiation in Donor Acceptor Systems91

as the resultant high crosslinked density during thermal curing, give thecured product its high heat-resistance, resulting in a high Tg and a highmechanical strength.

For the radiation curing of bismaleimides, comonomers such as2,2′-diallyl bisphenol A and 4-hydroxybutylvinyl ether have been tested.Unlike N-alkylmaleimide and N-phenylmaleimide, BMI does not reactwith vinyl ether without a photoinitiator. Triphenylphosphine oxide is asuitable photoinitiator.

2,2′-Diallyl bisphenol A, which is a good property modifier for BMIin thermal curing formulation, does not polymerize with either BMI or4-hydroxybutylvinyl ether, even in the presence of a photoinitiator. How-ever, 2,2′-diallyl bisphenol A is a co-initiator and speeds up the reaction ofa ternary system.91

11.3.2.4 Anionic Initiators

Several maleimides can be polymerized by nanometer sized Na+/TiO2

initiators. The temperature for the polymerization initiated by nanometersized Na+/TiO2 is lower than that for the radical polymerization. An an-ionic mechanism resulting from the catalysis by Na+/TiO2 as the counterion is proposed.92, 93

11.3.2.5 Diels-Alder Polymerization

A monomer suitable for Diels-Alder polymerization is shown in Fig-ure 11.14. The reaction between α,α′-dibromo-m-xylene (DBMX) andsodium 1,2,3,4,5-pentamethylcyclopenta-1,3-dienide gives the respectivepentamathylcyclopentadiene derivative,25 as depicted in Figure 11.14.


CH2CH2H3C

H3C H3C

CH3

CH3

CH3

CH3CH3

CH3

CH3

CH2CH2 BrBr

CH3H3C

H3C

CH3

CH3

Na+

CH3

CH3

H3C

CH3

CH3

Na+

Figure 11.14: Synthesis of Bis-1,3-methyl-1,2,3,4,5-pentamethylcyclopenta-2,4-diene benzene

The Diels-Alder polymerization is shown in Figure 11.15. The re-action must be performed in dimethylformamide at 140 to 150°C becauseof the low solubility of the BMI.

11.3.3 Interpenetrating Networks

11.3.3.1 Polyurethane Bismaleimide

In polyurethane/poly(urethane-modified bismaleimide-bismaleimide) in-terpenetrating polymer networks (PU/P(UBMI-BMI) IPNs) interpenetra-tion occurs at the hard segment domains of PU, which leads to an enhance-ment of the phase separation of PU. The dispersing tendency of the dis-persed phase increases.94

Poly(butylene adipate)-based polyurethane-crosslinked epoxy (BMI/PU-EP IPN) and bismaleimide from interpenetrating networks are pre-pared by using the simultaneous bulk polymerization technique. It wasdemonstrated that the bismaleimide was dissolved primarily in the polyur-ethane domains of the epoxy matrix to form a compatible system, therebyincreasing the mechanical strength of the BMI/PU-EP IPNs.95, 96

An epoxy based on poly(propylene oxide) has a better grafting effect


CH2CH2H3C

H3C H3C

CH3

CH3

CH3

CH3CH3

CH3

CH3

N R N

O

O

O

O

CH2CH2CH3CH3

N

O

ON

O

O

Figure 11.15: Diels-Alder Polymerization

due to higher compatibility between the BMI than poly(butylene adipate)epoxies.97

The incorporation of chain-extended BMI into polyurethane-modi-fied epoxy systems increases the thermal stability, and tensile and flexuralproperties, but decreases the impact strength and the glass transition tem-perature.98

11.3.3.2 Unsaturated Polyester Bismaleimide

A bismaleimide resin monomer can be readily dissolved in the uncuredpolyester matrix up to a concentration of about 20%.99 Spectroscopic in-vestigation during curing indicates that the crosslinking process is stronglyaffected by the presence of the bismaleimide in the system. The maleim-ide groups react preferentially with the styrene. The styrene radical reactswith both the unsaturated polyester and the maleimide moieties so that acrosslinked structure can emerge. When the maleimide groups are fullyconsumed, the curing proceeds as in a neat resin. The bismaleimide effectsan increase of the crosslinking density of the final product. Further, thebismaleimide increases the overall stiffness of the network.


11.4 PROPERTIES

In comparison to epoxy resins, BMI resins exhibit a higher tensile strengthand modulus, excellent chemical and corrosion resistance, better dimen-sional stability, and good performances at elevated temperature.

11.4.1 Thermal Properties

Among two high temperature adhesives, based on epoxy and bismaleimide,the bismaleimide-based adhesive shows a better high temperature perfor-mance and is more resistant to thermal aging than an epoxy based resin.100

There are relationships between structure and thermal properties of poly-mers.101

11.4.2 Water Sorption

In general, a disadvantage of thermoset resins is their tendency to absorbsignificant amounts of water when exposed to humid environments. Theabsorbed moisture has detrimental effects on material performance. Thetemperature dependence of moisture content in equilibrium is controver-sial. It has been reported that the equilibrium moisture content is inde-pendent of temperature,102 but also that it is dependent on the tempera-ture.103, 104

From the viewpoint of thermodynamics, the temperature dependenceof the solubility is governed by the enthalpy of dissolution

d lncs

d1/T= −∆Hs/R. (11.2)

During hydrothermal cycling experiments, the molecular network struc-ture of BMI appears to change.105, 106 It was concluded that in the courseof water absorption at elevated temperatures, a chemical degradation canoccur. This is part of an aging mechanism. IR spectra obtained by thereflection technique during water absorption show that the band at 1600cm−1 increases.107 This band is attributed to the N−H stretching of anamine and also of an amide. The hydrolysis reaction is shown in Figure11.16. It is assumed that the hydrolysis is similar to the reverse reaction offormation of a bismaleimide.107 When a BMI resin was stored in water attemperatures of up to 70°C for a period of 18 months, blistering and severemicrocracking occurred, leading to severe weakening of the materials.103


N

O

O

H2O

O

O

OH

OH N

H

H

O

O

N

H

OH

H2O

Figure 11.16: Hydrolysis of a Cured Bismaleimide Resin

The presence of about 10 to 15% of alkenyl-substituted cyanate indicyanate and bismaleimide blends leads to a marked reduction in moistureabsorption in comparison with an unmodified bismaleimide/cyanate blendcontaining a comparable amount of bismaleimide.

The modified samples display thermal stabilities that are indistin-guishable from cured resins that have not undergone immersion.108 Themoisture transport can be correlated to the glass transition temperature andthe network properties. The network structure can be systematically variedby the initial monomer composition and the conditions of curing.109

11.4.2.1 Multivariate Analysis

An analysis of samples subjected to accelerated ageing tests shows thatsimple near infrared spectroscopic measurements on virgin materials canpredict results otherwise obtained from dynamic mechanical thermal ana-lysis, and can provide correlations with thermogravimetric analysis.

Therefore, a rapid screening method for multivariate analysis hasbeen proposed, in conjunction with a combinatorial approach for the de-velopment of advanced composites.110


11.4.3 Recycling

Various styrene copolymers containing comonomers with a pendant furanring were subjected to Diels-Alder reactions with a monomaleimide or abismaleimide. When the materials are heated in the presence of excess of2-methylfuran, the retro Diels-Alder reaction is induced. The process israther a trans Diels-Alder reaction. The maleimides are released with thefuranic additive. Concomitantly the original copolymers can be recovered.The reaction is of interest because of the possibility of recycling cross-linked polymers by a simple thermal treatment.111

11.5 APPLICATIONS AND USES

11.5.1 Biochemical Reagents

Bismaleimides are used as reagents in biochemical investigations.112 Bis-maleimide is used as a crosslinking reagent for the synthesis of bifunc-tional antibodies. The use of a solid-phase reactor in the preparation of thebifunctional antibodies eliminates many time-consuming separation stepsbetween fragmentation and conjugation steps.113

11.6 SPECIAL FORMULATIONS

11.6.1 Adhesives

For high-temperature usage, i.e., above 200°C, either bismaleimides orpolyimides are suitable. These are supplied as films, with or without acarrier. Epoxies are not generally used at temperatures beyond 150°C, al-though there are some modified epoxies that can be used up to 200°C.114

11.6.1.1 Void Control

Polyimides have a higher service temperature than bismaleimides. Howev-er, bismaleimides offer some advantages as they do not generate volatilesduring cure. When volatiles are created during curing a high void contentin the adhesive can develop. There are several methods to control the voids.These include114

• Vacuum release technique. The joint to be bonded is placed inan oven under vacuum. The temperature is increased in order to


reduce the viscosity of the adhesive. When the vacuum is released,the voids collapse to a negligible volume.

• Another method uses an autoclave where hydrostatic pressure canbe applied. The hydrostatic pressure compresses the gas in a voidand reduces its volume.

11.6.1.2 Thermally Reversible Adhesives

A formulation of thermally reversible adhesives consists of a diepoxy com-pound and aliphatic diamines. The diepoxy compound is formed by theDiels-Alder reaction between epoxy-containing furans and a bismaleim-ide. The epoxy resin is cured with aliphatic diamines.115 At temperaturesabove 90°C the retro Diels-Alder reaction occurs, which leads to a signifi-cant loss in the shear modulus. The loss of the shear modulus is reversiblewith temperature. Therefore, the formulation can act as a thermally re-versible adhesive. The adhesive bonds are easily broken at elevated tem-perature where the modulus is low.

11.6.1.3 Adhesion Improvement

In order to improve the adhesion of Kevlar™∗ fibers to a 2,2-bis[4-(4-male-imido phenoxy)phenyl]propane (BMPP) resin, the surface of the fibers canbe chlorosulfonated. The fibers are immersed in a solution of chlorosulf-onic acid in dichloromethane at −10°C. After the chlorosulfonation, thesurface concentration of carbon decreases. In the subsequent reaction withethylene diamine, allylamine, the O/N ratio again decreases. On the otherhand, the O/N ratio was increased by hydrolysis treatment.

The interfacial shear strength (IFSS) is determined by pull-out ex-periments of the fiber from the matrix calculated by the relationship

τ =F

dL. (11.3)

τ Interfacial shear strengthF Pull-out forced Diameter of the fiberL Embedded length of the resin

The interfacial shear strength (IFSS) between Kevlar fibers and theBMPP resin increases slightly due to the chemical treatment.116, 117 In

∗Kevlar is a trademark of DuPont company


graphite/bismaleimide composites, the treatment with ammonia has beenshown to be promising for the improvement of adhesion.118

11.6.2 Phosphazene-triazine Polymers

Polyquinoline/bismaleimide blends are miscible thermosetting polymers.Thermogravimetry shows a 5% weight loss between 450 and 535°C forthin films at 5 to 60% of bismaleimide loading. The glass transition tem-peratures are between 275 and 360°C.119

11.6.3 Phosphazene-triazine Polymers

Phosphazene-triazine polymers can be obtained by curing a ternary blendof tris(2-allylphenoxy)triphenoxy cyclotriphosphazene (TAP), tris(2-allyl-phenoxy)-s-triazine (TAT) and bis(4-maleimidophenyl)methane (BMM).The maleimide component increases the thermal stability. The tensilestrength decreases and the modulus increases with increasing maleimide-content. Tensile properties improve for an allyl/maleimide ratio of two.120

11.6.4 Porous Networks

Network structures have been prepared by in-situ polymerization of amixture of N-phenylmaleimide and 1,1′-(methylene di-4,1-phenylene)-bismaleimide in 80% poly(vinylidene difluoride-co-hexafluoropropylene)(PVDH). The maleimide monomers are forming thermoreversible gelswith PVDH.

After polymerization, porous networks are obtained by removing thePVDH by solvent extraction. The poly(maleimide) networks are stable upto 380°C in an inert atmosphere. It is suggested that these networks maybe used for thermally stable membranes.121

11.6.5 Nonlinear Optical Systems

Thermally stable second-order nonlinear optical polymeric materials basedon bismaleimide contain chromophores with excellent thermal stability,such as the N-maleimide of Disperse Orange 3. The synthesis of the mono-mer is shown in Figure 11.17. A full interpenetrating polymer networkcan be formed by the simultaneous reaction of bismaleimide and a sol-gel process of the alkoxysilane dyes. The dynamic thermal and temporal


H2N N N NO2O

O

O

N N NO2

O

O

N

+

Figure 11.17: Synthesis of the Maleimide of Disperse Orange 3

stabilities of the interpenetrating network are much better than those ofcomparable non-interpenetrating networks.122

Azo chromophores with allyl groups at one or two ends of themolecules can be thermally cured with bis(maleimidodiphenyl)methane togive crosslinked and chromophore-modified bismaleimide resins. The re-sins show no appreciable decomposition up to 300°C. By incorporatinga chromophore into the network of a BMI resin, an improvement of thethermal stability of the materials is achieved.123

Examples of azo chromophore allyl compounds include (4-(N,N-diallyl)-4′-nitrophenyl)azoaniline, allyl-4-[(4-N-allyl-N-ethyl)aminophen-ylazo]-α-cyanocinnamate, and allyl-4-[(4-N,N-diallyl)aminophenylazo]-α-cyanocinnamate, c.f. Figure 11.18.


N

N

NCH2

CH

CH2

H2C

CH

CH2

NO2

N

N

NCH2

CH

CH2

CH

C CN

C O

O

CH2

CH

CH2

H2C

CH3

N

N

NCH2

CH

CH2

CH

C CN

C O

O

CH2

CH

CH2

H2C

CH

CH2

Figure 11.18: Azo Chromophore Allyl Compounds: (4-(N,N-Diallyl)-4′-ni-trophenyl)azoaniline, allyl-4-[(4-N-Allyl-N-ethyl)aminophenylazo]-α-cyanocin-namate, allyl-4-[(4-N,N-Diallyl)amino-phenylazo]-α-cyanocinnamate


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Reactive Polymers Fundamentals and Applications || Bismaleimide Resins