Bismaleimide Alloys

Chapter 18

Polybenzoxazine/Bismaleimide Alloys Z. Wang*,† and Y. Gu*,1 *Sichuan University, Chengdu, China, † North University of China, Taiyuan, China 1

Corresponding author: e-mail: [email protected]

Chapter Outline 1 Introduction 2 Benzoxazine/Bismaleimide Binary Alloys 2.1 Curing Behavior and Mechanism 2.2 Properties 3 X-Functional Benzoxazine/Bismaleimide Binary Alloys 3.1 Allyl-Functional Benzoxazine/Bismaleimide Alloy

301 302 302 306 308 308

1 INTRODUCTION Benzoxazine is a new kind of phenolic resin synthesized by Cope and Holly [1] in the 1940s. Benzoxazine can be polymerized through ring-opening by thermal invitation without the need for harsh catalysts. Polybenzoxazines not only have excellent mechanical properties, good thermal stability, and outstanding dielectric properties as traditional thermoset resin but also have some special characteristics, including extensive sources of raw materials, molecular design flexibility, and near-zero shrinkage upon polymerization [2–5]. Based on these features, polybenzoxazines are ideal materials for replacing traditional phenolic resin and epoxy resin in high-performance fields. However, as a kind of thermoset resin, polybenzoxazines have their shortcomings when used in high-performance fields; for example, a relatively high curing temperature is needed without catalysts, brittleness of cured products, and lower thermal properties. The main reason for the low thermal properties is the low cross-link density of cured products [6]. Three methods are used to solve this problem: (1) Synthesizing new benzoxazine monomers using amines and phenols that contain special functional groups, for example, introducing cyano groups [7–11], allyl groups [12–16], and maleimide groups [17–19] into the benzoxazine monomer. Among others, Liu [17], Ishida [18], and Jin [19] synthesized benzoxazine containing the maleimide group, and also researched the synthesizing process, the curing reaction, and the properties of cured products. The related results are embodied in the handbook [20], which show that the Tg of poly(maleimido-functional benzoxazine) reached above

3.2 Furan-Functional Benzoxazine/Bismaleimide Alloy 3.3 Nitrile-Functional Benzoxazine/Bismaleimide Alloy 3.4 Phenolic-OH-Functional Benzoxazine/Bismaleimide Alloy 4 Benzoxazine/Bismaleimide/Other Resin Tertiary Alloys 5 Conclusions

311 314 315 315 316

250°C (dynamic mechanical analysis, DMA, results); (2) Synthesizing high-molecular-weight benzoxazines using amines and phenols [21]. For example, Takeichi [22] synthesized high-molecular-weight benzoxazine, and the Tg of cured products reached 238°C; and (3) Blending with another kind of resin that has the high thermal properties. This method can effectively improve the thermal properties of polybenzoxazine based on the copolymerization and complementary effect between two resins. Compared with the two previous methods, the third method is widely applied by many researchers because of simple processing, easy industrialization, and favorable performance-to-price ratio. Epoxy [23–29], polyimide [30,31], and bismaleimide [32,33] have been used frequently as second components to blend with benzoxazine. For example, blending benzoxazine (BA-a) with epoxy [29] can effectively improve the Tg of polymer from 141°C (polybenzoxazine) to 156°C (copolymer containing 30% epoxy in BA-a/epoxy blends). As a second component of polybenzoxazine alloy, polybismaleimide [34,35] has high cross-link density, seldom contains defects, and has a high rigid molecular chain. These characteristics mean that the polybismaleimide has high strength, high modulus, and excellent thermomechanical properties. In particular, its outstanding thermal properties can satisfy the requirements of the aviation industry. The Tg of polybismaleimide is higher than 250°C, and for long-term use, its temperature is between 177°C and 232°C. In this chapter, the research on the bismaleimidemodified benzoxazine is summarized based on new research developments. Also the curing behaviors, the catalytic effect between benzoxazine and bismaleimide, and the

Advanced and Emerging Polybenzoxazine Science and Technology. http://dx.doi.org/10.1016/B978-0-12-804170-3.00018-4 Copyright © 2017 Elsevier Inc. All rights reserved.

301

PART IV Polybenzoxazine Blends and Alloys

effect of functional groups grafting benzoxazine on curing behavior are discussed. The curing mechanism and structure of cured products are reviewed as proposed by researchers. We show corresponding mechanical and thermal properties with the hope of enhancing the relationship between structure and properties. Also, we hope this chapter effectively supports the further study of bismaleimide/benzoxazine alloys.

2 BENZOXAZINE/BISMALEIMIDE BINARY ALLOYS Polybismaleimide has excellent thermomechanical properties. However, its monomer hardly dissolves in a common polar solvent, while its melting temperature is high. These features make processing bismaleimide difficult. Therefore, for bismaleimide-modified benzoxazine, a typical manufacturing process involves mixing a certain ratio of benzoxazine and bismaleimide into chloroform or acetone and stirring until both dissolve in the solution. The catalyst is dissolved in the same solvent and mixed with the blended solution. Then the solvent is evaporated by rotary evaporation to obtain the benzoxazine/bismaleimide binary resin. Of course, blending bismaleimide with benzoxazine by melting is also a common practice. Even though the two methods have some differences, the research results are the same and some consensuses have been formed.

of cured products. The DSC results of the blends of 3-phenyl-3,4-dihydro-2H-1,3-benzoxazine (P-a) and BMI showed that the Ti and Tp of pristine P-a were 198°C and 222°C, respectively. The enthalpy of the polymerization of P-a was 57 cal/g. However, BMI’s Ti, Tp, and enthalpy of polymerization were 171°C, 207°C, and 37 cal/g, respectively. Interestingly, the exothermic heat of the blends was larger than those of P-a and BMI, which suggested that some kind of reaction may exist beyond polymerization of individual benzoxazine and BMI. The same result was observed in Gu’s work. Then Takeichi et al. investigated the cure reaction further using Fourier transform infrared spectroscopy (FTIR), as shown in Fig. 2. The characteristic absorption of benzoxazine was located at 1037 and 952 cm1 assigned to the C–O–C stretching mode of the benzoxazine ring and the out-of-plane mode of the benzene ring. The band at 0.2 n(BOZ): n(BMI) = 1:1 Heat flow (W g–1)

302

Exo

2.1

0.0

n(BOZ): n(BMI) = 1:2 n(BOZ): n(BMI) = 2:1

–0.2

–0.4

–0.6

Curing Behavior and Mechanism –0.8

The curing behavior of benzoxazine/bismaleimide blend is the key issue of all research work related to this system. Moreover, a suitable understanding of the curing behavior of alloy is also a precondition to modifying the cured structures and properties. Therefore researchers have used different methods to study the curing behavior of benzoxazines/ bismaleimides alloys. Guo et al. [36] prepared 4,40 -diamine diphenyl methane (DDM) based benzoxazine (P-DDM)/1,10 -(methylenedi-4,1phenylene)-bismaleimide (BMI) blend using the melt method. The curing behavior of P-DDM/BMI alloy was studied via differential scanning calorimetry (DSC). Fig. 1 shows the DSC curves of P-DDM/BMI alloys at different mole ratios. The initial curing temperature (Ti) of the alloy (1:1) was 172°C, and its exothermal peak temperature (Tp) was 191°C. Compared with the DSC curves of P-DDM and BMI, the Ti of the alloy was lower than that of P-DDM (224°C) and close to that of BMI (176°C), but the Tp was lower than that of P-DDM or BMI. These findings illustrated that the curing reaction of P-DDM/BMI alloy moved to a lower temperature and that some catalytic effects were implied. Takeichi et al. [33] further proved the copolymerization between benzoxazine and bismaleimide and the structures

50

150 250 Temperature (ºC) FIG. 1 DSC curves of P-DDM/BMI blends.

350

FIG. 2 IR spectra of P-a/BMI blend at 20 wt% of BMI content after each cure stage at 50°C (A), 160°C (B), 200°C (C), and 240°C (D).

Polybenzoxazine/Bismaleimide Alloys Chapter 18

3101 cm1 is the characteristic absorption of BMI. These absorptions decreased simultaneously as the curing temperature rose and disappeared after the cure at 240°C. This means that the two components reacted at the same time. Meanwhile, the absorption at 1187 cm1, which is ascribed to the C–O–C structure, appeared and increased with the progress of cure. This indicates that the copolymerization occurred between the hydroxyl group of polybenzoxazine and the double bond of BMI and that the ether linkage was formed. The entire reaction process is shown in Scheme 1. In addition, phenol and N-phenyl maleimide were used as model compounds to prove the existence of the ether linkage. The 1H NMR and 13 C NMR spectra of cured phenol and N-phenyl maleimide blend were obtained to confirm the existence of the ether linkage, as shown in Fig. 3. Liu et al. [37] reported the curing behavior between P-a and various N-phenyl maleimides by DSC and FTIR. The results showed that the tertiary N atom from benzoxazine can catalyze the reaction of the C]C bond in bismaleimide. They found that, when the maleimide components contained an acid group, for example, 4-maleimidobenzoic acid (MI-COOH), the acid can catalyze the ring opening of benzoxazine and form the iminium ion (Scheme 2), which has a catalytic effect on the polymerization of an unsaturated group such as the maleimide ring. So iminium ion can lower the curing temperature of bismaleimide. This reaction also built up chemical linkages between the cross-linked networks of benzoxazine and maleimide. Also, the copolymerization between P-a and N-phenyl maleimide during the curing process was confirmed by FTIR. Wang et al. [38] discussed the curing mechanism of dimaleimide BA-a/N,N0 -(2,2,4-trimethylhexane-1,6-diyl) (TBMI) alloys in detail. 2,4-xylenol-based benzoxazine (DM-tma) (Scheme 3), triethylamine, N,N-dimethyl aniline,

FIG. 3 1H (A) and 13C (B) NMR spectra of I (DMSO-d6, 25°C). The asterisks denote the resonances from the solvent.

O

SCHEME 1 Possible thermal reaction between polybenzoxazine and bismaleimide (a), and the model reaction of phenol and N-phenylmaleimide (b).

OH

O +

N O

N

O

O

(A)

O OH

(B)

N

O +

Phenol

303

N O N-phenylmaleimide

Reflux, 8 h

O

O Ι

304

PART IV Polybenzoxazine Blends and Alloys

and bisphenol A were used as model compounds. The curing behavior was studied first by DSC. Fig. 4 shows the DSC curves of different alloys. The Tp of triethylamine/TBMI (1:1 in mole ratio) is 252°C (Fig. 4a) and a small exothermic peak of N,N-dimethylaniline/TBMI (1:1 in mole ratio) is

N O

N CH2

H R

R

Iminiumion SCHEME 2 Structure of iminium ion.

O O N

N O O N O

DM-tma TBMI SCHEME 3 Structures of DM-tma and TBMI.

around 150–240°C (Fig. 4c), which are both lower than that of TBMI (267°C). However, the curing reaction of bisphenol A/TBMI (1:1 in mole ratio) occurred at a higher temperature than that of TBMI, as shown in Fig. 4d. Therefore it can be inferred that the N atom from tertiary amines had an obvious catalytic effect on the curing reaction of TBMI; however, phenolic hydroxyl had no such effect. In order to illustrate the catalytic effect further, a DM-tma/TBMI blend with mole ratio of 1:1 was prepared. In Fig. 4B, the alloy showed that the Ti is 101°C and two exothermic peaks are 150°C and 267°C. Compared to Ti and Tp of TBMI, they clearly moved to lower temperatures. This meant that DM-tma clearly had a catalytic effect on the TBMI curing reaction. The negative oxygen ions formed during the ring-opening of DM-tma were thought to be the main reason for the catalysis; the detailed catalyzing process is shown in Scheme 4. This reaction also formed chemical linkages between the cross-linked networks of benzoxazine and maleimide. Gu and Liu’s conclusions were similar [37], but the types of negative ions were different. Negative oxygen ions were one type, and negative carbon ions were another type. Theoretically, both types of negative ions can effectively catalyze the BMI reaction because of unsaturation of C]C in BMI. Based on the preceding study of the catalyzing effect, the BA-a/TBMI blend with a mole ratio of 1:1 (BT11) was cured by stepwise temperature.

FIG. 4 DSC curves of (A): TBMI (a), TBMI/triethylamine (b), TBMI/N,N-dimethylaniline (c), and TBMI/bisphenol A (d); (B): TBMI (a) and DM-tma/ TBMI (b).

SCHEME 4 The proposed catalyzing mechanism of benzoxazine and maleimide.

Polybenzoxazine/Bismaleimide Alloys Chapter 18

The samples at different curing stages were characterized by FTIR, and the curing mechanism was proposed as shown in Scheme 5. BA-a and TBMI were self-polymerized first by thermal invitation, and then the negative oxygen ions from partly opened oxazine rings catalyzed the anionic polymerization of maleimide. Meanwhile, the phenolic hydroxyl

from the polybenzoxazine reacted with carbon the double bond of bismaleimide to form an ether linkage network. Furthermore, using DSC curves, Wang et al. [39] investigated the curing behaviors of BA-a/TBMI blends using imidazole as a catalyst. Fig. 5A shows the DSC curves of BA-a/TBMI alloy at 1:1 mole ratio with 3 wt% imidazole

O

O

O

O

160°C

N

N

N

N

O

O

O

(A)

O

O

−

O

N

+

160°C

N

−

O

O

N

+

N

(B) OH

OH

O

°C

160

N

O

n

160°C

N

N

O O O O

OH

n

OH

N

N

−

O

(C) O

+

N

N

O

−

O 160°C

O

O

N

+

N

O O

(F)

O O H

O O

+

160°C

O

HO

OH

O

N

N

−

N

N

O O

N

n

−

O

+

(D)

O H H

N

HO

OH N

OH

N

N O n

OH

n

(E) SCHEME 5 The proposed curing mechanism for BT11.

O O

O

N

N

O

O

O

O

O

n

OH

(G)

+

N

N

OH

305

−

O

O N O

306

PART IV Polybenzoxazine Blends and Alloys

FIG. 5 (A) DSC curves of BTI113 samples cured at 120°C for different time: (a) 0 min; (b) 60 min; (c) 80 min; (d) 100 min; (e) 120 min. (B) Conversiontime curves at 120°C: (a) BA-a with 3 wt% imidazole; (b) TBMI with 3 wt% imidazole.

as a catalyst (BTI113) after curing at 120°C for different time. Fig. 5B shows the conversion versus time curves of BA-a/3 wt% imidazole and TBMI/3 wt% imidazole at 120°C. Two exothermal peaks can be observed in Fig. 5A. The first peak became smaller and smaller as the curing continued at 120°C; however, the second peak had no visible change. This phenomenon illustrated that only one component in the blend cured at 120°C. On the other hand, in Fig. 5B, it can be seen that the conversion of BA-a/3 wt% imidazole had no visible change; however, that of TBMI/3 wt% imidazole increased dramatically. This proved that imidazole can obviously catalyze the selfpolymerization of TBMI; however, the reaction of BA-a hardly took place at 120°C. So, in BA-a/TBMI/imidazole alloy, TBMI reacted first followed by the reaction of BA-a. The copolymerization between the two components was rare, and the chain movement was slightly limited. It is noted that this system can form special phase morphology in the cured products and have excellent mechanical properties (see Chapter 35 for more on this topic).

2.2

Properties

The chemical structure and phase morphology structure of an alloy determine the properties of cured products. The properties of a cured benzoxazine/bismaleimide alloy integrates the advantageous properties of an individual resin. From a chemical structure perspective, the preceding discussion shows that self-polymerization and copolymerization can coexist and ether linkage can form in polybenzoxazine/ bismaleimide alloys. From the perspective of a phase

morphology structure, Takeichi et al. [33] thought that a homogeneous morphology formed in the cured benzoxazine/ bismaleimide alloy; however, Kumar et al. [32] thought an interpenetrating polymer network formed in this system. Recently, Gu et al. [39–41] introduced a reaction-induced phase separation into a benzoxazine/bismaleimide alloy and a prepared blending with special morphologies, such as seaisland structures, bicontinuous structures, and phase inversions in cured products, which will be discussed in the phase separation chapter. Guo et al. [36] studied the properties of cured P-DDM/ BMI alloys and corresponding glass fiber-reinforced alloy composites based on the curing behavior between P-DDM and BMI. The results showed that the Tg of cured products shifted to a higher temperature as the BMI content increased in alloys. For P-DDM/BMI at a mole ratio of 1:2, the Tg of cured alloy reached a maximum of 257°C, which was 50°C higher than that of polybenzoxazine. Then the P-DDM/BMI/DDM blends were used as a matrix to prepare the glass fiber-reinforced composites. The flexural properties of composites were improved with increased P-DDM content in the matrix. When BMI, P-DDM, and DDM were at a mole ratio of 8:16:1, the flexural strength of the composite was at a maximum of 648 MPa. Liu et al. [37] studied the thermal property of cured P-a/ N-phenylmaleimide alloys and found that the Tg of cured MI-COOH/P-a alloy was 223°C and that the char yield of the blend at 800°C was 52%, which is higher than the char yield of cured 4-hydroxyphenylmaleimide (MI-OH)/P-a alloy (char yield at 800°C is 50%). On the other hand, Takeichi et al. [33] found that, with the increase of BMI

Polybenzoxazine/Bismaleimide Alloys Chapter 18

content, the Tg of a polymer alloy shifted to a higher temperature, suggesting that adding BMI to benzoxazine is effective for improving the performance of polybenzoxazine, as shown in Table 1. When the BMI mass fraction reached 63%, the Tgs of BA-a/BMI and P-a/BMI reached the maximum values of 275°C and 244°C, respectively. Moreover, the Tgs of the polymer alloys with higher BMI contents were even higher than those of BA-a and BMI homopolymer. This phenomenon was caused by the copolymerization between benzoxazine and BMI. In addition, the 5% weight loss temperature (Td5), the 10% weight loss temperature (Td10), and the char yield at 800°C of the alloy were all higher than those of polybenzoxazine, as shown in Table 1. Takeichi et al. [42] synthesized a benzoxazine from bisphenol A and (poly(BA-hda)main) hexamethylenediamine as shown in Scheme 6 and blended it with BMI in NMP at various weight ratios. The cured

307

blends showed improved toughness and thermal properties. The detailed data of poly(BA-hda)main/BMI films are summarized in Table 2. Among all films, when the weight ratio of poly(BA-hda)main to BMI was 50:50, the tensile modulus, tensile strength, and elongation at break reached the maximum values, which were 4.7 GPa, 54 MPa, and 5.8%, respectively. In addition to the DMA results, the thermal stability of cured alloys was clearly improved compared to cured poly(BA-hda)main. The tendency of the CH3 O

O CH3

N

H2 N C

6

n

Poly (BA-hda)main SCHEME 6 Structure of poly(BA-hda)main.

TABLE 1 Thermal Properties of Benzoxazine/BMI Polymer Alloy DMA Benzoxazine P-a

BA-a

–

Softening Temperature (°C)

Tg from E (°C)

0

166

16

BMI Content (wt%)

TGA 00

T5 (°C)

T10 (°C)

Char Yield at 800°C (%)

146

344

370

46

179

167

352

381

49

30

220

198

353

386

51

42

253

211

363

396

52

53

256

237

379

409

53

63

–

244

386

414

54

72

–

243

395

424

55

80

–

240

421

431

55

87

–

–

425

434

53

0

172

154

310

338

45

8

213

214

315

342

47

16

231

241

321

351

50

25

248

250

327

360

51

34

262

268

332

366

52

44

254

272

333

367

53

54

–

275

338

366

57

64

–

275

351

379

57

76

–

268

380

399

55

87

–

–

428

435

53

222

222

477

482

51

100

308

PART IV Polybenzoxazine Blends and Alloys

TABLE 2 Tensile Properties and DMA Results of the Poly(BA-hda)main/BMI Films DMA Tg (°C) Poly(BA-hda)main/BMI (wt/wt)

E (GPa)

sb (MP)

Eb (%)

E0 (GPa)

E00

tan d

100/0

2.6

41

2.5

2.8

244

259

75/25

3.0

20

3.0

4.7

277

292

50/50

4.7

54

5.8

5.4

289

310

25/75

3.5

24

4.3

4.4

–

349

0/100

–

–

–

–

–

–

storage modulus was in agreement with the results of the tensile modulus. When the alloy weight ratio was 50:50, the storage modulus reached the maximum, 5.4 GPa, and further increasing the BMI content made the storage modulus decrease to 4.4 GPa. However, the Tg, Td5, and Td10 of the alloy increased with the increase of the BMI content in alloys. The detail value was located between that of cured poly(BA-hda)main and BMI.

3 X-FUNCTIONAL BENZOXAZINE/ BISMALEIMIDE BINARY ALLOYS Increased cross-link density of this system is necessary in order to obtain better thermal stability and mechanical properties, as compared to polybenzoxazine/bismaleimide alloys. More efforts have focused on the introduction of other functional groups (functional group X) into benzoxazine chemical structures. In particular, X groups are designed to react with the C]C of bismaleimide to increase cross-link density. The X-functional benzoxazines were synthesized first and then blended with bismaleimide. The curing behavior, chemical structure, and properties of the blends and their cured products are investigated here.

3.1 Allyl-Functional Benzoxazine/ Bismaleimide Alloy Many researchers [12–16] have reported on allyl-functional benzoxazine, mainly about including allyl phenol-based

benzoxazine (ALBA-a) and allyl amine-based benzoxazine (BA-ala); the corresponding structures are shown in Scheme 7. In these studies, the cured products showed improved thermal stability, and the curing behaviors were reported in a handbook [20]. However, rarely has a BA-ala blend with BMI been reported on. Therefore this section focuses only on the introduction of ALBA-a/ bismaleimide alloys. One reason that researchers may be interested in allylfunctional benzoxazine is the successful modification of an allyl group to a BMI system. Based on this, allyl-functional benzoxazine was blended with BMI in order to obtain resins with excellent properties. Kumar et al. [32] blended 2,20 -bis(8-allyl-3-phenyl-3,4-dihydro-2H-1,3-benzoxazinyl) propane (ALBA-a) with 2,20 -bis[4(4-maleimidophenoxy) phenyl] propane (BMI-1) at different mole ratios (structure is shown in Scheme 8). DSC was used to study the curing behavior of the alloys; the DSC curves are shown in Fig. 6. For the 1/0.5 (ALBA-a/BMI-1) blend, two exotherms are observed, at 228°C and 269°C, respectively. They assigned the first exotherm to the co-curing of allylbismaleimide and the second exotherm to the ring-opening polymerization of benzoxazine. The ALBA-a/BMI-1 alloy with an equal mole ratio has only one exothermic peak on the DSC curve, and corresponding enthalpy of polymerization is 190 J/g, which is larger than the sum of the enthalpy of polymerization of individual components (the curing enthalpy of ALBA-a is 40 J/g, and BMI-1’s is 130 J/g). This phenomenon illustrates that the curing mechanism was changed in ALBA-a/BMI-1 alloy. Combined with the results SCHEME 7 Structures of ALBA-a and BA-ala.

CH3 O N

O CH3

ALBA-a

N

CH3 O N

O CH3

BA-ala

N

Polybenzoxazine/Bismaleimide Alloys Chapter 18

N

N

O

O

O

O

309

N

N

BA-a

ALBA-a

O

O N

O

O

O

N O

BMI-1 SCHEME 8 Structures of ALBA-a, BA-a, and BMI-1.

FIG. 7 DSC thermograms of ALBA-a (A), TBMI (B), and BzT11 (C). FIG. 6 DSC thermograms of various formulations of ALBA-a/BMI-1 blends.

of the FTIR spectrum, the curing behavior of the blend was proposed. The allyl group reacted with BMI through the Alder-ene reaction involving an ene, Wagner-Jauregg, and Diels-Alder reactions at different temperature regimes. Benzoxazine polymerization was nearly complete after curing at 210°C/3 h, whereas allyl groups persisted even after that stage and completely vanished after curing at 240°C/3 h. The cured ALBA-a/BMI-1alloy had only one Tg, 274°C. This result indicates that the two components are miscible, which may be caused by copolymerizations between ALBA-a and BMI-1 limiting the movement of molecular chains. Conversely, the cured BA-a/BMI-1alloy had two Tgs, 145°C and 267°C. This phenomenon implied that a phase separated structure may be formed in cured BA-a/BMI-1 alloys. On the other hand, the Tg of cured ALBA-a/BMI-1 alloy was higher than that of cured BA-a/BMI-1 alloy. This may be due to more copolymerization of the ALBA-a/BMI-1

alloy leading to a higher cross-link density, compared to that of BA-a/BMI-1 alloy, which had only a few ether linkages. Wang et al. [43] reported the curing behavior of ALBAa/TBMI alloys. The curing behavior was examined by DSC. A peak centered at 276°C is observed for the reaction of ALBA-a, as shown in Fig. 7A. For TBMI, the exothermal peak centered at 268°C. The DSC curve of the ALBA-a/ TBMI alloy in a mole ratio of 1:1 (BzT11, where Bz represents the ALBA-a; T represents the TBMI; 11 represents the mole ratio of ALBA-a to TBMI, 1:1; BzT21 and BzT12 represent the mole ratio of ALBA-a to TBMI, which are 2:1 and 1:2, respectively) exhibits three exothermic peaks, and the peak temperatures are 226°C, 266°C, and 350°C, respectively, as shown in Fig. 7C. The exothermic peak at the highest temperature may represent the self-polymerization of residual allyl groups. Combined with the enthalpy of polymerization and FTIR analyses, the curing behavior was proposed. Authors proposed that ALBA-a reacted first;

310

PART IV Polybenzoxazine Blends and Alloys

then the Alder-ene reaction between the allyl group and BMI occurred. The self-polymerization of TBMI and the catalysis reaction of opened benzoxazine to TBMI proceeded at the same time during the cure of ALBA-a/TBMI alloys. After all the preceding mechanisms reacted, the selfpolymerization of residual benzoxazines occurred, as shown

O

HO

+

O

N

in Scheme 9. After studying the curing behavior, the Tgs of cured alloys with different ratios were evaluated and are shown in Fig. 8. With the decrease of TBMI content, the Tgs of cured alloys shift to a lower temperature. When the mole ratio reached 1:2, the Tg of cured alloy raised to 220°C.

OH

H

N

160°C

N

N

O

(A)

N O O N

O 160°C

160°C

Ene reaction

160°C O

Thermal cure −

O

N

Catalyzed by benzoxazine

+

N

−

O

O

N

+

N

O O N

N

−

O

N

O

O

− +

O

O O N

N n

O O

N

(B)

N

(C)

O N

O

(D)

HO

+

O

OH

H N

180°C/2 h 220°C/2 h

(E) SCHEME 9 The proposed curing mechanism for ALBA-a/TBMI blend.

N

N

Polybenzoxazine/Bismaleimide Alloys Chapter 18

Wang et al. [44] systematically reported the mechanical and electrical properties of ALBA-a/BMI alloys. The impact toughness and flexural strength are shown in Fig. 9. The impact strength improved largely when the ALBA-a content increased. When the weight ratio was 30%, the impact strength was 11.8 KJ/m2, which is improved by 71% compared to that of cured BMI. Meanwhile, the flexural strength was 124.1 MPa, which is improved by 47% compared to that of cured ALBA-a. Furthermore, the electrical property was also studied and is shown in Fig. 10. The dielectric constant of cured BMI and ALBA-a were 3.17 and 2.87, respectively. The dielectric constant of cured alloys decreased with increased ALBA-a content from 3.11 to 3.02 for the alloys with 10% and 40% ALBA-a content, respectively, suggesting the high cross-link density of cured ALBA-a/BMI alloys with increased ALBA-a content. Moreover, the dielectric constant deceased with increased frequency, which indicates that the ALBA-a/BMI copolymers had sound dielectric stability.

311

3.2 Furan-Functional Benzoxazine/ Bismaleimide Alloy Liu et al. [45] introduced furan ring into the benzoxazine structure and synthesized the bis(3-furfuryl-3,4-dihydro2H-1,3-benzocazinyl)isopropane (BA-fa), as shown in Scheme 10. Its curing behavior, chemical structure, and properties were characterized. The Tp of BA-fa was 247°C. The Tg of poly(BA-fa) was higher than 300°C. The char yield of poly(BA-fa) also was high at 800°C. Next they [46] mixed BA-fa with BMI at 80°C. A highmolecular-weight polymer (poly{(BA-fa)-co-(BMI)}) containing benzoxazine rings in the main chain was formed by a Diels-Alder reaction between the furan ring and BMI. The process for preparing the alloy is shown in Scheme 10. The curing behavior of poly{(BA-fa)-co-(BMI)} was studied by DSC. As shown in Fig. 11, poly{(BA-fa)-co-(BMI)} has an obvious endothermic peak and an exothermic peak, at 125°C and 200°C, respectively. The endothermic peak at 125°C was attributed to a retro Diels-Alder reaction, which has a clipping effect on polymer chains. The exothermic peak at about 200°C could be due to the ring-opening polymerization of benzoxazine groups and the polymerization of

0.0120 BMI BB19 BB28 BB37 BB46

Dielectric loss factor

0.0115 0.0110 0.0105 0.0100 0.0095 0.0090

30 40 50 60 Frequency (MHz) FIG. 10 The dielectric constant of ALBA-a/BMI alloys with different BMI contents. 10

FIG. 8 Tan Delta versus temperature of ALBA-a/TBMI alloys: BzT12 (A), BzT11 (B), BzT21 (C), and cured ALBA-a (D).

20

14 Flexural strength (MPa)

Impact strength (kJ m–2)

130 12 10 8 6

120 110 100 90 80

70 20 40 0 20 (A) (B) Mass fraction of ALBA-a (%) Mass fraction of ALBA-a (%) FIG. 9 The impact strength (A) and the flexural strength (B) of ALBA-a/BMI alloys with different ALBA-a contents. 0

40

312

PART IV Polybenzoxazine Blends and Alloys

O

N

O

O

CH3 C

O

O

+

N

N O

CH3

BMI 80ºC 120 h

THF

O O

CH3 C

N O

BA-fa

N

SCHEME 10 Preparation of poly{(BA-fa)-co-(BMI)} from Diels-Alder reaction.

O

H C H

O

N

O

O

H C H

N

N

O

O

O

n

CH3

Poly{(BA-fa)-co-(BMI)}

200ºC 0.4

Cross-linking reaction

(Heat flow (W J-1)

0.2

0.0

–0.2

–0.4 Retro Diels-Alder reaction Exo

125ºC –0.6 0

50

100

150 200 Temperature (ºC)

250

300

FIG. 11 DSC curve of poly{(BA-fa)-co-(BMI)}.

BMI as previously described. The molecular weight decreased from 19,000 to 7400 g/mol, and the polydispersity index changed from 1.81 to 3.52 when poly{(BA-fa)-co(BMI)} cured at 120°C for 3 h. Fortunately, Chou et al. [46] found the rate of the retro-DA reaction of poly{(BA-fa)co-(BMI)} was not high at 120°C in solid form. At 160°C the retro Diels-Alder reaction and cross-linking reactions performed simultaneously. Therefore, although the polymer chains of poly{(BA-fa)-co-(BMI)} might break down under heating, poly{(BA-fa)-co-(BMI)} still existed in “polymer” form, and corresponding cross-linked alloy (poly{(BA-fa)co-(BMI)}-R) was formed. To illustrate the effect of low temperature (80°C) on copolymerization between furan and BMI, the BA-fa/BMI mixture was directly cured at a high temperature, and corresponding

cross-linked alloy (poly{(BA-fa)-co-(BMI)}-BR) was formed. The properties of poly{(BA-fa)-co-(BMI)}-BR were inferior to those of poly{(BA-fa)-co-(BMI)}-R. The comparing data are summarized in Table 3. The Td5 and char yield at 800°C are 390°C and 52% for poly{(BA-fa)-co-(BMI)}-R, respectively. However, for poly{(BA-fa)-co-(BMI)}-BR, the Td5 and char yield at 800°C are 286°C and 48%, respectively. It was explained that some unreacted dangling ends may be formed during curing in a poly{(BA-fa)-co-(BMI)}-BR system. On the other hand, the Tg, storage modulus and Young’s modulus of both alloys showed the same tendency. Recently, Liu et al. [47] blended BA-fa with sulfonated 4,40 -bis(4-maleimidophenoxy) biphenyl and prepared a proton exchange membrane that could be used continuously in direct methanol fuel cells. This membrane had a high

Polybenzoxazine/Bismaleimide Alloys Chapter 18

313

TABLE 3 Thermal and Mechanical Properties of the Alloys Poly{(BA-fa)-co-(BMI)}-R and Poly{(BA-fa)-co-(BMI)}-BR TGA Analysis

Thermal and Mechanical Properties

Td5 (°C)

Char Yield at 800°C (wt%)

Tg Measured With DSC (°C)

Storage Modulus (GPa)

Young’s Modulus (MPa)

Tensile Strength (MPa)

Strain (%)

LOI (%)

Poly{(BA-fa)-co(BMI)}-R

390

52

242

5.2

907

31

3.7

29

Poly{(BA-fa)-co(BMI)}-BR

286

48

235

4.4

801

34

5.0

29

Polymer

initial modulus, 3000 MPa. Moreover, the modulus increased with a prolonged curing time and finally reached 4250 MPa. Compared to commercially available Nafion 117, BA-fa/ sulfonated 4,40 -bis(4-maleimidophenoxy) biphenyl membrane’s properties had outstanding advantages in direct methanol fuel cells. Its dimensional stability, proton conductivity at 60°C, and methanol permeability values were 1.6%, 48 mS cm2, and 2.52  107 cm1 s1, respectively. Meanwhile, its proton selectivity was 3.4 times the value measured with a Nafion 117 membrane. Conversely, Gaina et al. [48] synthesized the 4-allyl2-methoxyphenol and furfuryl amine-based benzoxazine (pALM-fa) and 2,20 -bis(8-allyl-3-furfuryl-3,3-dihydro-2H1,3-benzoxazine)isopropane (ALBA-fa), which contains a

O

H2 C

H2 C N

O C H

furan ring, an allyl group, and an oxazine ring at the same time, as shown in Scheme 11. Then BMI was used to blend with these new benzoxazines at 60°C at different ratios. The competing reactions among different functional groups and the influence of compositional changes on properties of cured products were investigated. The authors proposed the curing mechanism of this system. Retro Diels-Alder reaction came first when the curing temperature rose, followed by the copolymerization between the allyl group and BMI. Then the oxazine ring-opening reaction proceeded to connect to the furan ring. The final cross-linked network is shown in Scheme 12. The cured products had excellent thermal stabilities and high char yield, but the Tg was lower than 300°C. H3C

H2 C N

C H3

H2 N C

O

C H2

O

H2C CH H2C CH C H2 CH

O

2

OCH3

ALBA-fa

pALM-fa SCHEME 11 Structures of pALM-fa and ALBA-fa. SCHEME 12 The cross-linked structure of pALM-fa/BMI alloy in 1:1 mole ratio.

O H H2 C C

N

l

CH2

N

O O

OCH3 OH

O

O H H2 C C

N

O

m

n

CH2

N

H3CO OH

O

O

314

PART IV Polybenzoxazine Blends and Alloys

3.3 Nitrile-Functional Benzoxazine/ Bismaleimide Alloy

examined by DSC, FTIR, DMA, TGA, and a universal testing machine. From DSC results, the Tps of pCP-DDM and BMI were at 220°C and 247°C, respectively; meanwhile, the pCPDDM/BMI alloys showed two exothermal peaks, which were 200°C and 230°C when BMI content was lower than 50%. Combining the results of the DSC and FTIR studies, the authors proposed a curing mechanism. The exothermal peak at 200°C represented the self-polymerization and copolymerization of benzoxazine and bismaleimide. Another exothermal peak around 230°C came from the reaction of the nitrile groups. The detailed curing process is shown in Scheme 14. Thermal and mechanical properties were discussed further. It was found that the properties increased at first

Liu et al. [7–11] synthesized nitrile-functional benzoxazine as shown in Scheme 13. The curing behavior was studied. The results showed that the benzoxazine ring and nitrile group can respectively react at different temperatures, that the cured product has excellent thermal stability, and that the corresponding Tg is more than 300°C. To improve the thermal property of nitrile-functional benzoxazine, Lu et al. [49] blended nitrile-functional benzoxazine (pCP-DDM) with BMI via a solvent method. The curing behavior, thermal properties, and mechanical properties were

N C

O

C H2

N

N O

SCHEME 13 Structures of nitrile-functional benzoxazines.

O

N N

C

C N

H2 C

OH N

N

OH

CN

(a)

CN O

O N O

N

H2 C

N

O

pCP-ddm CN

CN

+ O

O

O O

N

N

O

H2 C

O

N

N

O O

O

N

O

(b)

O

O

N

H2 C

N

O

BMI

O

O

N

O

O

O

H2 C

N

N

O

O N

CN

(c) CN

OH

N

N (d)

N HO SCHEME 14 Possible reactions occurring in the pCP-DDM/BMI blends.

OH

Polybenzoxazine/Bismaleimide Alloys Chapter 18

and then decreased with the increase of BMI content in the alloys. When the mole ratio of BMI reached 80%, the Tg was at maximum, 334°C. When the mole ratio of BMI reached 40%, the tensile strength, flexural strength, and shearing strength of cured alloys reached their maximums of 69, 235, and 12.9 MPa, respectively, as shown in Fig. 12.

315

and BA-a was 2:1. BADCy was added to BA-a/BMI alloys by stirring to form a homogeneous liquid. The influence of BADCy on the thermal property and the mechanical property of cured alloys was discussed. The results showed that the impact strength and flexural strength increased at first and then decreased with increasing BADCy content. When the weight ratio of BADCy reached 30%, the impact strength and flexural strength of cured alloys reached their maximum value of 13.8 KJ/m2 and 142 MPa, respectively. Compared to cured BA-a/BMI alloys, the initial degradation temperature and maximum degradation temperature of cured tertiary alloys increased with the increasing of the BADCy content, and the moisture absorption of the alloy was found to be lower. These changes may be caused by different chemical cross-linked networks when introducing BADCy into BA-a/BMI alloys. Krishnadevi [52] reported acyclophosphazenereinforced BA-a/BMI alloys. The composites can be obtained by adding various weight percentages of acyclophosphazene (Cp) (5%, 10%, and 15%) to the BA-a/BMI blend (1:1 ratio) in tetrahydrofuran. It was found that Cp/ BA-a/BMI alloys went through a ring-opening reaction of

3.4 Phenolic-OH-Functional Benzoxazine/ Bismaleimide Alloy Lin et al. [50] first synthesized phenolic-OH-functional benzoxazine (BF-ap), as shown in Scheme 15. Then it was blended with BMI to prepare high-performance alloys. The curing behavior of alloys was examined by DSC and FTIR. The exothermic peak temperature of alloys decreased from 235°C to 185°C as the ratio of BF-ap increased from 0% to 75%. The measured enthalpy of alloys was higher than that calculated by a linear combination. For example, for a 25/75 blend, the enthalpy of polymerization measured was 39 KJ/mol, but the value calculated from BF-ap (59 KJ/mol) and BMI (17.3 KJ/mol) was only 27 KJ/mol. The larger difference between two values illustrated that the copolymerization existed in the alloy. Combining the FTIR results, the authors reached the same conclusion that the copolymerization existed in the BF-ap and BMI alloy. In this system, the additional phenolic-OH group can form more cross-links with BMI, which caused the thermal properties of cured BF-ap/BMI alloy to increase. For a 50/50 blend, the Tg and Td5 of cured BF-ap/ BMI alloy were 294°C and 410°C, which is higher than that of the cured bis(3,4-dihydro-2H-3-phenyl-1,3-benzoxazinyl) methane(BF-a)/BMI alloys (204°C and 375°C).

OH

N O

H2 C

O

N

4 BENZOXAZINE/BISMALEIMIDE/OTHER RESIN TERTIARY ALLOYS Wu et al. [51] introduced bisphenol A dicyanate ester (BADCy) into BA-a/BMI alloys. The weight ratio of BMI FIG. 12 Influence of BMI content on tensile strength, flexural strength, and shearing strength of cured pCPDDM/BMI alloys.

HO

BF-ap

SCHEME 15 Structure of BF-ap.

150 Flexural strength Tensile strength Impact strength 140 500 130 400 120

300

110

20

40 60 80 Molar content of BMI (%)

100

Impact strength (KJ m–2)

Mechanical strength (MPa)

600

316

PART IV Polybenzoxazine Blends and Alloys

NH2

H2N

N

O

+

N

O

O O P N N O OP P N O O

O

NH2

N

O +

O

N C H2

O NH2

H2N H2N

BA-a

Cp

BMI THF,

curing H2 C

O

O

N

OH N

H OH N H N

O P N N O P O P O NO

N

O

O

N H

O

O O

O

O N

N

N

C H2

O

O O

C H2

O

SCHEME 16 The proposed schematic representation of the Cp/BA-a/BMI alloy.

BA-a, that Cp copolymerized with BA-a, and that BMI polymerized with Cp by Michael addition under thermal curing. The chemical structures are shown in Scheme 16. Scanning electron microscopy and X-ray diffraction results implied that the Cp is homogeneously dispersed in the BA-a/BMI networks and that the tertiary alloys formed a homogeneous phase morphology. Furthermore, the thermal properties, mechanical properties, dielectric properties, and antibacterial properties were all improved with an increase in the Cp content in the alloys. When the weight ratio of Cp reached 15%, the Tg, tensile strength, tensile modulus, non-notched impact strength, hardness, dielectric constant, and dielectric loss at 30°C were 231°C, 87.5 MPa, 3543 MPa, 75.22 J/m, 93 HV, 2.7 F, and 0.1, respectively. Meanwhile, the tertiary alloys had a great effect against bacteria, and the antibacterial activity increased with increasing Cp content in alloys, which may be caused by the van der Waals force of interaction between the alloys and the bacterial surface.

5 CONCLUSIONS In this chapter, the curing behavior, structures, and properties of polybenzoxazine/bismaleimide alloys were introduced. The influence of additional functional groups attached

in benzoxazine monomers, such as allyl and the furan group, on curing behavior and properties of polybenzoxazine/ bismaleimide alloys were also discussed. The catalysis and copolymerization between various components were discussed first and then main curing processes were proposed. Some interesting results have been obtained. The copolymerization between benzoxazine and bismaleimide formed ether linkage in addition to the self-polymerization of each individual component. Furthermore, additional functional groups that grafted at benzoxazine can react with bismaleimide and form more connections between benzoxazine and bismaleimide. These reactions changed the cross-linked structure and increased the cross-link density. On the other hand, polybismaleimide has high cross-link density and excellent thermal properties, which have a positive effect on the properties of cured alloys. As a result, the cured alloys revealed higher tensile strength and elongation at break than that of pristine polybenzoxazine. The alloys also improved the Tg because of increased cross-link density. Moreover, the cured alloys showed a higher degradation temperature and char yield with increasing bismaleimide content. Based on their excellent properties, benzoxazine/bismaleimide alloys have strong potential for use in aeronautics, astronautics, and fuel cell industries.

Polybenzoxazine/Bismaleimide Alloys Chapter 18

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