Maleimide-Based Alder-Enes

12 Maleimide-Based Alder-Enes M. Satheesh Chandran and C.P. Reghunadhan Nair Polymers and Special Chemicals Group, Vikram Sarabhai Space Centre, Thiruvananthapuram, India

O U T L I N E Introduction to Bismaleimides

459

Shape Memory Alder-Ene Polymers

497

Maleimide-Based Alder
Ene Polymers

462

Kinetics of Alder-Ene Polymerization

499

Alder-Ene Polymers from Various Allyl-Maleimide Compounds

Outlook

505

463

Acknowledgments

505

Alder-Ene Polymer Blends

486

References

505

Nano-Modified Alder-Ene Polymers

495

Introduction to Bismaleimides Development of heat-resistant polymers and highperformance polymer matrix composites which can function above 250°C for long duration have been an area of interest for more than a decade [1
2]. Bismaleimides (BMIs) are a relatively new class of addition curable polyimides that have gained a prominent position among high-performance thermosets due to their unique features. The outstanding properties of this class of resin system amalgamates good mechanical and electrical properties, excellent physical property retention at elevated temperatures, non-flammability, excellent balance of properties at hot-wet environments, thermal stability, low cost, and good processability [3
4]. BMIs offer an intermediate material in temperature performance between epoxies and polyimides because of their capability of performing at temperatures up to 230°C, and their ability to be fabricated using epoxy-like conditions without the evolution of void-producing volatiles [5
6]. The BMIs were first synthesized from pyromellitic acid dianhydride, aromatic amine, and end-capper maleic acid anhydride in dimethylformamide (DMF) [7]. Presently among the variety of BMIs synthesized from various aromatic amines, 4,40 -biamaleimido diphenyl methane (BMPM) is reported to be the most often used one owing to its

low cost and excellent thermal stability when compared to the other BMI systems [8]. The structural composites of BMIs find major applications in the aerospace industry [9
10]. However, BMIs have exhibited a brittle mechanical response due to the high cross-link density formed during curing [11].

Synthesis and Structure-Property Relationship of BMIs Generally, BMIs are synthesized by reacting diamines with maleic anhydride to form bismaleiamic acid in the first step, followed by imidization in the second step. The first step reaction is fast and exothermic, and the latter can be carried out either thermally or chemically. The typical synthesis of BMI is represented in Scheme 12.1. It is reported that the synthesis leads to several by-products such as isoimides, acetanilides, maleimides, acetic acid adducts, and products with mixed functionalities which interns the yield to .70% [12
13]. The reaction mechanism of formation of bismaleiamic acid involves the nucleophilic attack of the amino group on the carbonyl carbon of the maleic anhydride group followed by the opening of the anhydride ring to form the amic acid group as illustrated in Scheme 12.2.

Handbook of Thermoset Plastics. DOI: http://dx.doi.org/10.1016/B978-1-4557-3107-7.00012-9 © 2014 Elsevier Inc. All rights reserved.

459

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NH O

+ H2N

O

R

THERMOSET PLASTICS

O

O

O

OF

R

NH

DMF

NH2

HO

OH

Toluene

O

O

Heat

Heat

HOAc DMF

DMF, NaOAc O

O

N

R

Ac2O

N

O

O

Scheme 12.1 Typical synthesis scheme of BMI.

O

O O

+

NH OH

H 2N O

O O

H2O N

O

Scheme 12.2 Reaction mechanism of imide formation.

Unlike the dianhydride, the nature of the diamine is of extreme importance for imide formation. Highly basic diamines (e.g. aliphatic diamines) are not suitable for the reaction due to their high tendency to form salts during the initial stages of the reaction, which prevents the amine groups from reacting with the anhydride by changing the stoichoimetry and molecular weight build-up [14
15]. The reactivity of diamines is dependent on their basicity, and diamines of low basicity do not exhibit sufficient nucleophilic character to form the polymer with anhydride and therefore, ideally, the diamine should have a basicity of pKb 4.5
6 for the bismaleiamic formation [16]. The exothermicity of the reaction is due to the strong acidbase interaction between the amic acid and the amide solvent and is the most important driving force of the forward reaction. Therefore, the more basic and more polar the solvent (typically N,Ndimethylformamide (DMF), N,N-dimethylacetamide (DMAc), N-methyl-pyrrolidone (NMP), etc.), the higher the rate of poly(amic acid) formation. The

thermal imidization is the direct thermal cyclodehydration of the bismaleiamic acid by heating the mixture to around 300°C; in the chemical imidization, a dehydration agent is added followed by a thermal imidization. The thermal process is the most costeffective and practical approach for commercial applications [17
18]. By selecting the appropriate aromatic amine it is possible to design BMIs with a wide variety of chemical structures that have high melting temperatures and crystalline structures. Generally, BMIs can be processed in a manner similar to epoxies, have low cure temperatures, and can cure without the evolution of by-products. Bismaleimides having incorporated the imide group in their repeating units gain the polymer backbone stiffness than epoxies. The high cross-link density of cured bismaleimides is advantageous as it results in improved thermal and hydrolytic resistant materials compared to epoxies [19]. Thus, when compared to the other polyimide systems, bismaleimides are outstanding owing to the effortless processability with

12: MALEIMIDE-BASED ALDER-ENES

461

high temperature resistance, and hence BMIs are intermediate in temperature performance compared to epoxies and polyimides (Figure 12.1). In the case of flammability, BMIs are intermediate to phenolic triazines (PT) and phenolics, and certainly better than epoxies, as shown in Figure 12.2. The mechanical and thermal properties of toughened BMIs are better than the conventional phenolics and epoxy resins. The thermo-mechanical properties of BMIs are compared to other high-performing resin systems and tabulated in Table 12.1. The major drawbacks of BMIs include: (i) brittleness due to their high cross-link density after curing, (ii) poor solubility in ordinary solvents leading to poor processability, (iii) high crystalline melting temperatures of the monomers, and (iv) a narrow temperature window for processing (the temperature difference between the melting point of bismaleimide monomer and its onset point of curing reaction) which necessitates only a solution processing method [20]. The simplest and most widely used bismaleimide resin (as stated in the Introduction) is Bismaleimidodiphenylmethane (BMPM). The commercial two-component BMI resin system (Matrimid 5292t) introduced in to the market by Ciba-Geigy is comprised of BMPM along with the reactive diluent, diallyl bisphenol A (DABA), which has found applications in various fields ranging medical to aerospace science (Scheme 12.3) [21
23]. The ability of these polymers to form rigid ring and regular backbone structures, along with their

Figure 12.1 Comparison of temperature performance of BMIs with epoxies and polyimides.

50

LOI (%)

40 30 20 10 0 PT

BMI

Phenolic

Epoxy

Figure 12.2 Comparative limiting oxygen index values of common thermosets.

Table 12.1 Comparative Properties of Various Thermoset Matrices (Neat Resin) Property

Epoxy

Phenolics

Toughened BMI

Cyanate Ester

Density (g/cc)

1.2
1.25

1.24
1.32

1.2
1.3

1.1
1.35

Max use temperature (°C)

180

200

B200

150
200

Tensile strength (MPa)

90
120

24
45

50
90

70
130

Tensile modulus (GPa)

3.1
3.8

3
5

3.5
4.5

3.1
3.4

Elongation (%)

3
4.3

0.3

3

2
4

Dielectric constant (1MHz)

3.8
4.5

4
10

3.4
3.7

2.7
3.0

Cure temperature (°C)

RT
180

150
190

220
300

180
250

Cure shrinkage (%)

.3

0.002

0.007

B3

TGA onset (°C)

260
340

300
360

360
400

400
420

Tg (°C)

150
220

170

230
380

250
270

54
100




160
250




0.60




0.85




2

GIC (j/m ) 1/2

KIC (MPa-m

)

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O

O N

CH

N 2

O

O

4,4' - Bismaleimidodiphenylmethane (BMPM)

CH3 HO

C

OH

CH3 2,2'-Diallyl bisphenol-A (DABA)

Scheme 12.3 Components of commercial BMI resin formulation, Matrimid 5292t.

network-forming capability, have resulted in these being widely used for producing high-performance polymer composites [24
26]. The methyl linkage group between the aromatic ring contributes to the highest cross-link density among all bismaleimides, and accordingly, the glass transition temperature (Tg) can reach as high as 450°C. However, the high crystallinity and the regular backbone structure imparts a high melting point (B150°C) to the system, which in practice impedes its applicability to the low-temperature Fiber Reinforced Plastics (FRP) processings such as Resin Transfer Molding (RTM) and vacuum assisted RTM (VARTM) [27
28]. The relatively difficult utilization of BMIs in high-tech fields arises from the fragility network of cured BMI resulting from homopolymerization of the maleimide functionality. The greatest advantage of BMI is its versatile reaction capability of the maleimide functionality, which offers the possibility to modify BMIs with components such as reactive diluents, additives, comonomers, and viscosity modifiers so that tailor-made BMI systems are possible [29
30].

Modification Approaches of BMIs Considerable efforts are being devoted to improve the processing ability of BMIs and decreasing the brittleness of the cured resins [31]. In view of this, the versatile reaction capability of the maleimide groups is utilized to form an array of high-performance polymeric systems. Maleimides

OF

THERMOSET PLASTICS

can undergo a Michael addition reaction with nucleophiles such as primary and secondary amines, phenols, thiols, etc. [32]. BMI being a bisdienophile can undergo Diels-Alder reactions with dienes [33]. Allylphenol reacts with BMI via an ‘ene’ reaction [34]. Vinyl and allyl type ethylenic double bonds with maleimides are also reported [35]. Anionic polymerization of the maleimide double bond is facilitated by tertiary amines and imidazoles [36]. Other approaches for improving the toughness include modification with highperformance thermosets such as epoxy resins, cyanate esters, benzoxazines, etc., and incorporation of engineering thermoplastics [37].

Maleimide-Based Alder-Ene Polymers Among the variety of co-reactants used for BMI modification, olefinic compounds (especially allyl compounds) have been the most successful since these systems do not affect the high-temperature properties and processability of the BMIs significantly. These compounds form what is called an Alder-ene polymer network with maleimides during curing via undergoing an addition polymerization by an ene reaction at low temperatures and subsequently a Diels-Alder reaction at higher temperatures.

Reaction Sequence of Alder-Ene Polymerization The general reaction sequence of polymerization between an allylphenol and a maleimide moiety takes place as per the following steps [38]. 1. In the 100
200°C range, the maleimides and allyl monomers react via an ene reaction to form an ene adduct. The ene adduct is multifunctional with carbon
carbon double bonds and is capable of chain extension and crosslinking. In addition, the hydroxyl groups undergo etherification by hydroxyl dehydration. 2. The unsaturated ene adduct intermediate undergoes a further Diels-Alder type reaction with BMI to give the bis and tris adducts. The

12: MALEIMIDE-BASED ALDER-ENES

463

Scheme 12.4 Reaction sequence of Alder-ene cross-linking and network formation [68]. (Reprinted with permission from Thermochimica Acta 514, 44
50, 2011, Elsevier.)

intermediate step (Diels-Alder) is sometimes referred to as the Wagner-Jauregg reaction. 3. The principal cure reactions occur in the 200
300°C range via the carbon
carbon double bonds together with etherification occurring above 240°C. 4. Along with the Alder-ene reaction, at higher temperatures, the homopolymerization of maleimides and allyl monomers also occur. The reaction sequence of Alder-ene polymerization is presented in Scheme 12.4.

Alder-Ene Polymers from Various Allyl-Maleimide Compounds BMI and Allyl PhenolFormaldehyde Alder-Ene Systems Researchers have extensively co-reacted allyl phenol (AP) and phenyl maleimide groups to form a variety of polymer systems with useful properties. In such reactive blends, further improvement in properties is possible by way of structural modification of either the BMI or the phenolic ring. The properties of the resultant matrix depend on the molecular

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HANDBOOK

OH

OH

OH

N

O

OF

THERMOSET PLASTICS

O

1. n

*

* COCl

2.

THF

O

Br

K 2CO3

N

Acetone O= C O

Cross-linked Product

O OH

O

*

n

Scheme 12.5 Preparation and curing of the BMI-modified novolac resin prepolymer.

structure and relative ratio of the two reactants, and the extent of cure [39]. Novolac can be allylated by reacting with allyl halide and then co-reacted with BMI as shown in Scheme 12.5, and used as super abrasives [40]. The high allyl content in the system improves the heat resistance at the cost of bending strength. Among the various combinations of the resin, the best resin system with allyl novolac of molecular weight of 450 and allyl content of 50% showed increased thermal stability (372° to 400°C). Similar to the previous work, BMI modified by allyl Novolac resins (extent of allylation: 39%, 48%, and 59%) with different BMI content were also synthesized as shown in Scheme 12.5 [41]. When the degree of allylation is increased from 48 to 59%, the BMImodified novolac resin transited from a singlephase structure to a phase-separated structure. This group of researchers observed the maximum thermal and mechanical properties with a 48% allylated novolac
BMI system (Tg: 274°C; and modulus: 3.53 GPa at RT). The higher allyl content caused a decrease in the thermal properties and mechanical properties of the resin as expected. Nair et al. [42] synthesized phenolic novolac resins bearing maleimide groups. From the DSC results, the exotherm around 275°C was attributed to the addition polymerization reaction of the maleimide groups, and the exotherm at around 150°C to 170°C was ascribed to the condensation reaction of the methylol groups formed in minor quantities on the phenyl ring. As a continuation of the same work, the authors reported the reactive blending of maleimide-functional phenolic resin and allylfunctional novolac in varying proportions [43]. The allyl and maleimide incorporated resin systems envisaged include: (i) Phenol
hydroxyphenyl

(Maleimide)
Formaldehyde (PMF) Resin, (ii) Phenol
Allylphenol
Formaldehyde (PAF) Resin, and (iii) Phenol
Maleimidophenol
Allylphenol
Formaldehyde (PMAF) Resin. The systems underwent a multi-step curing process over a temperature range of 110
270°C (Scheme 12.6). It was observed that increasing the allylphenol content decreased the cross-linking in the cured matrix, which led to enhanced toughness of the resultant silica laminate composites. The thermal stability of the cured network improved with an increase in the maleimide content for different compositions of resins as shown in the Figure 12.3. Good mechanical performance was observed at an optimum ally phenol:maleimide molar ratio of 1:3 when a partially allylated novolac was cured with BMI in the presence of allyl phenyl ether diluent [44]. The system retained a flexural strength and flexural modulus of 65% and 78% at 300°C and 200°C, respectively. From TGA, the residual weight at 500°C and 700°C was 80% and 51%, respectively. The resin system showed excellent processing properties for RTM, such as low viscosity and long pot life at the injection temperature with no volatile release upon curing. The same research group [45] developed different thermosetting resin systems with high glasstransition temperatures (.300°C) based on BMIs and allylated novolac to form BMAN resin systems. The resin system with a molar ratio of allyl to maleimide groups of 100:15 (BMAN15) was found to be appropriate for RTM. The cured BMAN15 resin showed a very high Tg at 418°C, and the modulus retention was over 90% up to 350°C. By virtue of the high Tg of the matrix resin, the composite exhibited a modulus retention rate of around 89% and a strength retention rate of 46.4% at 350°C. Table 12.2 shows the DMA results of the developed systems.

12: MALEIMIDE-BASED ALDER-ENES

465

OH

O OH

+

N

O

80–160°C

N

O

Ene Mono-adduct O PMF

PAF

O

Wagner-Jauregg 160–220°C

N O

OH

OH

O

O

N O

N O

O

O

N

N

O

N

O

N O

O

Diels-Alder 225–275°C

O O

Di-adduct

Tri-adduct 250–300°C 250–300°C

Thermal rearrangement

Further cross-linking O

O

250–300°C

N O

O

O

N

N

O O

Tri-adduct

Scheme 12.6 Alder-ene reaction mechanism of maleimide and allyl-modified novolacs and structure of cured network [42
43].

Figure 12.3 Thermal stability and char residue for the resin systems synthesized by Nair et al. [43].

In yet another work in the same area [46], allyl novolac (AN) was co-polymerized with BMPM in different weight ratios (1:1, 3:2, and 7:3). The modified BMI resin was stable up to 484°C. The hot/wet properties of the cured copolymer were investigated by aging in boiling water. After aging for 100 h, water absorption and heat deflection temperature (HDT) were 3.2% and 277°C, respectively. In the case of Short Beam Shear (SBS) strength of the composites, 86% and 78% of the original room temperature value was retained during testing at 230°C and 250°C, respectively. Similarly, flexural strength was also (85% and 72%) retained during testing at 230°C and 250°C. Allyl-functional novolac (AN) resins with varying degrees of allylation (from 32.4 to 114.6%) were blended with BMPM at a weight ratio of

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THERMOSET PLASTICS

Table 12.2 Tg and Modulus Retention at 350°C for the BMAN Resins [45] Code

Molar Ratio of Allyl to Maleimide Groups

Tg by DMA (°C)

Modulus at RT (GPa)

Modulus Retention at 350°C (%)

BMAN 15

100:15

419

2.43

87.6

BMAN 20

100:20

407

2.47

77.7

BMAN 25

100:25

409

2.06

76.7

BMAN 30

100:30

417

2.20

79.7

(Reprinted with permission from Journal of Applied Polymer Science, Vol. 97, 443
448, (2005), John Wiley and Sons Publishers.)

Table 12.3 Typical TGA Results of BAMN Resins [47]. System

Allylation Degree (%)

Tonset (°C)

T0.05 (°C)

Residual Mass at 800°C (%)

BMAN 324

32.4

368.8

375

55.0

BMAN 515

51.5

405.5

395.2

44.3

BMAN 656

65.6

407.8

400.6

38.5

BMAN 846

84.6

411.5

406.5

37.7

BMAN 1146

114.6

422.3

416.1

36.1

(Reprinted with permission from Journal of Applied Polymer Science, Vol. 104, 2822
2829, (2007), John Wiley and Sons Publishers.)

AN1146 BMANI146 BMAN846 BMAN656

Heat flow

BMAN515 BMAN324

100

200

300

400

Temperature, °C

Figure 12.4 Shift in peak cure temperature with increase in allylation degree [47]. (Reprinted with permission from Journal of Applied Polymer Science, Vol. 104, 2822
2829, (2007), John Wiley and Sons Publishers.)

2.50:1 to form Alder-ene resins [47]. DSC results showed that as the allylation degree increased, the peak cure temperature shifted slightly to a lower

temperature as shown in Figure 12.4. TGA and DMA showed that (Table 12.3) as the degree of allylation increased, thermal stability of cured resins showed an enhancing trend because of an increase in cross-linking density, but char yield of the above resins at 800°C decreased in turn. The effect of incorporation of polydimethylsiloxane (PDMS) on the toughness of allylated novolac/BMPM blend has been investigated (Scheme 12.7) [48]. Incorporation of PDMS improved the impact strength at lower molecular weight of PDMS. However, the toughening effect decreased with an increase in the number-average molecular weight of PDMS. Thus when PDMS of the number-average molecular weight 1000 was used in 5 wt.%, the impact strength increased by over three times that of the parent AN/BMPM resin (2.38 kJ/m2). For PDMS with a number-average molecular weight of 3000, the impact strength was close to the parent resin. The incorporation of PDMS had little effect on thermal stability of the cured resin. The same research group investigated the curing behavior of the system in the presence of PDMS [49]. Incorporation of PDMS into the backbone of AN was found to favor the Claisen rearrangement reaction during curing. The total enthalpy of the curing reaction for the PDMS-modified AN/BMPM resins

12: MALEIMIDE-BASED ALDER-ENES

467

OH CH3

OH

OH

Si

+ H CH2

[

]

CH3

1

CH3

n

Tolune/80 °C

CH2

CH2

CH2

Si CH3

H

CH3

H2PtCl6

CH3

OH

Si

O

CH3 O

Si 1

OH CH2

CH2

CH2

CH3 CH2

CH2

Scheme 12.7 Synthesis of PDMS-modified allylated novolac [48]. (Reprinted with permission from European Polymer Journal 42, 580
592, (2006), Elsevier Publishers.)

Table 12.4 DSC Parameters of the Curing of PDMS Incorporated AN/BMPM [49]

Samples

PDMS content (wt.%)

Tp (°C)

DHRXN (J/g)

AN/BDM

0

234.73

392.25

PDMSAN/BDM

5

234.18

357.28

PDMSAN/BDM

10

233.67

354.40

PDMSAN/BDM

15

233.37

334.50

PDMSAN/BDM

20

231.59

312.16

(Reprinted with permission from Journal of Applied Polymer Science, Vol. 107, 554
561 (2008), John Wiley and Sons Publishers.)

decreased due to the reduction in the degree of allylation when compared to the parent AN/BMPM, as shown in Table 12.4. The presence of PDMS did not change the Alder-ene reaction pathway, as concluded from the dynamic DSC and FTIR results. An Acetyl-Capped Paraformaldehyde (ACPF)
BMI-AN resin system has been developed that is suitable for RTM [50]. The viscosity time


temperature profile (Figure 12.5) of the 0.5% ACPFincorporated system after 4 h at 100°C showed a viscosity of ,0.5 Pa•s, which implied pot life sufficiently long for RTM. Incorporation of 5 wt.% of ACPF gave rise to enhancement of HDT by 18°C, a glass transition temperature (Tg) of 34°C, and retention of flexural strength at 90 and 65% when tested at 200°C and 300°C, respectively. The high percentage of ACPF had an effect on the processability of the resin. It is seen that the onset temperature of decomposition increased marginally from 402°C to 416°C with the incorporation of 5 wt.% of ACPF, and the temperature for 10% weight loss was increased from 445°C to 458°C. The char yield at 700°C also increased by around 5%.

Bismaleimide
Allyl Phenol Systems The use of allyl derivatives of phenol for the synthesis of bismaleimide based Alder-ene polymers began as early as 1969. Cross-linked polymers which contain imide groups by reacting polymaleimides with alkenylphenols or alkenyl phenol ethers (preferably at temperatures of 100
250°C), and hardenable compositions comprising bismaleimides, alkenyl phenols, and phenol diallyl ethers have been reported [51
52]. N-(Meth)-allyloxyphenylmaleimides were pre-polymerized with bis-imides

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HANDBOOK

(a)

1.5

(b)

THERMOSET PLASTICS

1.4 1.2

Viscosity (Pa. s)

1.2 Viscosity (Pa. s)

OF

0.9 0.6

1 0.8 0.6 0.4

0.3 0.2 0

0 70

90

110

130

150

0

2

4

6

8

10

Time (hour)

Temperature (°C)

Figure 12.5 (a) Viscosity
temperature profile, and (b) change in viscosity with time at 100oC of (’) 5% ACPF containing resin and (¢) resin system without ACPF [50]. (Reprinted with permission from Journal of Applied Polymer Science, Vol. 86, 1265
1271 (2002), John Wiley and Sons Publishers.)

to an imidazole compound, useful for producing shaped articles with improved mechanical strength, reported by Zahir et al. [53]. Stenzenberger et al. [54] synthesized a series of diallylethers from selected bisphenols. The bisphenols, such as 2,20 -bis(4-hydroxyphenyl)hexafluoropropane and bis(4-hydroxyphenyl)-p-diisopropylbenzene, were converted to the corresponding o,o0 -diallylbisphenols. These compounds were thermally copolymerized with bismaleimides to give tough, high glass transition (Tg) copolymer networks. Later, the same group [55] synthesized a series of bis[3-(2-allylphenoxy)phthalimides] from bis(3-nitrophthalimides) and o-allylphenol via a nucleophilic displacement of the nitro group. Blends of bis(allylphenoxyphthalimides) and BMI thermally copolymerized to tough, high Tg copolymer networks. Intermolecular cyclization of diallyl 2-substituted succinates with maleic anhydride [56] revealed that the types of cyclizations in the network include 2-alkenyl (22%), 2-aralkyl and 2-isobutyl (37%), and long-chain Z-(n-alkyls) (22%). The best overall properties in terms of flexural strength and modulus, Rockwell hardness A186 and HDT at 100°C were exhibited by 2-aralkyl polymers. Allyl nadic imides were formed via Diels-Alder and “ene” reactions as depicted in Scheme 12.8. Thus, starting from dicyclopentadiene, a feasible route for the synthesis of allyl nadic imides was demonstrated by Alfred Renner et al. [57] Bis- and tris-allylnadic-imides realized by this route were viscous liquids resins or low-melting solids that are soluble in common solvents. Upon

heating to 250°C cross-linked solid networks with high Tg were obtained with good thermo-oxidative and environmental stability and reduced burning rate. These cured allylnadic-imide resins were excellent high-temperature insulators and low permittivity dielectrics. Prepolymers of addition curable imide oligomers end-capped with methylnadic, hexachloro-nadic, acetylene, n-propargyl, cyclohexene, and maleic end groups have also been synthesized [58]. The solubility order of the synthesized polymers is as follows: INCREASING SOLUBILITY Maleic <

<

N-Propargyl

Cl6-Nadic

Nadic

Cyclohexane

< Ch3-Nadic

Acetylene

The acetylene, n-propargyl, and cyclohexenecapped materials displayed lower melting temperature (by 20
30°C) than the nadic end-capped polyimide (LARC-13), which makes them potential candidates for hot-melt impregnation. The curing, Tg, and melt flow of all the resin combinations are tabulated in Table 12.5. The thermal stabilities of the cured polyimides were similar to that of LARC-13 except for the hexachloronadic-capped polymer, which showed signs of decomposition as early as 275°C. The acetylene-capped polyimide proved to have the greatest stability after aging for 1000 hours at an elevated temperature of 232°C.

12: MALEIMIDE-BASED ALDER-ENES

469

Crivello et al. [59] have copolymerized mixtures of bismaleimides and bisvinyl ethers/bispropenyl ethers using a wide variety of free radical initiators. At temperatures greater than 100°C homogeneous melts of these monomers were formed, which, in the presence of free radical initiators such as azobisisobutyronitrile (AIBN) or benzoyl peroxide, rapidly polymerized to hard glassy masses. It was observed that the mechanical properties of silica-

filled specimens of these resin systems remain essentially the same even at 180°C. Zahir et al. [8] extensively studied the curing of BMI with DABA with varying mol.% of DABA; the composition-dependent variation in properties is represented in Table 12.6. The ene reaction and Diels-Alder mechanism were proved by using monofunctional compounds, N-phenylmaleimide and 2-allylphenol by

360 °C

+

Na

Na

0.5 t-BuOH

Cl CO

O

O NH2

R n

O

+

CO CH2

CH

CH2

O

O

(

N

)

R

+ nH2O

n

O

Scheme 12.8 Formation of allyl nadic imide [57]. (Reprinted with permission from Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 27, 1301
1323 (1989), John Wiley and Sons Publishers).

Table 12.5 The Cure and Melt Flow Properties of Addition Curable Imide Oligomers with Different End-Caps [58] Oligomer End Group

Exotherm Onset °C

Exotherm Maximum °C

Apparent Tg of Cured Polymer °C

Melt-Flow Range °C

Nadic

275

357

248

160
25

CH, Nadic

Gradual

No peak

208

152
285

CI, Nadic

275

315

227

190
200

Acetylene

175

250

248

140
160

N-Propargyl

250

330

235

130
160

Cyclohexene

280

415

189

140
185

Maleic

240

317

220

185
212

(Reprinted with permission from Polymer engineering and science, January, Vol. 22, No. I (1982). John Wiley and Sons Publishers.)

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THERMOSET PLASTICS

Table 12.6 Neat Resin Properties of BMI/DABA of Different Molar Ratio [8] BMI/DABA (1/1)

BMI/DABA (1.2/1)

Property (MPa)

RT

149°C

204°C

RT

149°C

204°C

BMI/DABA 0.87/1 (RT)

Tensile strength

85

53

41

97

72

74

80

Tensile modulus

4430

2529

2100

4030

2943

2418

4243

Elongation (%)

2.3

2.6

2.3

3.0

3.05

4.6

2.3

Flexural strength

172







192







160

Flexural modulus

4207







4140







4114

Compressive strength

21







220










Compressive modulus

2485







2570










HDT (°C)

273







285







295

Tg (°C, DMA)

295







310










GIC (J/m2)

195







217










Table 12.7 Properties of BMI/DABA System (Cure Schedule: 175°C/3h 1 230°C/4h) [62] BMI/DABA Molar Ratio Property

1.2/1

1.5/1

2/1

3/1

Flexural Strength (MPa)

186

188

174

131

Flexural Modulus (GPa)

4.02

3.94

4.05

4.14

Deflection (%)

7.78

7.30

5.53

3.50

KIC (MN/m1.5)

0.97

0.86

0.80

0.64

GIC (J/m2)

197

158

133

83

Tg (°C)

279

282

288

288

Reyx et al. [60]. The studies indicated that although the rate of allyl phenol consumption varies with the reaction temperature, the complete reaction does not occur even at 250°C for 6 hours. It is also reported that N-phenylmaleimide is consumed very rapidly even at low temperature (180°C) through an Alderene reaction that leads to the di-adduct which can be an endo or exo isomer. This transient intermediate is simultaneously consumed by a retro Wagner-Jauregg reaction or Diels-Alder reaction to give the triadduct. Cunningham et al. [61] isolated and characterized the products from the ene reaction between allyl-substituted aromatics with different enophiles

such as maleic anhydride, maleimide, N-phenylmaleimide, and N-(4-phenoxyphenyl) maleimide; a semi-quantitative assessment of ene and enophile reactivities has been achieved. Stenzenberger [62] observed that generally the best property of the blend is obtained when the allyl to maleimide ratio is 1:2 with a slight compromise in thermal properties. Under a given cure schedule, BMI enhances the thermal capability at the cost of toughness and flexural strength. Table 12.7 represents the cure properties of the BMI/DABA system with varying ratios of maleimide to allyl groups. An addition curable diallyl bisphenol A
formaledehyde resin (ABPF) was synthesized as shown in Scheme 12.9 and co-reacted with Bisphenol A-based bismaleimide (BMIP); its adhesive characteristics were investigated by Nair et al. [63]. The adhesive properties determined by LSS on aluminum substrates at different cure conditions were not commendably good (Table 12.8), but it retained more than 100% at 150°C. As a continuation of the previous work, the effect of BMI structure on thermal stability of co-cured networks of diallyl bisphenol A
formaldehyde resin (ABPF) with (i) bis(4-maleimido phenyl) methane (BMIM), (ii) bis(4-maleimido phenyl) ether (BMIE), (iii) bis(4-maleimido phenyl) sulfone (BMIS), and (iv) 2,2-bis 4-[(4-maleimido phenoxy) phenyl] propane (BMIP) was also studied [64]. The probable network structures of BMIP
ABPF cure reactions are shown in Scheme 12.10. When the TGA of BMIP
ABPF was compared with its monomeric

12: MALEIMIDE-BASED ALDER-ENES

471

CH3 HO

CH3

Br

C

O

OH K2CO3

CH3

O

C CH3

Bisphenol A

Diallyl ether of bisphenol A Heat

CH3 HO

C CH3

OH

CH3

HCHO

CH2

HO H+

Diallyl bisphenol A-formaldehyde resin (ABPF)

C

OH

CH3 Diallyl bisphenal A (DABA)

Scheme 12.9 Synthesis of diallyl bisphenol A
formaldehyde resin (ABPF) [63].

Table 12.8 Adhesive Properties of BMIP-ABPF (1:1) Under Different Cure Conditions [63] Cure Conditions (Temperature for Time: °C for h)

LSS at RT (MPa)

LSS at 150°C (MPa)

Retension of LSS at 150°C (%)

160 for 4

1.9 6 0.5

2.6 6 0.4

137

160 for 0.5 1 200 for 3

2.8 6 0.4

3.4 6 0.6

121

160 for 0.5 1 200 for 0.5 and 250 for 2

4.1 6 0.6

4.8 6 0.4

117

160 for 0.5 1 200 for 0.5 and 250 for 6

3.0 6 0.5

4.0 6 0.4

133

(Reprinted with permission from Polymer International, Volume 50, pp. 403
413 (2001), John Wiley and Sons Publishers.)

counterpart BMIP
DABA (for 1:1 stoichiometry), the ABPF system showed a slightly lower initial decomposition temperature (Ti) by around 15°C as compared to DABA, which could be due to the degradation triggered at the methylene cross-bridges (absent in DABA). The degradation initiation temperature (Ti) and char residue of the ABPF system with different bismaleimide backbones is compared to the conventional DABA/BMIP system in Figure 12.6. The ternary blend of Novolac epoxy (EPN), 2,2’diallyl bisphenol A (DABA), and bisphenol A bismaleimide (BMIP) was studied with varying molar concentration of BMI (from 0.5 to 2) [65]. The rheological evaluations revealed that the major cure reactions are completed by around 200
210°C. The reaction of ternary blend with BMIP is shown in Scheme 12.11. From the DMA analysis, the Tg of the system was also found to increase systematically with an increase in BMI content (106°C to 225°C, Figure 12.7). The temperature for completion of decomposition reaction showed a systematic increase with an increase in BMI content (605°C to 679°C).

Adhesive formulations comprised of allyl novolac and DABA with a new bismaleimide containing polymethylene flexible groups and aramide
arylate mesogen groups (BMILC, Scheme 12.12) that have low melting temperature (which upon cross-linking form a liquid crystal ordered state) have been developed [66]. The relationship between structure, thermal properties, and liquid crystal structure formation for a typical adhesive formulation, BEA811 (eutectic mixture of BMILC B8 and B11 (BE811) and DABA) was studied by Polarized Light Microscopy (PLM) and revealed the nature of the states of matter upon different thermal treatments (Figure 12.8). The authors observe that the transformation that occurs between 105°C and 110°C is due to complete melting of DABA, and material continues to reflect polarized light and exist as a suspension of crystalline BE 811 in amorphous DABA liquid. At 150°C the formulation BEA811 showed a strong decrease of polarized light reflection and disappeared entirely at 180°C, which is due to the complete melting of

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THERMOSET PLASTICS

Scheme 12.10 Probable network structures of BMIP
ABPF cure reactions [64].

BE 811 in DABA. Beyond 180°C, the BEA 811 formulation is completely amorphous. Finally, from 230°C, the BEA 811 formulation showed again a reflection of polarized light and intensity of this polarized light increased up to 250°C. The authors attribute this phenomenon to the presence of B8, which could foster the emergence of the crystalline order in the system and the BEA811, BE 811, and B11 cross-linking phenomena, which induced an order in the amorphous state. The aging in natural and laboratory environments of the iron
iron adhesives of this system

revealed that these formulations were more efficient than the industrial formulation MATRIMIDE 5292 (Table 12.9). This is attributed to molecular structures and their cross-linking ability in the liquid crystal state. In another study, BMPM was chain extended with 2,4-di(allylphenoxy)-6-(2-naphthyloxy)-1,3,5triazine (DAPNPT) [67]. The structure of DAPNPT is shown in Scheme 12.13. When the molar ratio of DAPNPT/BMPM was 1:2, the copolymer matrix could improve the impact strength by 9.9 times and the shear strength by 3.1 times compared to the

12: MALEIMIDE-BASED ALDER-ENES

473

neat system. All the co-polymers showed Tg in the temperature range of 280 to 390°C. When another BMI containing phthalide cardo structure (Scheme 12.14) was co-cured with DABA [68], the resultant system showed a Tg of 274°C and char residue of 39.1% at 600°C. The FTIR spectrum revealed that below 200°C the alternating copolymerization of maleimide and allyl groups was more favored than the homopolymerization of Ti (°C) 415

44.8

Char residue (%) 420

420

400

BMI monomer and the isomerization reaction of the allyl to the propenyl form. Copolymerization of BMPM with DABA and 9,10-dihydro-9-oxa-10-phosphaphenanthrene- 10-oxide (DOPO) resulted in a modified bismaleimide resin system (BDP) with significantly improved flame retardancy and decreased dielectric loss [69]. When the content of phosphorus is as low as 0.5 wt.%, the flame retardancy of BDP resin is evaluated to be UL94 V-0 level, while that of BD resin (without DOPO) is classified as UL94 V-1 level. The flame retardant properties are compiled in Table 12.10. BDP resins exhibited improved dielectric properties; specifically, the dielectric constant and loss at 1 GHz of BDP resin with 19.7 wt.% of phosphorus are 2.90 and 0.0058. The 1,3-cycloaddition reaction (click reaction) [70] of azido-imides to bispropargyl monomers containing allyl groups was used to produce poly (1,2,3-triazole)s functionalized with allyl groups. Co-curing of BMPM with these allylated polytriazoles resulted in Alder-click cure networks (Scheme 12.15) with enhanced thermostability and dielectric properties (dielectric constant at 1 kHz ranged between 3.09 and 3.39)
except for the

42.1

46.9

380

46

39.1

BMIP-DABA BMIP-ABPF BMIM-ABPF BMIE-ABPF BMIS-ABPF

Figure 12.6 TGA data from different systems synthesized by Nair et al. [64].

O O HO

+

OH

O

+

O N

DABA

O

O

EPN

R

O N

BMIP

Heat

O O N

O

N O O O

O

O

O OH

Scheme 12.11 Reaction scheme of EPB ternary system [65].

N O

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O

O

THERMOSET PLASTICS

the ene and Diels-Alder reactions. Thus, the cure reaction between BMPM, DABA, and DPD involves: (i) the addition of allyl groups and the maleimide ring, and (ii) amine groups to the maleimide ring. Resultant networks were found to be compact with few defects. Moisture absorption studies after aging for 120 h in boiling water gave very low water absorption (2.6%). The properties of the cured neat resin are given in Table 12.11. Electron-beam curing was attempted for the BMPM/DABA system with various reactive diluents, such as N-vinylpyrrolidone (NVP) and styrene [73]. It was observed that BMPM/NVP can be initiated easily by low-intensity E-beam without thermal curing. About 70% of the reaction occurred in the BMI/NVP system with 200 kGy dosage exposure at 10 kGy per pass. A new BMI system named 4504 with good processability was formulated with BMPM/DABA and

Figure 12.7 Change in Tg with increase in BMI content in the ternary blend [65].

N

OF

O C NH

C O

O

O

(CH2)n

O C

NH C

BMI LC

O

O

N O

Scheme 12.12 Structure of BMILC [B8 (n 5 8), B11 (n 5 11)] [66]. (Reprinted with permission from International Journal of Adhesion & Adhesives 42 51
59, (2013), Elsevier Publishers.)

polymer with an aliphatic segment instead of the imide group, which has a higher dielectric constant. BMPM/DABA matrix resin was extensively studied as a matrix resin for carbon fabric reinforced composites by Morgan et al. [71]. The ene reaction resulted in a small molecular weight increase after 16 h at 100°C, and viscosity-timetemperature observations indicated that the viscosity increase at a significant rate only above 180°C. The sequence of chemical reactions in the temperature range of 200
300°C can be as follows: (a) The ‘ene’ reaction occurs at a significant rate, (b) BMPM homopolymerization reaction occurs at around 250°C, and (c) Diels-Alder reactions involving ene-BMI systems. A fully cured composite with minimal BMI possessed a Tg near 350°C. In another research work, BMPM was co-reacted with DABA and N,N0 -diallyl p-phenyl diamine (DPD) [72] to form an RTM-processable resin system. It was observed that Michael addition reaction, which can occur between the amine group in DPD and BMPM, tends to take place easily compared to

catalysts like imidazole and diisopropylbenzyl peroxide [74]. The 4504/T300 prepreg has an out-time in excess of two weeks, which exceeds the generally accepted out-time requirements of 10
14 days minimum. The system exhibited a gel time of 50
55 min at 120°C favorable for good fiber wetout and compaction of the prepreg. Good retention of short beam shear (SBS) strength at 230°C (51% of the original value at RT) and 80% retention of SBS were observed after aging 100 h, with water absorption of only 0.8% by weight. The comparison of this new system with the BMPM/DABA is shown in Table 12.12, and the merits of the 4504 are quite evident. (Meth)allyl compounds such as triallyl isocyanurate (TAIC), o,o0 -dimethallyl bisphenol A (DMBA), and trimethallyl isocyanurate (TMAIC) were used to modify two component BMI resin system [75] as shown in Scheme 12.16. In the ternary blends of BDM/DBA/TAIC, the fracture toughness (KIC) and flexural strength decreased with increasing TAIC content though thermal

12: MALEIMIDE-BASED ALDER-ENES

475

(a)

(b)

(c)

(d)

(e)

(f)

Figure 12.8 PLM thermal evolution of BEA 811- (5°C/min; G 5 3 200). T 5 30°C (A); T 5 105°C (B); T 5 161.1°C (C); T 5 230.4°C (D); T 5 240°C (E); T 5 250°C (F) [66]. (Reprinted with permission from International Journal of Adhesion & Adhesives 42 51
59, (2013), Elsevier Publishers.)

properties of the cured resins did not deteriorate. In the case of the ternary blend of BDM/DBA/DMBA, KIC and flexural modulus for the cured resins increased and their glass transition temperatures decreased with an increase in DMBA content as expected. Flexural strength increased up to a 70% content of DMBA in the blend and then decreased. In the ternary blend of BDM/DBA/TMAIC (1.0/0.5/0.5), KIC for the blend increased to 15%, with retention of flexural property and Tg. In the ternary BDM/DMBA/TMAIC (1.0/0.5/ 0.5) blend, the cured resin had balanced properties, and its KIC increased to 50% compared to the cured Matrimidt (Commercial BMI from Ciba Geigy) resin.

Bismaleimides and bis-citraconimides with bisallyl groups were synthesized by Viorica Gaina et al. [76]. The ene-addition reaction of the allyl-maleimide group takes place at lower temperatures in the bisallyl system than the allyl-citraconimide system. The Polymer Decomposition Temperature (PDT) of the synthesized polymers was in between 390
450°C and the char yield varied from 32 to 42%. Blends of BMI with allyl diglycidyl ether of bisphenol A (AE) provided a resin with a large processing window [77]. The neat resin exhibited excellent thermal stability up to a temperature of 515°C with the PDTmax at 564°C and a char yield

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Table 12.9 Comparison of the Natural Aging of Adhesive Formulations [66] VIETNAM Test Time (months)

BEA 811

MATRIMIDE 5292

Strength (N)

Elongation (mm)

Strength (N)

Elongation (mm)

0

4300

0.40

2994

0.28

3

3413

0.25

123

0.06

6

1323

0.09

142

0.01

MAROCCO 0

4300

0.40

2994

0.28

3

2999

0.23

2808

0.22

6

2660

0.19

2195

0.15

9

2099

0.16

3675

0.28

12

1912

0.158

1120

0.098

(Reprinted with permission from International Journal of Adhesion & Adhesives 42 51
59, (2013), Elsevier Publishers.)

O

N O

N N

O

DAPNPT

Scheme 12.13 Structure of DAPNPT [67]. (Reprinted with permission from Polymer, 42, 7595
7602, (2001), Elsevier Publishers.) O

O N O

O

O O

N O

O

Scheme 12.14 Bismaleimide containing phthalide cardo structure [68]. (Reprinted with permission from Thermochimica Acta 514, 44
50, (2011), Elsevier Publishers.)

of 33% at 800°C. After aging for 100 h in boiling water, moisture absorption was found to be 3.6 wt.% with a concomitant reduction in HDT from 292 to

248°C. The composites retained SBS strength to 75 and 60% when tested at 200 and 230°C . Cyclomatrix phosphazene
bismaleimide polymer (PZ-BMM) was synthesized (Scheme 12.17) by the Alder-ene reaction between tris(2-allylphenoxy)triphenoxycyclotriphosphazene (TAP) and BMPM [78]. In the case of PZ-BMM and PZ-TZ-BMM (cyclomatrix phosphazenetriazine-based polymers) polymers, AO resistance is found to be dependent on both the structure and phosphorus content of the polymer. It was interesting to learn that PZ-TZ-BMM-15, which has less phosphorus content (3.4%), has better AO resistance when compared to PZ-BMM-15, which has higher phosphorus content (4.5%). The authors attribute this observation to the presence of triazine moieties in the former. A comparison of the mass loss versus AO fluence for phosphazene-based polymers is presented in Figure 12.9. An unreactive monomer spirodilactam bisallylether was made to undergo Claisen rearrangement by heating above 200°C and then reacted with BMPM by ene reaction as shown in Scheme 12.18 [79]. Despite the high water absorption of 5 to 7%, the resins retained high hot/wet flexural properties owing to the high Tg (.300°C). Such high water absorption was attributed to more polar spirodilactam moeities, high cross-link density, and the high free volume associated with the high Tg. The flexural modulus increases from 3.93 to 4.48 GPa when the spirodilactam bisallylether content was increased from 20 to 50 mol.%.

12: MALEIMIDE-BASED ALDER-ENES

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Table 12.10 Typical Results of Cured BD and BDP Resins from Cone Calorimeter Tests [69] BD (BMPM: DABA:DOPO, 140:70:0)

BPD1 (BMPM: DABA:DOPO, 140:70:8.1)

BPD2 (BMPM: DABA:DOPO, 140:70:13)

BPD3 (BMPM: DABA:DOPO, 140:70:16.2)

BPD4 (BMPM: DABA:DOPO, 140:70:19.7)

P content (wt.%)

0

0.50

0.80

1.0

1.2

N content (wt.%)

5.2

5

4.9

4.8

4.7

UL94 Level

V-1

V-0

V-0

V-0

V-0

PHRR (kW/m2)

452

214

205

191

257

THR (MJ/m2)

74

45

38

41

42

TTI (s)

193

159

160

158

184

590

575

510

525

525

850

670

565

490

480

TSR (m /m )

2743

1050

977

943

935

EHC (MJ/kg)

23.21

22.47

21.69

21.47

19.62

CO/CO2 (w:w)

0.0307

0.0190

0.0170

0.0148

0.0137

Weight Ratio of Resin

TOF (s) 2

SEA (m /kg) 2

2

Abbreviations: THR, total heat release; HRR, heat release rate; MLR, mass loss rate; TTI, time to ignition; TOF, time of flameout; SEA, specific extinction area; EHC, effective heat of combustion.

In order to understand the cure reaction of BMI in the presence of acetylene bonds, bis-propargyl ether bisphenol A (PBPA, Scheme 12.19) was blended with BMPM at different molar ratios [80]. The onset cure temperatures of the blend were about 20
30°C lower than that of pure PBPA, and the cure exothermic enthalpy of the resins was significantly reduced from 1320 (PBPA) to 493 J/g (PBPA
BMPM (1.0:2.0)). The thermal stability and Tg of the resins improved markedly with the increase in BMPM content. With the increase in BMPM content, T5% for the cured blend resins improved from 354°C (cured PBPA
BMPM (1.0:0.5)) to 396°C (cured PBPA
BMPM (1.0: 2.0)). Char yield was also increased from 40% (cured PBPA
BMPM (1.0:0.5)) to 46% (cured PBPA
BMPM (1.0:2.0)) at 900°C. Modified BMI resin matrices with enhanced processing characteristics were realized from BMPM and allyl phenyl compounds, allyl epoxy resins, and epoxy acrylate resins [81]. The modification improved the impact strength of the systems by 200% (5.8
15.2 kJ/m2). Scanning electron microscopic (SEM) analysis of the fracture surfaces indicated that modified BMI resin matrix composites exhibited typical toughness rupture with good

interfacial adhesion with reinforcement. After 100 h in boiling water, water absorption of composites is less than 1.1% and the flexural strength and shear strength properties were retained by more than 85% and 90%, respectively. The fundamental effects of vacuum thermal cycling on unidirectional carbon Fiber (T 300) reinforced BMPM/DABA composites have been investigated by Qi Yu et al. [82]. It was inferred that the cumulative vacuum and thermal cycling could improve the crosslinking degree and the thermal stability of resin matrix to a certain extent and later on induce matrix outgassing and thermal stress, which in practice leads to the mass loss and the interfacial debonding of the composite. This observation was substantiated by the DMA results of the composites in which Tg shifted from 233.54°C to 250.55°C as thermal cycles increase to 283 times. This observation narrates the initial (up to 95 thermal cycles) thermal stability improvement of the composites. The damage mechanism through AFM studies (Figure 12.10) revealed that the interface between fiber and matrix (marked as A, B, and C) gets damaged as more hackle marks distributed on the surface can be seen on exposure to 48 cycles (Figure 12.10b) caused by the outgassing behavior of the resin matrix under high vacuum surroundings, and hence mass loss when

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THERMOSET PLASTICS

O

O

O

O

N R N N3

OF

N3 O

O

N N

R N

N

CuBr

CH

O

O

N

N CH2 O

O

CH2

N

N

C

n

HC

C

CH2 O

O

CH2

CuBr DBU

O

CH

OH N3

H2 *

C

O C

C H2

OH

CH CH2 O

OH CH CH2 N

O

CH2 C C H H2

N3

OH

N N CH

N CH2

O

O

CH2

N N CH

C CH H2

CH 2

n

Scheme 12.15 Click reaction of azidoimides and bispropargyl monomers [70]. (Reprinted with permission from J Polym Res, 19:9968, (2012), Springer Publishers.)

compared to the unexposed specimens (Figure 12.10a). It can be observed from Figure 12.10c that numerous fibers are isolated without any matrix adhesion, indicating that fiber/matrix debonding occurs along the circumference of the fiber for the specimen exposed to 283 thermal cycles. The transverse tensile strength of the composite fell by approximately 9%, and flattened off after 95 cycles. The variations of the flexural strength and the ILSS (Figure 12.11) were similar in tendency, increasing firstly due to the thermal cycling induced crosslinking and then falling back to a plateau value after 198 cycles. A novel bismaleimide 2,20 -bis[4-(4-maleimidophenoxy)phenyl]-hexafluoropropane (BAPOFP-BMI) containing fluorine (Scheme 12.20) in the backbone has been synthesized [83]. The system was readily soluble in common solvents (Table 12.13). Compared with conventional Alder-ene resin (BMPM/DABA) composite, the glass transition temperature of BAPOFP-BMI composite was higher

(322°C and 339°C), while the storage modulus was lower. The BAPOFP-BMI composites possessed much lower dielectric constant and dissipation factors than that of BMPM/DABA/glass composites. The dielectric constant of BAPOFP-BMI composites was 3.06 at 1 MHz and 2.99 at 1 GHz, respectively, while that of BMPM/DABA was 4.01 and 3.91. A dissipation factor of BAPOFP-BMI composites is 0.009 at 1 MHz and 0.005 at 1 GHz, and that of BMPM/ DABA composites is 0.012 and 0.007. The reason being that fluorine substitution lowers the K value by decreasing the polarizability and the moisture absorption and by increasing the free volume.

Alder-Ene Polymers with Other Allyl Compounds Apart from the allylated versions of novolac and phenols, a variety of compounds containing allyl

12: MALEIMIDE-BASED ALDER-ENES

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Table 12.11 Properties of BMPM/DABA/DPD Cured Neat Resin System [72] System

Properties

Value

BMPM/DABA/DPD Cured Resin

Tensile strength (MPa) at RT

96.2

Tensile modulus (GPa) at RT

4.8

Elongation at break (%) at RT

2.5

Flexural strength (MPa) at RT

121.4

2

Impact strength (kJ/M ) at RT

10.9 2

Critical strain energy release (Gic, J/m )

198

Flexural strength (MPa) at 100°C

92.5

Flexural strength (MPa) at 200°C

76.3

Tg by DSC (°C)

285

Heat distortion temperature (HDT, °C)

456

Maximum decomposition temperature (T max, °C)

515

Char yield at 800°C (Yc%)

43

Water absorption in boiling water for 120 h (%)

2.6

HDT after absorption in boiling water for 120 h (°C)

236

(Reprinted with permission from Journal of Applied Polymer Science, Vol. 80, 2245
2250 (2001), John Wiley and Sons Publishers.)

Table 12.12 Cured Resin 4504 Properties [74] Properties

Cured 4504 Resin

BMPM/DABA (100/80 w/w)

Tensile strength (MPa) at RT

72

73

Tensile modulus (GPa) at RT

3.82

3.67

Elongation at break (%) at RT

1.93

2.2

Flexural strength (MPa) at RT

110

112

9

13

Critical strain energy release (Gic, J/m )

182

210

Tg by DSC (°C)

290

268

Heat distortion temperature (HDT, °C)

317

274

Initial decomposition temperature (°C)

456

426

Maximum decomposition temperature (T max, °C)

510

447

Char yield at 700 °C (Yc%)

29.4

21.1

1.23

1.23

2

Impact strength (kJ/m ) at RT 2

3

Density (g/cm )

Reprinted with permission from Journal of Applied Polymer Science, Vol. 62, 799
803 (1996), John Wiley and Sons Publishers.

groups have been reacted with BMIs to form Alder-ene polymers. Three novel allyl
maleimide single-component monomers, namely A2B, AB, and AB2, were developed by varying the allyl to maleimide content as

shown in Scheme 12.21 [84]. A2B and AB showed fairly low melting temperatures (Tm , 90°C) and a wide processing window ranging from 90°C to 260°C. Polymers of A2B and AB2 showed high glass transition temperatures (Tg . 270°C) and

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CH3 C CH3

HO

OH

e

8

6

4

2

CH3 C CH3

HO

d b

OH

O,O’-dimethallyl bisphenol A (DMBA)

O,O’-diallyl bisphenol A (DBA)

THERMOSET PLASTICS

10

Mass loss (mg/cm2)

their corresponding composites showed a high bending modulus (E0 . 1900 MPa). Char yield of the cured allyl
maleimide resins at 700°C was in the range of 30
60% under nitrogen atmosphere. The polymers exhibited relatively low water uptake (,2.6%, 120 h in boiling water). Modification of bisphenol A-based bismaleimide resin (BMIP) with an allyl-terminated hyperbranched polyimide (AT-PAEKI, Scheme 12.22) was reported by Haihu Qin et al. [85]. The resultant blend

OF

c a

0 0

5

15

10

20

30

25

35

Fluence (atoms/cm2) × 10−20

O

O

O

N

O

N

VCP-1

: vinylic polymer containing fluoroalkoxy- substituted phosphazene

PZ-BMM-25

: bis (4-maleimido-phenyl)methane (BMM) with allyl to maleimide content 1:2.5

PZ-TZ-BMM-15 : cyclomatrix phosphazene-triazine based ones with allyl to maleimide content 1:1.5

N

N

N

N

O

O

Trially isocyanurate (TAIC)

Trimethallyl isocyanurate (TMAIC)

Scheme 12.16 Structure of meth allyl compounds [75].

PZ-TZ-BMM-25 : cyclomatrix phosphazene-triazine based ones with allyl to maleimide content 1:2.5

Figure 12.9 Mass loss versus AO fluence for phosphazene-based polymers (a) VCP-1, (b) PZBMM-25, (c) PZ-TZ-BMM-15, (d) PZ-TZ-BMM-25 and (e) Kapton film [78]. (Reprinted with permission from J Mater Sci 41, 5764
5766, (2006), Springer Publishers.)

PZ O

O

O

CH2

CH

CH2

N

N

O

N

O

CH

CH

CH2

CH2

Δ >200°C

O

O

Spirodilactam bisallylether O

O HO CH2

P

PZ O O

CH

N

CH2

CH2

N

O

O Where

CH2

O

N

N

O

N

N

O

O

P N

OH

P O

Scheme 12.17 Cyclomatrix phosphazene
bismaleimide polymer [78].

O,O’ -Bisallyl spirodilactam diphenol

Scheme 12.18 Synthesis of o,o’-bisallyl spirodilactam diphenol [79]. (Reprinted with permission from Polymer Engineering and Science, MID January, Vol. 34, No. 1, 1994 John Wiley and Sons Publishers.)

12: MALEIMIDE-BASED ALDER-ENES

481

Br KOH HO

O

OH

O

C2H5OH, 75–80 °C/ 4h PBPA

BPA

Scheme 12.19 Synthesis of bis-propargyl ether bisphenol A [80].

Figure 12.10 AFM images of the carbon/BMI composites exposed to different thermal cycles: (a) unexposed, (b) 48 thermal cycles, (c) 283 thermal cycles [82]. (Reprinted with permission from Materials Chemistry and Physics 130 1046
1053, (2011), Elsevier Publishers.)

exhibited reduced processing viscosity, a wide processing window, high glass transition temperature, modulus, hardness, and toughness. The properties were co-related to the macromolecular architecture of the network. Thus, it is assumed that AT-PAEKI facilitated viscosity reduction and increased Tg and modulus, whereas the allyl group was conducive for increasing the fracture toughness of the system. A series of allyl compounds containing boron (XB, XA, BQ) were prepared (Scheme 12.23) by esterification of allyl-phenol compounds and boric acid [86].

Enthalpy of the curing was moderate (2161.9 to
236.8 kJ g21). It is seen that all three systems have good thermal properties, which are manifested in a high HDT (262
319°C) and initial degradation temperature (495°C). The cured resins exhibited good mechanical properties and excellent thermal stability. Their char yield was more than 54% higher compared with that of a conventional BMI system. The same group has reported [87] the means to improve ablativity of BMIs by co-curing allyl compounds containing boron in their molecular

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OF

30 ILSS Change rate

20

110

10 0 90 −10 20

Change rate (%)

ILSS (MPa)

100

−20

10

−30 −40

0 unexposed 14 cycles

95 cycles 198 cycles 250 cycles 283 cycles

48 cycles

No. of thermal cycles

Figure 12.11 ILSS of the carbon/BMI composites subjected to different thermal cycles [82]. O

O CF3 N

C

O

O

N

CF3 O

O

Scheme 12.20 Structure of 2, 2’-bis[4-(4-maleimidophenoxy) phenyl]-hexafluoropropane (BAPOFP-BMI) [83].

Table 12.13 Solubility of BMPM and BAPOFP-BMI in Various Solvents [83] Resin

Toluene

Benzene

Acetone

DMF

DMAc

BMPM

1

1




11

11

BAPOFP-BMI

1

1

11

11

11

Soluble (11); Soluble at Heating (1); Insoluble (2).

O

O O

N O

O

N

N O O

O

O

O

O

O

O

O C

O C

O O AB

A2B

Scheme 12.21 Allyl-maleimide single component monomers [84].

AB2

N O

O

12: MALEIMIDE-BASED ALDER-ENES

483

O O

O

N F O

O O

n

Scheme 12.22 Allyl-terminated hyperbranched polyimide, AT-PAEKI [85].

structure with BMI. A comparative evaluation of these systems with the conventional BMPM/DABA system revealed that although the processing characteristics of the pre-polymers and mechanical properties of the cured resins is comparable (Table 12.14), the ablativity and thermal stability is superior to the BMPM/DABA system. The authors attribute this observation to the bond energy of C
O
B (185 kcal/mol), which is higher than the C
O bond (80 kcal/mol), and the probable formation of a thermal protective coating of boron carbide that has a high melting point and can prevent heat from continuing to diffuse and conduct inside. Polymerization and degradation studies of the bismaleimide/propargyl-terminated resin systems were also investigated [88]. The activation energy required for curing of BPEBPA and its blend with BMIE is reported to be 177.2 and 179.4 kJ mol21 respectively. The detailed isothermal pyrolysis studies of cured BPEBPA showed the formation of phenols and several substituted phenols, which can be explained due to the competitive C
C and C
O scissions of the chromene ring formed via the Claisen rearrangement of the aryl propargyl ether system present in BPEBPA. The Alder-ene reaction was investigated in the modified ethylene
diene copolymers with maleic anhydride (MAH) via a Lewis acid-catalyzed reaction [89]. The introduction of anhydride functionality increased intermolecular interaction in the species, reduced mobility of the chains, and thereby increased the Tg. Adhesion properties of the grafted MAH copolymer were greatly improved (by a factor of 3
6) when compared with commercial polyethylene (PE) owing to high polarity introduced by the anhydride functionality. A co-cured product of Tung oil (TO)/BMI prepolymer (cured at 200°C for 2 h) gave a bio-based

high-performance cross-linked TO/BMI product (Scheme 12.24) [90] with C-C ratio from 1/1 to 1/4. To evaluate the reaction of TO and BMI, the model reaction products of TO and N-phenylmaleimide (PMI) in DMI were analyzed by 1H-NMR spectroscopy. The NMR data of the reaction products of TO/ PMI with the C-C ratio 2/1, 1/1, 1/2, 1/3, and 1/4 at 150°C for 24 h revealed that the Diels-Alder reaction preferentially occurred at 2/1, and that the ene reaction and other reactions such as radical homoand copolymerization gradually increased with a decreasing C-C ratio of TO/PMI. Site-specific functionalization for the preparation of polar polyolefins could be achieved by the copolymerization of ethylene with a non-conjugated diene, followed by the reaction of the copolymer with maleic anhydride (MAH) via the Alder-ene reaction [91]. Spectroscopic studies confirmed that the functionalization of the ethylene/5,7-dimethyl1,6-diene (EDMO) copolymer proceeds mainly through the Alder-ene mechanism. By using this route, the use of peroxides can be avoided, which reduces/eliminates the degradation and other undesirable reactions such as chain scission, crosslinking, and coupling usually associated with free radical processes. Alder-ene functionalization of vinylidene-terminated polyisobutene (PIB) with stoichiometric amounts of maleic anhydride at 200°C was investigated by NMR spectroscopy [92]. The investigation indicated the formation of a noteworthy amount of a not-expected structure generated by the maleination of the less reactive PIB “endo” β-form provided by the isomerization of residual PIB “exo” α-form during the functionalization reaction. The evidence of unexpected molecular structures suggested an unconventional reactivity pattern of the precursors. Polyaralkyl-phenolic resin (Xylok) was allylated and co-polymerized with bismaleimidodiphenyl methane by Aijuan Gu et al. as shown in Scheme 12.25 [93]. At 120°C, it maintained a working life of 90 minutes. When the prepolymer was heated at 150° and 200°C, gelation occurred in 35 and 2 min, respectively. No weight loss was observed when heated to 490
500°C under nitrogen atmosphere. The cured resin property of Xylok is represented in Table 12.15. The system exhibited very good thermal stability and mechanical strength. The system showed a decomposition temperature of 530°C, and char yield of about 25% at 800°C. After aging for 100 h,

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Scheme 12.23 The synthesis reactions of allyl compounds containing boron [86]. (Reprinted with permission from Journal of Materials Processing Technology, 89
90, 544
549, (1999), Elsevier Publishers.)

water absorption and heat deflection temperature (HDT) were 2.3% and 280°C, respectively. For the corresponding glass-reinforced composites, the flexural strength, when tested at 200 and 250°C, 83 and 67% retention of the original room temperature value was observed.

In order to merge the advantages of the modification of BMI by amine and allyl compounds, N-allyl diaminodiphenylether (ADDE) was synthesized, and the modified BMI system contained ADDE [94]. The system exhibited a low curing temperature (200°C) and the optimum mechanical

12: MALEIMIDE-BASED ALDER-ENES

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Table 12.14 Mechanical and Thermal Properties of Boron-Containing Alder-Ene Polymers [87] BMPM/ XB

BMPM/ XA1

BMPM/ XA2

BMPM/ XA3

BMPM/ BQ1

BMPM/ BQ1

BMPM/DABA (1:0.8)

Tensile Strength (MPa)

83.6

83.3

80.7

79.5

77.8

74.4

73

Tensile Modulus (GPa)

3.72

3.77

3.7

3.78

3.78

3.79

3.61

Elongation (%)

2.3

2.32

2.45

2.25

2.25

2.17

2.2

Flexural Strength (MPa)

124

139

129

122

116

98

112

Impact Strength (kJ/m2)

12.6

10.9

9.6

17.7

11.8

8.9

13

Tg (°C)

277

282

278

283

285

330

274

Ti (°C)

489

490

487

495

495

499

426

Char-yield (%)

57

56

54

60

58

61

21

Property

Diels-Alder reaction N

N

O

H2C O C

Ene reaction

O

+

HC O C

O

O

O

H

N

N O

O

H2C O C

CH

CH

N

O

CH

CH

H C

O

O

O

O

O

)n

( O

O

homopolymerized (cross-linked) TO and / or co-polymerized TO-PMI

+

N homo/copolymerization

Scheme 12.24 Bio-based high-performance cross-linked TO/BMI polymer [90].

CH2

CH

H2C

OH

OH

OH CH3 CH2

CH2

CH

H2C

CH3

CH2

CH2 n

Scheme 12.25 Allyl polyaralkyl-phenolic resin (Xylok) [93].

CH2

CH

H2C

CH2

CH3

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Table 12.15 Cured Resin Properties of Allyl Xylok [93] Value Dry Property

Wet (100 hr in boiling water)

Tensile strength (MPa) at RT

78

63

Tensile modulus (GPa) at RT

3.9

3.2

Elongation at break (%) at RT

2.7

2.2

Flexural strength (MPa) at RT

114

92

Flexural strength (MPa) at 200°C

93

70

Flexural strength (MPa) at 250°C

78

55

Impact strength (kJ/m2) at RT

17.6

14

Critical strain energy release (Gic, J/m2)

169

130

Tg by DSC (°C)

490

478

Heat distortion temperature (HDT, °C)

310

280

Initial decomposition temperature (°C)

490




Maximum decomposition temperature (T max, °C)

530




Char yield at 800° C (Yc %)

25




Density (g/cm3)

1.23




(Reprinted with permission from Journal of Applied Polymer Science, Vol. 59,975
979 (1996),John Wiley and Sons Publishers.)

and thermal properties of the resin were obtained with the mole ratio of BDM:ADDE of 1:0.55. As the mole ratio of BDM was increased continually, the thermal properties leveled off.

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Alder-Ene Polymer Blends Thermoset Incorporated HighPerformance Alder-Ene Blends A novel approach for developing highperformance thermost polymer is to introduce a second thermost into the first polymer matrix phase; by this method it has been possible to produce a variety of high-performing matrices by the formation of linked interpenetrating networks (LIPNs). The method of thermal co-polymerization of two thermosets can also turn out highperforming resin systems with a favorable balance of both thermal and mechanical properties. Bisphenol A-based benzoxazine (Bz-A) and BMIP were thermally polymerized in varying proportions by Nair et al. [95]. It was observed that polymerization of maleimide was catalyzed by benzoxazines and that the maleimides did not influence the ring opening of the benzoxazines. The Tp, where the processing of the polymer is usually carried out, decreased for the blend to 211°C in comparison to those of BMI (270°C) and Bz-A (218°C). Moreover, the final cure temperature (Tf) of the blend was 284°C compared to 339°C for BMI. A wide cure regime between 142 and 284°C was observed for the processing of the blend as given in Table 12.16. The thermal stability and Tg of the Bz-A/BMI were significantly increased relative to polybenzoxazine; this was attributed to intermolecular H-bonding and network interpenetration. Cure studies of allyl-functionalized benzoxazine and bismaleimide blends (Bz-allyl/BMI) indicated the co-reaction between the two by an Alder-ene type reaction formed a single-phase system (Scheme 12.26). Hamerton et al. [96] co-reacted propenylsubstituted cyanate ester with bismaleimide and modeled the reaction mechanism using simple model compounds (1-cyanato-2-(2-propenyl)-4-tert-butylbenzene (geometric isomers) and N-(4-phenoxy)phenylmaleimide) to reduce the likelihood of crosslinking. The thermally initiated co-reaction between blends of the two model compounds when analyzed by Raman spectroscopy revealed that, as the thermal reaction proceeds, there was a pronounced decrease in the alkenyl CQC stretch band at 1655 cm21, which is accompanied by a concomitant decrease in the vinylidene band at 3010 cm21. Authors confirm that in the absence of a dedicated catalyst, the cyanate cyclo-trimerization is slow and the cure reactions

12: MALEIMIDE-BASED ALDER-ENES

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Table 12.16 DSC Results of Benzoxazine/BMI Blend [95]

Samples

Ti (°C)

Tp (°C)

Tf (°C)

CCW (°C)

Bz-A

156

218

270

BMI

195

270

Bz-A/BMI (1/0.5)

128

Bz-A/BMI (1/1) Bz-A/BMI (1/3)

Heat of Reaction (J/g)

Cure Window (°C)

Experimental

Theoretical

62

114

257




339

75

144

130




212

293

81

165

225

209

142

211

284

69

142

180

191

130

224

342

118

212

174

157

Ti, Initial cure temperature; Tp, peak cure temperature; Tf, final cure temperature; CCW, cure controllable window (Tp 2 Ti); cure window, Tf 2 Ti.

initially follow the co-reaction between the alkenyl group and the maleimide ring. This observation was substantiated by the molecular modeling of mechanisms (Figure 12.12) using semi-empirical and ab initio methods, which revealed the formation of a trans-ene adduct consistent with calculated and observed vibrational frequencies. A hybrid polymer network based on DABAmodified BMPM and 1,10 -bis(4-cyanatophenyl) ethane (BCyPE) with excellent dielectric properties has been reported [97]. The three formulations [BMPM/DABA: BCyPE,70:30 (F1), BMPM/DABA: BCyPE,80:20 (F2), BMPM/DABA: BCyPE,90:10 (F3)] developed by the researchers have good processing characteristics, such as low viscosity (220
411 cps) at 90°C, suitable pot life (.4 h), excellent glass transition temperature (Tg 300°C), and good reactivity; this renders them suitable for the RTM technique. The system possessed a lower dielectric constant (Figure 12.13) and dielectric loss than the current commercially available bismaleimide systems (3.5
4.5 3 105 Hz). A novel allylphenoxytriazine monomer, 2,4-di(2-allylphenoxy)-6-N,N-dimethylamino-1,3,5triazine (DAPDMT) (Scheme 12.27), was synthesized and reacted with BMI in different molar ratios [98]. The results showed that the copolymerization could help in improving the mechanical performance of the resin system. Compared with the neat BMPM matrix, the DAPDMT/BMPM copolymer matrix could improve the impact strength by 7.3 times. Researchers have developed a novel allylfunctionalized dicyanate ester resin bearing sulfoxide linkage (BACS, Scheme 12.28); it was co-reacted with bismaleimide at various ratios in the uncatalyzed condition [99]. From DSC it was observed that the pure allyl cyanate gives two distinct

exothermic peaks and up on incorporation of BMI only a single exothermic peak was observed at 260
280°C due to the Alder-ene reaction. The kinetic data reveal that a considerable extent of cure takes place in about 200 min at 190°C, and the cure attains near completion in about 3 h at this temperature. A high performance matrix blend was developed using bisphenol A dicyanate (BADCy), bismaleimide, diallyl phthalate (DAP), and cobalt(III)acetylacetonate/nonyl phenol (NP) as a complex catalyst system [100]. The flexural strength of the ternary blends reached its maximum value of 139.3 MPa when 15 phr DAP was added, which was 1.14 times as strong as the original BADCy. SEM analysis showed rough surfaces that suggest that the addition of DAP and BMI can effectively improve the toughness of BADCy, which is in agreement with the results of impact properties. The BADCy/ BMI/DAP blends showed an increasing trend in the values of E’ with increasing BMI content. Existence of two Tg was attributed to microphases separated in the blends. Char yield of BADCy/ BMI/DAP blends have increased with BMI loadings (33
35%), though the values were lower than that of cured the BADCy network. Interpenetrating polymer networks (IPN) of DABA-modified BMPM and cyanate ester in different ratios [101] showed excellent thermal stability (5% weight loss at 409
423°C), and maximum decomposition temperature ranging between 423 and 451°C. The modified systems showed less activation energy for the main degradation step than their neat counterparts. A single compound maleimide epoxy system containing both maleimide unit and allyl ether group has been synthesized using (N-(p-carboxyphenyl)maleimide (CPMI) and allyl glycidyl ether

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Scheme 12.26 Cure reaction between allyl-benzoxazine and maleimide [95].

(AGE) as shown in Scheme 12.29 [102]. This compound was designed to allow cure reactions analogous to those of Matrimid 5292t, but in a simplified, one-component resin system. The ether linkages and the higher aliphatic content within the

monomer were also expected to impart additional degrees of flexibility to the cross-linked network, which improves the toughness. CPMI-AGE underwent an initial weight loss at 205°C; the 5% weight loss was observed at 375°C. The exothermic

12: MALEIMIDE-BASED ALDER-ENES

Mechanism for the possible ene reaction between N-(4-henoxy)phenylmaleimide and 1-cyanato-2-(2-propenyl)-4-tert-butylbenzene.

Mechanism for the possible Diels-Alder reaction between N-(4-phenoxy)phenylmaleimide and 1cyanato-2-(2-propenyl)-4-tert-butylbenzene.

489

HOMO (top) and LUMO (bottom) for the product of the ene reaction between N-(4-phenoxy)phenylmaleimide and 1-cyanato-2-(2-trans-propenyl)-4-tertbutylbenzene.

HOMO (top) and LUMO (bottom) for the product of the Diels-Alder reaction between N-(4-phenoxy) phenylmaleimide and 1-cyanato-2-(2-transpropenyl)-4-tert-butylbenzene.

Figure 12.12 Reaction mechanism and molecular modeling of the model compounds [96]. (Reprinted with permission from Reactive & Functional Polymers 72, 279
286, (2012), Elsevier Publishers.)

transition in the DSC scan of the CPMI-AGE adduct beginning at 200°C is comparable to the cure exotherm seen in the Matrimid 5292t system. In another study, new co-monomers were synthesized by reacting epoxy resin with 2-allyl4-methylphenol (AE) and with BMI [103]. The catalyst used to accelerate the difficult reaction between phenolic hydroxyl and the epoxy ring was Mg (BF4)2. Although epoxy-modified BMI resins possess improved toughness and good tack and drape

properties, their poor solubility in common solvents and insufficient heat stability limit their scope. The moisture absorption of the polymers after aging for 100 hrs at around 250°C ranged between 3.2 to 3.8%. The mechanical properties such as SBS and flexural strength were also retained at 75 and 60% of the original value when tested at 200 and 230°C. Epoxy acrylate resins and allylepoxy resin were co-cured with BMPM/DABA [104]. The modified BMI resin systems exhibited two times improved

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4.5

Base resin

4.0 Dielectric constant

F3# F2#

3.5

F1# 3.0

2.5 BCyPE 2.0 0

50

100

150

200

250

300

350

Temperature, °C

Figure 12.13 Dielectric constant of BMPM/DABA BCyPE system [97]. (Reprinted with permission from Composites Science and Technology 66 1749
1755, (2006) Elsevier Publishers.)

Cl

Cl N

o-allylphenol (2 eqiv.)

N

N

acetone/H2O, aq.NaOH

Cl

N

Cl

N N

O

O

Me 2NH acetone/H2O aq.NaOH

H3C

N

N O

CH3 N

N

O

Scheme 12.27 Synthesis of allylphenoxytriazine monomer [98]. (Reprinted with permission from Journal of Applied Polymer Science, Vol. 86, 2279
2284, (2002), John Wiley and Sons Publishers.) NCO

O

OCN

S

Scheme 12.28 Dicyanate ester resin bearing sulfoxide linkage, BACS [99].

impact strength without a great decrease in dielectric properties when compared to the neat system. The physical and mechanical properties of the modified resin are given in Table 12.17. Both of the modified BMI matrix composites reinforced by glass fabric and quartz fabric possess good mechanical properties (Figure 12.14). The composites exhibited excellent dielectric constant in the range of 3.24B3.28 and

12: MALEIMIDE-BASED ALDER-ENES

491

O

O O

O

OH

O

TEA

O

+

N

THF reflux

N O O

O

HO

O

Scheme 12.29 Synthesis of single compound maleimide epoxy system [102]. (Reprinted with permission from Polymer Bulletin 46, 339
344 (2001), Springer Publishers.)

Table 12.17 Properties of the Allyl Epoxy-Modified BMI Resin [104]. Properties

Modified BMI Resin

Neat BMPM Resin

Tensile strength (MPa)

78

55

Tensile modulus (GPa)

3.6

3.8

Tensile elongation (%)

2.3

1.6

Flexural strength (MPa)

108

100

Flexural modulus (GPa)

3.7

3.9

Impact strength (kJ/m2)

15.2

5.83

HDT (Dry)

265

283

HDT (aged 100 h in boiling water)

231

234

Tg (DSC, °C)

274

293

Td (°C)

424

438

Water absorption (%)

2.3

2

Tan δ (10 GHz)

0.012

0.013

ε (10 GHz)

3.14

3.09

(Reprinted with permission from Polymer Bulletin 59, 269
278, (2007), Springer Publishers.)

2.92B2.98, respectively, and the dielectric loss was 0.012B0.014 and 0.0083B0.0096, respectively. Poly(phenylene oxide) microcapsules filled with epoxy resin (PPOMCs) systems have been used to realize self-healing BMI resin systems comprised of

BMPM/DABA [105]. The PPOMCs had a certain effect on the BMI system such as enhancement of the reactivity of BDM/BA owing to the
OH and amine groups in PPOMCs. The addition of PPOMCs has greatly influenced the fracture toughness enhancement of the BMPM/DABA system (with 5 wt.% PPOMCs; a KIC value which increased by 34
56% as compared to BMPM/DABA). PPOMCs attain their self-healing nature due to the presence of epoxy resins in the core material, which upon crack formation are released into the crack surface and polymerize in the presence of
OH and amine groups under controlled temperature; the crack surfaces of the matrix can be bonded together by the polymerized epoxy resins. The self-healing efficiency of BDM/BA is influenced by the contact areas between the crack surfaces, the size, and the content of PPOMCs. When the fractured samples are fixed using a clamp, BMPM/DABA with 10 wt.% PPOMCs can have the self-healing efficiency of 57
79% after heat treatment at 220°C for 5 h.

Thermoplastic-Toughened Alder-Ene Polymers Generally, engineering thermoplastics, if blended with bismaleimides, are expected to impart great improvement in toughness. The drawbacks of thermoplastic modification include: (i) induced viscosity increase by blending, which hampers processability of the blend; and (ii) low Tg associated with thermoplastics, which would be unfavorable for the hightemperature resistance of the blend. There have been some approaches for the thermoplastic modification of Alder-ene polymers by various groups. Diallyl bisphenol A
formaldehyde co-polymer (ABPF, Scheme 12.30) was addition-cured with bisphenol A
bismaleimide (BMIP) via the Alderene reaction by Nair et al. [106]; its adhesive

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600

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490 512

500 410 405 400

356 350

300 173 157

200 100 25

45

28

43

0 Tensile strength (MPa)

Tensile modulus (GPa)

Flexural strength (MPa)

Modified BMI/glass fabric

Impact strength (kJ/m x m)

ILSS (MPa)

Compressive strength (MPa)

Modified BMI/quartz fabric

Figure 12.14 Mechanical properties of allyl epoxy-modified BMI glass and quartz composites [104].

CH3 C

HO

CH3

OH CH2

Diallyl bisphenol A-formaldehyde resin (ABPF)

(a) O

O CH3 N

O

C

O

N

CH3 O

O

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

Scheme 12.30 Diallyl bisphenol A
formaldehyde co-polymer [106].

characteristics were studied by determining lap shear strength (LSS) and T peel strength. One-hundred percent retention of the LSS at 250°C was observed for the system at an optimized 1:1 ratio of allyl to maleimide. The same matrix, when modified with thermoplastic toughening agents such as polysulfone (PS) and polycarbonate (PC), reduced the Tg from 350
380°C to 190
260°C. Among these thermoplastic modifiers (compared to PS), the PC-modified system was less stable at lower temperatures, but showed better performance at temperatures above 600°C. The adhesive characteristics of the different combinations are shown in Table 12.18.

A three-component Alder-ene resin composed of BMPM, DABA, and o,o’-dimethallyl bisphenol A (1.0/0.3/0.7 eq ratio) was modified with polyetherether-ketone (PEEK) by Takao Iijima et al. [107]. Considerable improvement in toughness was achieved by the incorporation of PEEK as it provided a co-continuous phase structure from a single phase structure, as shown in Figure 12.15. Thus, when 15 wt.% of PEEK was incorporated (44 mol.% TP, MW 23,400), the KIC value for the modified resin increased to 30% with no penalty in the mechanical and thermal properties. Also, the KIC increased to 95% when compared to the value for a commercial bismaleimide (Matrimid 5292t) resin. In a similar work, a BMPM/DABA system was incorporated with amorphous thermoplastic bisphenol A polysulfone (PSF), polyether ketone (PEK-C), and polyether sulfone (PES-C) bearing a phthalidylidene group [108]. The absence of endothermic peak, or the decrease in the area of the endothermic peak of thermoplastic components in the aged blends, was attributed to the formation of semi-interpenetrating polymer networks that restricted the segmental mobility of thermoplastic components. In another study, a BMPM/DABA system was modified with 4,4’-bismaleimide diphenyl ether of biphenyl A (MEBMI), allyl phenol epoxy (APE), and thermoplastic-modified polyether-ketone PEK-C [109]. The authors found that the impact strength of the BMI resin depended strongly on the amount of PEK-C incorporated. The typical MBMI/DABPA component system modified with MEBMI, AE, and PEK-C exhibited outstanding impact toughness,

Table 12.18 Thermoplastic-Modified (PS and PC) Adducts of BMIP:ABPF (1:1) [106] Relative Increase in LSS (%)

LSS (MPa) System

250°C

150°C

200°C

250°C

TPS (k Nm21)




117

122

127

poor

212

79

51

33

0.32 6 0.05

120

23

86

57

33

0.38 6 0.04

185

114

6

93

72

37

0.40 6 0.04

176

98

68

54

84

74

71

poor

5.4 6 0.6

115

44

24

4

78

70

61

poor

3.5 6 0.5

102

40

10

233

81

66

42

poor

RT

150°C

200°C

250°C

RT

150°C

200°C

Unmodified

4.1 6 0.6

4.8 6 0.4

5.0 6 0.4

5.2 6 0.3










PS-10

13.9 6 0.7

11.0 6 0.6

7.1 6 0.6

4.6 6 0.5

239

129

42

PS-20

19.3 6 0.7

16.5 6 0.8

11.0 6 0.6

6.4 6 0.5

371

244

PS-30

14.8 6 0.6

13.7 6 0.7

10.7 6 0.6

5.5 6 0.6

261

PC-10

11.3 6 0.8

9.5 6 0.7

8.4 6 0.7

8.0 6 0.5

PC-20

8.8 6 0.8

6.9 6 0.7

6.2 6 0.6

PC-30

8.3 6 0.7

6.7 6 0.6

5.5 6 0.6

Cured at 160°C for 30 Min, 200°C for 30 Min, and 250°C for 2 H.

Relative Retention of LSS (%)

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Figure 12.15 Scanning electron micrographs of the fracture surfaces for the (a) neat, and (b) 15 wt.% PEEK modified BMPM/DABA system [107]. (Reprinted with permission from Journal of Applied Polymer Science, Vol. 82, 2991
3000, (2001), John Wiley and Sons Publishers.)

Scheme 12.31 Structures of elastomeric additives [112].

excellent thermal stability, relatively high Tg, and good processability. Polyetherimide (PEI) synthesized from different diamines was also used to toughen BMI [88]. All the modified compositions exhibited two relaxation peaks or shoulders corresponding to the relaxation transition of the two-blend components, whereas the unmodified BMI resin gave only one tan δ peak at 293°C. The incorporation of the second component induced phase separation in the system. The GIC values of some of the modified systems were nearly three times higher than the neat resin. The change in the light-scattering profile with curing time indicated that the phase separation mechanism depended on the modifier concentration [110]. Thus, phase

separation took place via the spinodal decomposition mechanism in the PEI 15-phr- and 20-phrmodified system and the fracture energy (GIC) increased with PEI content in the modified system. In the PEI 15-phr-modified system, the GIC value was three times greater than that of the unmodified BMI resin. Toughening of commercial BMI resin with varying proportions of poly(phthalazinone ether ketone) (PPEK) was also reported [111]. The activation energy of the reaction indicated that the reaction mechanism remained the same even after the incorporation of PPEK. The morphology of the cured resin changed from a dispersed structure to a phase-inverted structure with the increase of PPEK content. Compared to the neat resin, the fracture toughness of the modified resin exhibited a moderate increase when PPEK was incorporated. Maleimide functional novolac phenolic resin (PMF), self-cured and co-cured with a novolac epoxy resin, was modified as a function of the varying concentrations of the additives, ranging from 10 to 30 phr of the base resin by three thermoplastic elastomers: (1) two grades of carboxyl terminated butadiene acrylonitrile copolymer (CTBN) of different molecular weights; (2) a low-molecular-weight, epoxidized hydroxylterminated polybutadiene [EHTPB]; and (3) a highmolecular-weight acrylate terpolymer containing pendant epoxy functionality [EPOBAN] (Scheme 12.31). The adhesive characteristics were studied by determining T peel strength and LSS on aluminum adherends by Nair et al. [112]. All the elastomers were effective in increasing the adhesive properties at ambient temperature of the brittle, less cross-linked, self-cured PMF. CTBN-S (high-molecular-weight Mn-65000g/mol) was the most effective additive with

12: MALEIMIDE-BASED ALDER-ENES

495

O O N O O Si N

O

O O

O

Si O

O O

N

O O

O

O Si

Si

N

O Si O

O

Si

O O

O

O

O

O Si

O

Si

O

O N N O O

Scheme 12.32 Structure of OMPS [113].

a good phase-separated morphology. For the more rigid, less ductile, epoxy-cured PMF system, the adhesive properties were marginally improved by the highmolecular-weight CTBN, whereas the other elastomers were practically ineffective.

Nano-Modified Alder-Ene Polymers Organic
inorganic nanocomposite materials are promising and fast developing research areas in materials chemistry. These materials are important as they have potential applications in fields such as chemistry, electronics, optics, and biomedicine. There are a few efforts to study the effect of nanomaterial incorporation on Alder-ene polymers. Octamaleimidophenyl polyhedral silsesquioxane (OMPS) was investigated as a co-curing agent for allyl-functional novolac (AN) and BMPM resin system [113]. Scheme 12.32 depicts the structure of OMPS. The intensity of the characteristic bands of maleimide groups at 3078 cm21, 826 cm21, and 690 cm21, and allyl groups at 923 cm21, was decreased on curing (Figure 12.16), and the appearance of a new band at 1190 cm21 (succimide group) proved that OMPS could be used as a co-curing reagent to prepare the POSS-modified AN/BMPM resins. SEM images showed no discernible phase separation for the OMPS-modified AN-ABMI systems. At

the same time, POSS-modified systems showed an improvement in thermal stability (Ti improved from 414°C to 426°C) and char residue (31% to 41%). In yet another attempt to develop novel highperformance hybrids, copolymerization of bismaleimide with the caged octa(aminopropylsilsesquioxane) (POSS-NH2) was attempted and then co-reacted with 2,2’-bis(4-cyanatophenyl)isopropylidene (Scheme 12.33) [114]. POSS
NH2/BT hybrids have lower curing temperatures than BT resin (by about 13°C) because of the additional reactions occurring between the
OCN and amine groups. Dielectric loss of the hybrids is only 25% that of BT resin at the frequency lower than 105 Hz, whereas all hybrids and BT resin have almost equal dielectric losses when the frequency is higher than 105 Hz. It was also observed that the char yield improved from 24 to 32% by incorporation of 0.1 by weight of POSS-NH2. Bismaleimide-modified novolac resin/silsesquioxane (BMI-PN/SiO3/2) nanocomposites were prepared via the sol
gel process by Hamerton et al. [115]. When the BMI/PN ratio was kept constant, the introduction of SiO3/2 phase of nanoscale domain size remarkably enhanced the Tg and the thermal resistance of the material. FESEM and AFM images (Figure 12.17) showed that BMI-modified novolac thermoset was a homogeneous material, whereas BMIPN/SiO3/2 nanocomposites were two-phase materials. From TGA, thermal decomposition of the nanocomposite occurred in two stages, and the introduction of the inorganic phase improved the Tmax and the weight retention at 700°C. In another study, Bisphenol A-based bismaleimide (BMIP) was blended with various nanoclays [116], among which Closite 15 B had a strong influence on the cure (onset of the cure exotherm shifted to 40°C). Except for Cloisite 15A and nanoclay DK4, all the other nanoclay incorporated BMIP showed very little influence on the enthalpy of cure, which actually decreased. Incorporation of the nanoclay in the BMIP system increased the thermal stability of the system and this was reflected in the char residue value (42 to 52%). BMPM/DABA/organoclay (OLC) nanocomposites were synthesized, and the effect of nanoclay on the Alder-ene curing reaction was studied [117]. It is reported that the ene reaction was accelerated to different degrees depending on the acidity of the modifier and the accessibility of the organoclays used, as evidenced by a lowering in the activation energy for curing of the nanoclay-incorporated systems.

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1130 D

THERMOSET PLASTICS

926 827 691

1185

3078

OF

C

Transmittance

B

A

3500

4000

2500

3000

1500

2000

Wavenumber

1000

500

(cm−1)

Figure 12.16 FTIR spectra of the AN/BMPM resin incorporated with 12.7 wt.% OMPS at different curing stage: (A) uncured resin, (B) 170°C 4 h, (C) 200°C 4 h, (D) 250°C 6 h [113]. (Reprinted with permission from Journal of Applied Polymer Science, Vol. 104, 3903
3908, (2007), John Wiley and Sons Publishers.)

O

O R O R Si

O

Si O

Si

O

O

N O

O R

Si

O Si

R1

O O

O O Si O R Si O R

N

NH

CH2CH2CH2

Si

R O

O

O

N

CH2CH2CH2NH O

R1

O HN

N

O

Si

R

O

Si

H2NH2CH2C Si O O

O

Si

O Si O Si O

R

O

O O R Si R O O Si R

O

O O

O R

CH2CH2CH2

N

NH O

R1

R1

CH2CH2CH2

N O

N

NH O

R1

HNCH2CH2CH2

N O

CH2

Scheme 12.33 Structure of POSS-NH2-BDM [114]. (Reprinted with permission from Journal of Applied Polymer Science, Vol. 120, 360
367, (2011), John Wiley and Sons Publishers.)

12: MALEIMIDE-BASED ALDER-ENES

497

Figure 12.17 FESEM photographs of the BMI-PN/SiO3/2 nanocomposites (a) neat BMPM/DABA, (b, c) POSS incorporated BMPM/DABA [115]. (Reprinted with permission from Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 41, 2599
2606 (2003), John Wiley and Sons Publishers.)

O HOOC

NH2

O

Acetic anhydride/ Sodium acetate

O

+

N

COOH

THF

Para-aminobenzoic acid

O Maleic anhydride

O Maleimidobenzoic acid SOCl2

O N

O

O O

O O

O

n

THF

HO

O

H n

N

N

COCl

THF/TEA

O

O Maleimide-end-capped PTMO (BMI-PTMO)

O Maleimidobenzoyl chloride

Scheme 12.34 Synthesis of BMI-PTMO bismaleimide-end-capped poly(tetramethyleneoxide) [119].

Because of the presence of cations of OCLs, the electron-rich oxygen atom of the carbonyl group may interact with the cations, and this makes the dienophile, BMPM, more electron-deficient, thus reducing the activation energy. The consumption of 2-phenylphenol (a styrene-like group) can be realized by thermal re-aromatization after conversion of all maleimide into succinimide in the presence of OCL. In another nano-modification attempt, BMPMmodified novolac resin/titania nanocomposites were prepared by the sol
gel process involving tetrabutyl titanate co-adducted in the presence of BMPM-modified novolac prepolymers using acetyl acetone (AcAc) as a stabilizer [118]. Obviously, incorporation of TiO2 nanoparticles had no effect on the improvement of the Tg of the nanocomposites. However, thermal resistance of the material was lowered due to the incomplete decomposition

of AcAc coordinated with TBT. Evaluation of morphology revealed that the BMI-PN/TiO2 nanocomposites were two-phase materials with the continuous phase constituted by BMIPN and the dispersed phase constituted by TiO2 nanoparticles.

Shape Memory Alder-Ene Polymers Alder-ene resins having shape memory characteristics have been synthesized by Nair et al. [119]. A bismaleimide
allyl phenol polymer system was synthesized by co-reacting 2,2’-bis 4[(4-maleimidophenoxy)phenyl]propane (BMIP) and o,o’-diallylbisphenol A (DABA) with varying proportions of bismaleimide-end-capped poly(tetramethyleneoxide) (BMI-PTMO) (Scheme 12.34) that possessed a transition temperature of up to 220°C.

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O

O O

O

THERMOSET PLASTICS

O

O

N

O

O

OF

O

N

n

+

O

BMI-PTMO

O

O

N

O

O

BMIP O

O

O

N

OH

HO

N

DABA

O

O

N

O

OH

Ene reaction (110–140°C)

BMIP O

O

Diels-Alder (200°C)

O

N

N

O

CH2 CH2

O O N

O

O

BMI-PTMO O

N

O

O

N

O

N

(200°C) O

O

HO HO O

O

N

O O n

O

O

O

N

O

Scheme 12.35 Possible reaction mechanism of BMIP/BMI-PTMO/DABA system [119].

Flexural strength and modulus of the carbon fabricreinforced composites of BMI
PTMO resin decreased with an increase in the concentration of PTMO (from 220 MPa to 100 MPa for a change of PTMO weight percentage of 55 to 69 wt.%). The decrease in flexural strength was due to the incorporation of PTMO segments in the matrix that decreased the effective cross-link density of the BMI
allyl system by matrix dilution, and by plasticizing the network, which imparted the shape memory effect to the system. The probable cure mechanism of the BMIP/BMI-PTMO/DABA system is represented in Scheme 12.35. The authors estimated the shape memory behavior by a bending test between the temperatures Ttrans 1 20°C and Ttrans 2 20°C. Ttrans is the tan δ peak temperature. The original (permanent) rectangular shape was heated at Ttrans 1 20°C, and the

sample was deformed into a U shape and subsequently, when cooled under load, these deformed temporary shapes were fixed. It was observed that on reheating above Ttrans 1 20°C again, the samples recovered their original rectangular shape and all systems in the present case showed shape retention (fixity) of B94%. The recovered shape (Figure 12.18) was nearly indistinguishable from the original shape, which confirmed the excellent shape fixity and recovery. The extent of shape recovery was a direct function of the concentration of PTMO (Table 12.19). Shape recovery increased and recovery time decreased with an increase in PTMO content. The reason is that highly crosslinked structures have strong constraining forces on their segments, which need a large free volume and more energy, thus necessitating higher temperature to accomplish shape recovery.

12: MALEIMIDE-BASED ALDER-ENES

499

Figure 12.18 Shape recovery of the composite PTBMI-3 at different time intervals at 160°C [119].

Table 12.19 Mechanical and Shape Recovery Properties of Different Compositions [119]

Polymer

PTMO (wt.%)

Molar % of Components, BMIP/DABA/ BMI-PTMO

PTBMI-1

55

36/46/18

220 6 10

221.7

9

PTBMI-2

63

32/46/22

160 6 7

180.8

13

92 6 1

55

94 6 1

PTBMI-3

69

27/46/27

100 6 2

140.5

18

98 6 1.2

42

94 6 1

Flexural Strength (MPa)

Ttrans (°C)

Kinetics of Alder-Ene Polymerization There have been a number of research efforts to comprehend complex curing reactions and network formation of Alder-ene polymers where the reaction proceeds via an ene reaction at the lower temperatures and an intermediate Wagner-Jauregg reaction followed by Diels-Alder reaction at higher temperatures. Cure kinetics of a toughened bismaleimide [120] revealed that a lower temperature exotherm was due to a chain-extension reaction between the

Modulus Ratio (Eg/Er)

Percentage of Shape Recovery at Ttrans 120°C

Recovery Time(s)

Percentage of Shape Fixity at Ttrans 2 20°C

88 6 0.5

65

94 6 1

maleimides and the toughening agent, and the higher temperature exotherm was due to bismaleimide homopolymerization. Total heats of polymerization varied over the range 196.5
231.4 J/g with a mean value of 209.4 J/g. The authors reported that the curing process followed the first-order cross-linking reaction during curing. Two major exotherms were identified with activation energies in the range 72
85 kJ/mol. In another study, 2,20 -methylene-bis[4-methyl-6(2-propenyl)] phenol (MBMPP) was used to toughen BMPM [121]. Kinetic analysis of the

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BMPM - DABPA (1:1 Molar) BMI Cure Reactions Maleimide consumption (820 cm-1)

100

Allyl consumption (915 cm-1)

90 80

Reaction percentages

70 60 50 Ether appearance (1183 cm-1)

40 30 20 10 0 0.5

1

1

1

1

1

3

9

TIME, HRS

130

130

150

175

200

225

250

250

TEMP, °C

Accumulative cure conditions

Figure 12.19 The consumption of the maleimide and allyl groups and appearance of the ether group during isothermal cure conditions [122]. 400

Tg (°C)

300

200

100 300°C 280°C 250°C

200°C 180°C 150°C

130°C

0 0

5

10

15

20

25

30

35

Time, hr

Figure 12.20 Tg vs isothermal cure time for temperatures ranging from 130 to 300°C for the BMPM-DABPA (1/1 molar) BMI system [122]. (Reprinted with permission from Polymer Vol. 38 No. 3, pp. 639
646, 1997, Elsevier Publishers.)

curing through DSC proved that the reaction accorded with a first-order reaction with an activation energy of 74 k J/mol. Roger J. Morgan [122] characterized a BMPM/DABA system curing

through FTIR and DSC as a function of isothermal exposures from 130 to 300°C in intervals from 1 to 14 h. The allyl, propenyl, and maleimide double CQC bonds of the ene adduct were completely reacted after 3 h at 250°C (as evidenced by FTIR). However, only 50% of the hydroxyl groups of the ene adduct formed cross-links via dehydration at 250°C over long durations, which resulted in the increase of Tg. Further post-curing at 300°C for 9 h resulted in a further B10% increase in ether crosslinks as monitored by the decrease in intensity of the hydroxyl band at 3473 cm21, and an associated increase in Tg from 280°C to 350°C. The consumption of the maleimide and allyl groups, and appearance of the ether group for the BMPM DABPA system, as during isothermal cure conditions, is shown in Figure 12.19. Tg as a function of isothermal exposures at different temperatures for variable time periods is shown in Figure 12.20. The degree of cure (α) as determined from DSC was found to be less sensitive to the dehydration reaction of the ene adduct because of a lower heat of reaction as compared to cure reactions involving a CQC bond. The fluorescence behavior observed during the cure of BMI/DABA resin was utilized to study the

12: MALEIMIDE-BASED ALDER-ENES

cure reaction [123,124]. The model compounds gave fluorescence signals that were red-shifted by 40 nm or more from the emission maximum in DABA resin, while no fluorescence was observed from the BMI. BMI was found to quench the fluorescence from DABA. The DABA resin component was found to have the highest quantum yield and was likely to be responsible for most of the fluorescence near 356 nm when the resin was excited near 280 nm. A succinimide derivative that arose from a Diels-Alder-ene reaction sequence was found to have a higher quantum yield than other succinimides investigated in the study. The results obtained in this work indicated that fluorescence signals from phenolic-like structures occurred at shorter wavelength regions than those arising from phenyl succinimide moieties. During curing, phenylmaleimide units were converted to phenylsuccinimide via ene, Diels-Alder, and copolymerization reactions. The absorbance band associated with C-N-C maleimide (centered at 1149 cm21) decreased, while a new absorbance band at 1180 cm21 corresponding to C-N-C succinimide increased. Correlation of the extent of conversion with fluorescence intensity and cure time indicated that the largest increase in fluorescence intensity was observed after conversion of approximately 70
80% of the maleimide units to succinimide. UV reflection spectra of BMI/DABPA resin before and after “full cure” showed an overall decrease in the percentage reflectance near 230 nm and 295 nm with a simultaneous overall increase in the percentage reflectance near 266 nm. The parameters affecting the Alder-ene reaction for the synthesis of a telechelically anhydridefunctionalized polypropylene was investigated by Thompson et al. [125]. A temperature range of 220
250°C at maleic anhydride concentrations from 2
12 mol equivalence with respect to the vinylidene group were selected for the study. The group observed that increasing the temperature and maleic anhydride concentration assisted in incorporation of the succinyl anhydride moiety at the terminal site in polypropylene. They also observed that ruthenium chloride was found to be a better catalyst for the anhydride incorporation in polypropylene as compared to Stannous chloride. The kinetics of curing of three different bismaleimides with tris(2-allylphenoxy) triphenoxycyclotriphosphazene via an Alder-ene reaction was studied by Nair et al. [126]. The bismaleimides used for the study included: (i) bis(4-maleimidophenyl) methane (BMM), (ii) bis(4-

501

maleimidophenyl) ether (BME), and (iii) bis(4-maleimidophenyl) sulphone (BMS). DSC of all the blends manifested two major exotherms due to ene reaction followed by Wagner-Jauregg and DielsAlder reactions. The Wagner-Jauregg reaction was found to be disfavored by the electron-withdrawing nature of the maleimide group, while the DielsAlder reaction was facilitated. When the stoichiometry of BMI in the blend increased beyond 1:1.5, an apparent change in the reaction mechanism was observed. The total reaction sequence was found to be faster in the case of BMM through a predicted isothermal DSC profile. At lower temperatures the network was mostly a result of the Wagner-Jauregg reaction, and at high temperatures it was from the Diels-Alder reaction. The kinetic parameters obtained for these resin systems are given in Table 12.20. The effect of BMI structure on the kinetics of thermal degradation of the reaction of additioncured blends of diallyl bisphenol A
formaldehyde resin (ABPF) with four different BMIs was also investigated [64]. The kinetic parameters E and A were nearly the same for BMIM and BMIE systems, and non-isothermal kinetic analyses of the different systems showed the decomposition occurred in at least two kinetic steps. Contrary to the apparent single-step degradation manifested in TGA, the kinetic plots of the different BMI
ABPF during non-isothermal evaluations showed two major sectors for all the systems. The authors attributed the first step (between 415
455°C) to the degradation of the methylene and allyl group-derived structures present in the network. The second step (470
525°C) was most likely due to the degradation of the maleimide part and volatilization of the degraded products of the first step. The activation energy for the first step was enhanced by the greater number of crosslinks present in systems with higher maleimide content, and the activation energy for the second step was only around 40% of that for the first step; the pre-exponential factor was also very low, which indicated the dominance of volatilization in the kinetics of degradation. The results of the kinetic analysis of degradation are presented in Table 12.21. Kinetics of the BMPM/DABA cure reaction and the model compounds phenyl maleimide/DABA were examined [127]. It was established that polymerization proceeded via the step-wise ene reaction and consecutive/parallel chain polymerization of maleimide and propenyl groups. Homopolymerization

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Table 12.20 Kinetic Parameters for Different Steps of the Reactions [126] Kissinger

Osawa

System

Reaction Step

E (k J/ mol)

A (s

BMPM(1:2)

I

54.6 6 7.1

II BMPM(1:1.5)

BMPM(1:1)

BME (1:2)

BMS (1:2)

21

E (kJ/mol) for Homopolymerization

E (k J/ mol)

A (s21)

3.62 3 103

54.9 6 7.1

4.02 3 103

113.5 6 8.9

6.47 3 10

8

113.7 6 8.9

6.88 3 10

I

75.3 6 10.4

7.81 3 10

5

75.5 6 10.4

6.42 3 105

II

84.3 6 2.5

1.09 3 10

6

84.7 6 2.5

1.21 3 106

I

75.9 6 17.0

1.31 3 106

75.3 6 17.0

1.52 3 106

II

96.3 6 4.5

2.15 3 107

94.3 6 4.4

1.88 3 107

I

85.9 6 11.3

9.15 3 106

86.2 6 11.3

9.81 3 106

II

81.7 6 3.4

4.83 3 10

5

82.0 6 3.4

5.14 3 10

I

175.7 6 13.2

2.78 3 10

17

175.85 6 6.1

2.9 3 1017

II

53.3 6 13.2

4.51 3 102

53.5 6 6.2

4.75 3 102

)

76.9

8

119.6

5

(Reprinted with permission from Thermochemica acta, 374, 159
169, (2001), Elsevier Publishers.)

Table 12.21 Kinetic Parameters of Degradation of BMIP-ABPF Systems at Varying Maleimide to Allylphenol Stoichiometry and BMIP-DABA Systems at 1:1 Stoichiometry [64] First Stage (415
455°C)

Second Stage (470
525°C)

Mass Loss Range (%)

E (kJ/mol)

A (s21)

Mass Loss Range (%)

E (kJ/mol)

A (s21)

BMIP-ABPF (1:1)

6.0
15.1

177.7 6 10.3

9.4 3 109

19.0
37.4

85.6 6 1.7

1.1 3 103

BMIP-ABPF (2:1)

5.3
17.5

190.7 6 11.4

1.4 3 1011

22.4
38.6

71.7 6 0.8

1.5 3 102

BMIP-ABPF (3:1)

4.2
17.2

199.0 6 10.4

5.4 3 1011

24.4
43.4

77.0 6 2.4

4.2 3 102

BMIP-DABA (1:1)

5.2
24.5

175.33 6 11.9

1.1 3 1010

31.7
51.3

66.5 6 1.2

6.9 3 101

System

(Reprinted with permission from European Polymer Journal 38, 503
510, (2002), Elsevier Publishers.)

proceeded auto-catalytically, and mediated by free radicals generated by the thermal decomposition of maleimide
propenyl group donor
acceptor pairs. A dehydration reaction was also observed in the curing system and proceeded with mandatory participation of 1:1 ene adduct. The heat of opening the double bond of maleimide groups in both the ene reaction and radical polymerization was found to be 92 kJ/mol. Yet another study dealt with the cure kinetics of bismaleimide modified with DABA, with ratios of 1,10 -(methylene di-4,1-phenylene) bismaleimide and diallylbisphenol A [128]. The activation

energies of both modified BMI formulations increased with conversion from 65 kJ/mol to a maximum at 90 kJ/mol at higher conversions, and remained fairly constant thereafter. The plot of activation energy versus percentage conversion is presented in Figure 12.21. The activation energy for pure BMI, within the conversion of 10% to 25%, was constant, with a value of 92
96 J/g. From the isothermal cure analysis, the conversion was found to increase rapidly with the curing time, but reached a maximum asymptotically. This cure profile evidenced the change of the reaction mechanism from a chemically controlled process to a

12: MALEIMIDE-BASED ALDER-ENES

503

160 140 pure BMI

E (J/g)

120

E at exothem peaks DABA-1

DABA-0.5

100 80 60 40 10

0

20

30

40 α (%)

50

60

70

80

Figure 12.21 Dependency of activation energy on conversion [128]. (Reprinted with permission from Journal of Applied Polymer Science, Vol. 90, 2229
2240, (2003), John Wiley and Sons Publishers.)

100

80

α (%)

60

40

20 170°C 190°C 210°C

0 0

10

20

30

40

180°C 200°C 220°C 50

t (min)

Figure 12.22 Plot of conversion vs. isothermal reaction temperature [130]. (Reprinted with permission from Journal of Macromolecular Science, Part A: Pure and Applied Chemistry, 43:1687
1693, 2006 Taylor & Francis publishers.)

diffusion-controlled process. Once the maximum conversion was reached for the reaction, vitrification occurred, and stopped the reaction almost completely. For pure BMI, the cure reaction at the respective temperatures yielded only a 30
40% conversion. At higher conversions, the diffusioncontrolled reaction rate was too sluggish to be detected because of the high cross-link density of the network. The activation energy obtained by the multi heating-rate method was higher than that obtained by the isothermal method.

The Tg-conversion relationship after isothermal curing at various temperatures for 1,10 -(methylene-di-4, 1-phenylene) bismaleimide (BMI), modified with DABA, was investigated by Y. Xiong et al. [129]. The DiBenedetto equation was used to model this relationship for the formulation of DABA-1 (BMI:DABA, 1:1). Based on this model, the Tg-conversion relationship of the formulation containing BMI: DABA (1:0.5) was modeled. The high consistency between the model curve and experimental data showed that the change of Tg, attributed to copolymerization between BMI and DABA in DABA-0.5, in the lowconversion regime, was the same as that in DABA-1. This also verifies that, for the formulation DABA-0.5, copolymerization and homopolymerization did not overlap with each other. It was also observed that the Tg increase caused by BMI homopolymerization was much faster than by BMI
DABA copolymerization. A series of isothermal differential scanning calorimetry (DSC) run from 170°C to 220°C provided information about the cure of the BMPM/DABA resin system [130]. From the dynamic DSC experiment, after the isothermal run, the total heat of reaction was found to be 288.44 J/g. These measurements indicated that complete cure was never achieved when the cure temperature was below 220°C. The plot of conversion versus isothermal reaction temperature is presented in Figure 12.22. The frequency factor determined was 4.95 3 108 min21 and activation energy was 83.31 kJ/mol. From the isothermal DSC data, it was found that the first-order kinetic expression was appropriate to analyze the cure characterization of the system. When the cure progressed, the reaction rate decreased gradually because of diffusion-controlled reactions. The curing behavior of allylated novolac/BMPM modified by PDMS was studied by FTIR and DSC [49]. The decrease in the degree of allylation was accounted for the decrease in the total enthalpy of the curing reaction, and the authors concluded that the presence of PDMS did not apparently change the reaction pathway. The kinetic evaluation of the curing process revealed that the curing process of the PDMS-modified AN/BMPM resins and the parent AN/BMPM resin followed an nth order mechanism. The isothermal curing temperature and PDMS content were found to influence the curing kinetics. For allyl-functionalized dicyanate ester resin bearing a sulfoxide linkage, when reacted with bismaleimide at various ratios [99], the pure allyl cyanate gave two distinct exothermic peaks. When the BMI

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Figure 12.23 (a) The conversion of maleimide group at 1149 cm21, and (b) conversion of ally group at 995 cm21 at isothermal cure at different temperatures [68].

Figure 12.24 Dependence of the activation energy on the extent of conversion [68].

was incorporated into the cyanate ester, only a single exotherm peak was observed, which was due to the Alder-ene reaction. The kinetic data revealed that a considerable extent of cure took place in about 200 min at 190°C, and that the cure attained near completion in about 3 h at this temperature. Cure behavior of a bismaleimide-containing structure (BMIP) and DABA was studied using FTIR and DSC [68]. The polymerization of BMI was observed by the decrease of the peak at 1149 cm21. This was due to the C
N
C stretching vibration of the maleimides. Gradual disappearance of the absorbance band centered at 3,100 cm21,

which was assigned to the QCaH stretching due to the consumption of the BMI unsaturation. The characteristic peak of allyl unsaturation arose due to trans
HCQCHa wagging at 995 cm21 and at 912 cm21, which was due to vinyl CH2 wagging. The latter, which is often used to evaluate the conversion of allyl groups, was heavily masked in the present system and hence the change in the absorption at 995 cm21 was used to study the consumption of DABA. The methyl absorption at 2964 cm21 did not change during the cure process, and was used as the internal reference band. The conversion of maleimide and allyl groups at different temperatures is given in Figure 12.23. The authors point out that the consumption of maleimide and allyl groups showed a fairly strict 1:1 correlation below 200°C, and the reaction progressed through alternating copolymerization of maleimide and allyl groups; the homopolymerization and isomerization reaction of BMI and allyl groups at this temperature were negligible. The calculated Ea versus percentage conversion is shown in Figure 12.24 for the second step of the curing reaction, which is prominent compared to the first; the Ea values varied between 84
98 kJ/mol. The initial decrease in Ea could be explained by the diffusion control which was induced by melt viscosity; the molecular weight increased slowly at low conversion, but the viscosity decreased drastically with increasing temperature. This in turn could result in an increase in mobility of the molecular chains. The increase in Ea above 70% was due to the change of reaction kinetics from chemical kinetic control to diffusion control induced by the vitrification.

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Outlook This chapter analyzed the recent literature on maleimide
allyl phenol-based Alder-ene polymers, and took into account various aspects such as synthesis, polymerization, blending, reaction mechanisms, reaction kinetics, properties, and applications. Bismaleimide resins constitute a leading class of thermosetting polyimides that exhibit a balance of thermal and mechanical properties. These evince a lot of interest in the aerospace industry due to their high performance-to-cost ratio and excellent thermal and mechanical properties. A considerable amount of research has been devoted to improving shortcomings like brittleness after curing, poor processability due to poor solubility in ordinary solvents, high crystalline melting temperatures of the monomers, and narrow processing windows of the systems. Olefinic compounds are successfully used as modifiers to enhance the toughness of bismaleimides. Allyl-type compounds have been the most successful modifiers since they offer less severe adverse effects on the high-temperature properties and processability. The allyl group co-reacts with the maleimide group via the ene reaction followed by the Diels-Alder reaction to form the final cross-linked product. Allyl groupincorporated phenol
formaldehyde resins (Novolac) have been used as co-curing agents with BMIs to form a variety of polymer systems with useful properties. The properties of the resultant matrix depend on the chemical structure, relative ratio of the two reactants, and the cross-link density. Among the allyl group-incorporated phenolic derivatives, diallyl bisphenol A is the most popular reactive diluent. Researchers have also developed a variety of allyl phenolic derivatives to form Alder-ene polymers with better toughness and mechanical performance, all without sacrificing much of the thermal and thermomechanical performance of the resulting matrix. There are efforts to further modify the Alderene polymers with other modifiers such as highperformance thermosets (cyanate esters, benzoxazines, and epoxies) and engineering thermoplastics (polyether imide, polysulfones, etc.) to improve the prospects for applications in several engineering areas. There have been a few reports on the effect of nanomaterials on the properties of Alder-ene polymers recently. It was observed that thermal properties of the Alder-ene adducts were enhanced considerably by the introduction of nanomaterials into the matrix.

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There has been only a single report on shape memory Alder-ene polymers. The kinetics of Alder-ene polymerization was extensively investigated by researchers employing different spectroscopic and calorimetric methods. The proposed mechanisms divulged that along with the ene and Diels-Alder reactions, the Claisen rearrangement, Wagner-Jauregg reaction, and homopolymerization of maleimides (at higher temperature) were also possible. Tailor-ability of the structure and compositions (and hence properties) offers hope of deriving high-performing polymer thermosets for related engineering applications. This renders the Alder-ene polymers ideal candidates for high-performance composite matrices.

Acknowledgments The authors thank director, VSSC, for granting permission to publish this chapter. Special thanks to the editors of Handbook of Thermoset Plastics, Third Edition for inviting us to write this chapter.

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