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Thermal properties of an epoxy cresol-formaldehyde novolac/diaminodiphenyl sulfone system modified by bismaleimide containing tetramethylbiphenyl and aromatic ether structures.

INTRODUCTION

Epoxy resins possess good adhesive, physical, dielectric, and mechanical properties and excellent processability, which are required for an ideal matrix resin in structural composites for aerospace applications. However, their poor high-temperature properties and brittle nature restrict their use in advanced composites. Bismaleimides (BMIs) can provide a higher service temperature than epoxy resins because of their excellent thermal properties over a wide range of temperatures (1), and have a high performance-to-cost ratio. Generally, BMI-modified epoxy resins are thermoset/thermoset blends with a class of intercross-linked or interpenetrated polymer networks (IPNs) (2-10). In particular, for the BMI-modified epoxyamine system, a Michael addition reaction of the diamine to the double bond in the maleimide groups occurs during the curing process (6). This leads to improved thermal and mechanical properties of the resultant composite (9).

Various BMIs have been synthesized and used to prepare advanced composites which improved the mechanical and thermal properties (11-15). A BMI monomer (11) and epoxy monomer (12), (13) with a tetramethylbiphenyl structure showed higher thermal resistance and lower moisture absorption than a common 4,4'-bismaleimidodiphenylmethane (BDM) and bisphenol A epoxy resin, respectively, because of their biphenyl structure and tetramethyl groups. Two novel BMIs containing an epoxy backbone, a diglycidyl ether of bisphenol A, and TDE-85 epoxy, showed improved toughness and good thermal properties (14). Numerous attempts have been made to increase the toughness of epoxy resins with multiple functional groups, e.g., tetraglycidyl diamino-diphenyl methane (TGDDM) cured with diaminodiphenyl sulfone (DDS), but these led to a loss of heat resistance and processability. Introducing a BMI monomer into the epoxy-diamine system (15) could lead to a thermoset/thermoset blend and avoid the loss of heat resistance and processability.

The aim of the present study was to improve the thermal properties of an epoxy cresol-formaldehyde novolac resin (ECN)/DDS system. We report on a BMI monomer containing tetramethylbiphenyl and aromatic ether structures. 4,4'-bis(4-maleimidophenoxy)-3,3',5,5'-tetramethyl biphenyl (BMITB) (16), which was synthesized from 2,2',6,6'-tetramethyl-4,4'-biphenol by a facile method, and describe our attempts to improve the thermal properties of this BMITB-modified ECN/DDS system.

EXPERIMENTAL

Materials

2,2',6,6'-Tetramethyl-4,4'-biphenol, p-chloronitrobenzene, maleic anhydride, acetic anhydride, triethylamine, DDS, hydrazine hydrate (85 wt% aqueous solution), potassium carbonate and iron(III) chloride were purchased from Sinopharm Chemical Reagent Co. (Shanghai, China) and used without purification. Dimethylformamide (DMF), tetrahydrofuran (THF), and ethanol were purchased from Hangzhou Shuanglin Chemical Reagent Co. (Hangzhou, China). Ethanol and THF were used as received, whereas DMF was purified by distillation under reduced pressure over calcium hydride before use. ECN (EEW 200 g/eq, softening point (85[degrees]C)) was purchased from Changchun Resin Co. (Shanghai, China).

Synthesis of BMITB

The general procedure for synthesis of BMITB (see Fig. 1) was as follows. 4,4'-Bis(4-nitrophenoxy)-3,3',5,5'-tetramethyl biphenyl (BNTB) was synthesized by a nucleophilic elimination reaction between 2,2',6,6'-tetramethyl-4,4'-biphenol (24.20 g, 0.10 mol) and p-chloronitrobenzene (34.67 g, 0.22 mol) in DMF (200 ml) in the presence of [K.sub.2][CO.sub.3] (27.64 g, 0.20 mol) for 10 h at 145-150[degrees]C under an [N.sub.2] atmosphere. BNTB (48.40 g, 0.10 mol) was then reduced to 4,4'-bis(4-aminophenoxy)-3,3',5,5' -tetramethyl biphenyl (BATB) in ethanol (200 ml) using hydrazine hydrate (85 wt%, 100 ml)/[FeC1.sub.3] [6H.sub.2.O] (0.6 g)/activated carbon (2.0 g) for 4 h at 80[degrees]C under [N.sub.2]. This reduction system was reported to be easily prepared and effective for this reaction compared with the [NH.sub.2]-[NH.sub.2]./Pd-C system (17). Ring-opening addition reaction of BATB (21.20 g, 0.05 mol) and maleic anhydride (10.78 g,0.11 mol) under [N.sub.2] (4 h at room temperature and 1 h at 60[degrees]C) yielded 4,4'-bis(4-maleamido-phenoxy)-3,3',5,5'-tetramethyl biphenyl (BMATB). The target compound, BMITB, was obtained from intramolecular dehydration of BMATB (9.30 g, 0.015 mol) in DMF (100 ml) in the presence of triethylamine (0.909 g, 0.009 mol), cobalt acetate (0.01 g) and acetic anhydride (6.64 g, 0.065 mol) for 3 h at 65[degrees]C in 94.0% yield.

[FIGURE 1 OMITTED]

Preparation of Cured ECN/BMITB/DDS Blends

The structures of ECN and DDS are shown in Fig. 2. Cured ECN/BMITB/DDS blends were prepared as follows:

ECN/BMITB mixtures (weight ratios of 1:0, 4:1, 3:2, and 2:3) were prepared by mixing the components in [CH.sub.2][Cl.sub.2] to form a homogenous mixture, which was then combined with DDS in 1:1 stoichiometry for the amine hydrogen groups to the combined amount of double bonds in BMITB and epoxy groups in ECN (corresponding BMITB weight ratios in the blends were 0, 15.5, 31.3, and 47.2 wt%, respectively). The mixtures were stirred and then dried at room temperature under vacuum. These samples were stored in a freezer (-10[degrees]C) for differential scanning calorimetry (DSC) or cast in molds for curing at 160[degrees]C for 1 h and 210[degrees]C for 4 h, followed by postcuring at 260[degrees]C for 4 h.

[FIGURE 2 OMITTED]

Instrumentation

Infrared spectra were recorded on a Vector 22 FTIR spectrophotometer using the KBr pellet method. [.sup.1]H and [.sup.13]C NMR data were obtained on an Avance DMX 500 NMR spectrometer at 35[degrees]C using [CDCl.sub.3] as solvent and tetramethylsilane as internal standard. Elemental analysis was performed on a Flash EA1112 CHN-O-Rapid elemental analyzer using acetanilide as standard.

Samples of 3.0-5.0 mg were placed in aluminum pan and analyzed on a Perkin-Elmer DSC 7 thermal analyzer at a heating rate of 10[degrees]C/min to determine the curing exotherm. Nitrogen was used as carrier gas at a flow rate of 40 ml/min. The glass transition temperature ([T.sub.g]) of the cured resins was determined by DSC at a heating rate of 20[degrees]C/min under [N.sub.2]. Thermogravimetric analysis (TGA) of the cured resins was carried out on a Perkin-Elmer Pyris 1 instrument under [N.sub.2] from 50 to 850[degrees]C at a rate of 10[degrees]C/min.

A specimen of 60 mm in length, 12.6 mm in width, and ~ 1.6 mm in thickness was used for dynamic mechanical analysis (DMA) on a Perkin Elmer 7 DMA instrument. The storage modulus G' and loss factor tan [delta] were determined for samples subjected to temperature scanning at a rate of 3[degrees]C/min from room temperature to ~330[degrees]C at a frequency of 1 Hz. [T.sub.g] data were also determined from the tan [delta] peak. Water absorption tests were carried out according to ASTM D570-1998.

RESULTS AND DISCUSSION

Characterization of BMITB

Figure 3 shows IR spectra of BNTB, BATB, BMATB, and BMITB (curves a-d, respectively). Curve a shows IR absorption peaks at 1337 and 1237 [cm.sup.-1] attributed to [-NO.sub.2] and aromatic ether linkages, respectively. The lack of an absorption band above 3000 [cm.sup.-1] indicates complete reaction of the bisphenol. In comparison, in curve b the [NO.sub.2] absorption peak disappeared completely and new absorption bands (3435 and 3352 [cm.sup.-1], [[nu].sub.NH2]; 1628 [cm.sup.-1], [[delta].sub.NH2]) due to primary amine appeared. The primary amine bands disappeared after the reaction between BATB and maleic anhydride, whereas irregular and wide IR absorption bands at 3400-2500 [cm.sup.-1] due to -NH- and -COOH and at 1709 [cm.sup.-1] due to carbonyl groups appeared (curve c). Curve d reveals that the IR absorption bands at 3400-2500 [cm.sup.-1] in curve c disappeared completely on complete intramolecular dehydration of BMATB.

[FIGURE 3 OMITTED]

Figures 4 and 5 show [.sup.1]H and [.sup.13]C NMR spectra of BMITB. The chemical shifts of all protons and carbons have been assigned. The proton chemical shifts for methyl, alkylene (maleimide ring) and phenyl groups in BMITB were at 2.2, 6.8, and 6.9-7.3 ppm, respectively. The [.sup.13]C NMR spectrum of BMITB shows 12 peaks that are consistent with the amount of carbons in different chemical environments in the BMITB molecule. Furthermore, the elemental analysis results for BMITB [[C.sub.13][H.sub.28][N.sub.2][O.sub.6] (584.62): Calcd., C 73.96%, H 4.83%, N 4.79%; Found, C 73.13%, H 4.92%, N 4.98%] confirm the structure of the target product. From the above results, we conclude that BMITB was synthesized successfully.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

Curing Behavior of ECN/BMITB/DDS Blends

Figure 6 shows DSC thermograms of BMITB and BMITB/DDS blend (1:1 stoichiometry of amine hydrogen groups in DDS to the amount of double bonds in BMITB). An endothermic melting peak and a wide exothermic peak are evident at 239 and 270[degrees]C, respectively. The endothermic melting peak resulted from melting of BMITB, whereas the exothermic peak resulted from the self-curing reaction involving double bonds of the maleimide groups (see Fig. 7). For the BMITB/DDS blend, two individual endothermic peaks were observed at 176 and 239[degrees]C that can be attributed to the melting of DDS and BMITB, respectively. This means that DDS and BMITB were not miscible. Two exothermic peaks were observed 252 and 300[degrees]C. The peak at 252[degrees]C can be attributed mainly to the self-curing of BMITB, as it is close to the peak due to BMITB homopolymerization in curve 1. The peak at 300[degrees]C can be regarded as a result of the Michael addition reaction between BMITB and DDS (see Fig. 8) (15).

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

ECN, BMITB, and DDS were blended at given weight ratios and the curing behavior of these ECN/BMITB/DDS blends was evaluated by dynamic DSC (see Fig. 9). For curing of ECN/DDS resin during dynamic heating (curve a in Fig. 9). the main reaction was the addition reaction of amine and oxirane groups (15) to afford secondary hydroxyl groups (see Fig. 10) and cross-linking points. The onset temperature of curve 1 was ~ 156[degrees]C and the temperature of maximum conversion rate in the DSC curve ([T.sub.p]) was 227[degrees]C. This is a typical DSC curve for epoxy-amine systems during dynamic heating. The DSC exothermic peak for the ECN/BMITB (15.5 wt%)/DDS blend was the same as that for ECN/DDS resin (curve a), indicating that small amounts of BMITB in this system have no clear influence on the curing behavior. Furthermore, no endothermic melting peaks were observed in curing curves for ECN/BMITB/DDS blends with 31.3 and 47.2 wt% BMITB (curves c, d), indicating that the three components in this system were miscible. The tetramethyl structure and flexible ether linkages of BMITB may confer good miscibility with the epoxy-amine matrix, even though BMITB has a rigid biphenyl structure.

[FIGURE 9 OMITTED]

[FIGURE 10 OMITTED]

For BMITB weight ratios of 31.3 and 47.2 wt% in the system, a small second exothermic peak at ~284[degrees]C was observed (curves c and d in Fig. 9). Both exothermal peaks may result from the heat enthalpy of self-curing of BMITB. The self-curing of BMITB might be retarded by the epoxy-amine network formed in advance, and thus the corresponding exothermal peak is shifted to higher temperature. As a result of the epoxy and BMITB networks formed and the consumption of DDS, the Michael addition reaction between DDS and BMITB would not be significant (15), and thus the corresponding heat enthalpy may be too low to be observed in the DSC curve. In this case, a thermoset/thermoset blend with an IPN might be formed.

Furthermore, the [T.sub.p] of the blends shifted to slightly lower temperature with increasing BMITB weight ratio. Thus, the curing behavior of the ECN/DDS system was not markedly changed by the introduction of BMITB. This means that the ternary resin system can be cured using the same process as for the ECN/DDS system.

Thermal Properties of Cured ECN/BMITB/DDS Blends

DSC Analyses. The [T.sub.g] values for all cured resins were obtained from a second DSC curve (calculated from the thermal capacity transition, Fig. 11) measured at 20[degrees]C/min. It is clear that the cured ECN/DDS resin (curve a) has only one [T.sub.g] (260[degrees]C) that starts at 226[degrees]C and ends at 297[degress]C, whereas DSC curves for the cured ternary blends show two adjacent but distinct [T.sub.g] values. The first [T.sub.g] of the cured ternary blends with 15.5 and 31.3 wt% BMITB (264 and 259[degrees]C, curves b and c, respectively) was approximately the same as for the cured ECN/DDS resin. This [T.sub.g] is attributed to the epoxy-amine network formed by the addition reaction between amino groups in DDS and oxirane groups in ECN (see Fig. 10) in the cured resins. However, the [T.sub.g] of the ternary blend with 47.2 wt% BMITB (251[degress]C, curve d) was broad and irregular and started at 180[degrees]C. This might be the result of a decrease in cross-linking density for the uniform epoxy-amine network on introduction of a greater amount of BMITB. The second [T.sub.g] values are at ~309-321[degrees]C and mainly resulted from self-curing of BMITB at high curing temperatures. The blends with greater BMITB content had low second [T.sub.g] values, as the reaction of BMITB and DDS led to the production of chain-extended BMITB, which may decrease the cross-linking density of the resultant thermoset (18). Cured ternary blends with two [T.sub.g] values indicate that thermoset/thermoset blends with an IPN structure might have been produced. This structure improved the thermal stability of the cured resins and may improve the fracture toughness of resulting thermosets (7), (11).

[FIGURE 11 OMITTED]

TGA Results

The thermal stability of cured ECN/BMITB/DDS blends was assessed by TGA in terms of the following parameters: the 5 wt% decomposition temperature ([Tsub.d.5%]). the char residue ([Y.sub.c]) at 850[degrees]C under [N.sub.2], and the integral procedure decomposition temperature (IPDT). [T.sub.d,5%] is a measure of the apparent thermal stability of epoxy thermosets. IPDT, as proposed by Doyle (19), can be considered a measure of the overall thermal stability of a resin, including the char residue of resulting thermosets at high temperature. According to the TGA results for cured ECN/BMITB/DDS blends, IPDT was calculated using the following equation (19):

IPDT([degrees]C) = A*K*([T.sub.f] - [T.sub.i]) + [T.sub.i] (1)

where A* is the area ratio of the total experimental curve to the total TGA thermogram, K* is the coefficient of A*, and [T.sub.i] and [T.sub.f], are the initial and final experimental temperature, respectively.

Figure 12 shows TGA thermograms of cured ECN/BMITB/DDS blends for different BMITB contents. Corresponding values of [T.sub.d.5%], [Y.sub.c], and IPDT are listed in Table 1. The [T.sub.d.5%] values for the cured ternary resins were all >395[degrees]C and higher than that for the ECN/DDS resin (381[degrees]C). This demonstrates that the BMITB addition to the epoxy-amine system led to an improvement in the initial decomposition temperature of the resultant thermosets. The char yield ([Y.sub.c]) of the cured ECN/BMITB/DDS blends (31.13-32.50%) was higher than that of the cured ECN/DDS resin (28.77%). This is because the biphenyl moiety and cyclic maleimide structure of BMITB can facilitate the formation of char residue during decomposition (20), (21). According to Van Krevelen's theory (22) and reports by Lin and Pearce, (23) and Chen et al. (24), an increase in char formation decreases the flammability of the material, and a high char residue favors improvement of its limited oxygen index and nonflammability. The increase in IPDT with increasing BMITB content indicates that BMITB addition dramatically improved the inherent thermal stability of the resultant epoxy thermosets. However, 47.2 wt% BMITB in this system was excessive.

[FIGURE 12 OMITTED]
TABLE 1. Thermal properties of the cured ECN/BMITB/DDS resins with
various weight ratios.

   BMITB content (wt%)   [T.sub.g1]    [T.sub.g2]      [T.sub.d.5%]
                       ([degrees]C)  ([degrees]C)  [([degrees]C).sup.a]

a          0                260            -                381
b         15.5              264           321               395
c         31.3              259           316               404
d         47.2              251           309               398

   [Y.sub.c](%, 850[degrees]C)  IPDT [([degrees]C)sup.b]

a             28.77                      1018.0
b             31.13                      1082.3
c             32.50                      1130.0
d             32.50                      1126.6


TGA curves of the cured ECN/BMITB/DDS blends exhibited two decomposition stages (the turning point was at 500-550[degrees]C). The latter decomposition stage may be related to the network structure of BMITB homopolymerization, which might further evidence of an IPN structure in the resultant thermoset. This IPN structure favors an improvement in the thermal properties of cured ECN/BMITB/DDS blends. This conclusion was also confirmed using a TGDDM/BMI/DDS system (25).

DMA Results

DMA can provide information about the microstructure and thermomechanical properties of cured blends. Unfortunately, DMA specimens of ECN/BMITB/DDS blends with high BMITB content ([greater.than.or equal to]10 wt%) were not easy to prepare, as the high viscosity of the ternary mixtures meant that bubble removal was difficult in the curing process. Thus, cured ECN/BMITB/DDS blends with 5 and 10 wt% BMITB were prepared to investigate the influence of BMITB on the dynamic mechanical properties of the blends.

Figure 13 shows the dynamic mechanical behavior of cured ECN/BMITB/DDS blends using a heating rate of 3[degrees]C/min from room temperature to 330[degrees]C at a frequency of 1 Hz. The storage modulus (G') of the cured ECN/BMITB (5 wt%)/DDS blend was almost the same as that of the cured ECN/DDS resin up to 150[degrees]C. This indicates that small amounts of BMITB had no clear influence on G' at low temperature. The cured ECN/BMITB (10 wt%)/DDS blend had a slightly lower G' up to 150[degrees]C in comparison with the 5 wt% samples. However, it is clear that BMITB improved the storage modulus of cured ternary blends at high temperature (>150[degrees]C). The [T.sub.g] value of these samples increased from 278.8 to 291.8[degrees]C with increasing BMITB content. This is because the rigid biphenyl and cyclic maleimide structures retard the movement of the chain segment in the cured resins. Furthermore, only one tan [delta] peak was observed for ternary thermosets in Fig. 13, indicating that no phase separation was detected in DMA of cured ECN/BMITB/DDS blends with small amounts of BMITB.

[FIGURE 13 OMITTED]

Moisture absorbed by a composite can plasticize the resin, causing a decrease in [T.sub.g] and in turn affecting the mechanical response. The interaction between absorbed water molecules and networks in a tetrafunctional epoxy resin (TGDDM) was investigated in detail by Musto et al. In their research, the equilibrium water content of cured TGDDM/DDS/BMI blends was 4-5 wt% (20), (26). In the present study, moisture absorption for cured ECN/BMITB/DDS blends with 5 and 10 wt% BMITB was 0.24 and 0.23 wt%, respectively, which is lower than for cured ECN/DDS blends (0.40 wt%) and can be attributed to the introduction of hydrophobic tetramethyl groups. These values are also lower than the absorbed moisture determined for tetramethylbiphenyl and naphthalene structures containing BMI monomer (0.48-0.55 wt%) and epoxy monomer (0.58-0.80 wt%) (11).

CONCLUSIONS

The following conclusions can be drawn from the study results. BMITB was synthesized in high yield (94%) via a facile four-step reaction. Introducing BMITB into the ECN/DDS system improved its thermal stability and decreased its moisture absorption. Cured ECN/BMITB/DDS blends with higher BMITB content (15.5-47.2 wt%) had two distinct [T.sub.g] values >250[degrees]C, indicating that an IPN structure may have formed. The initial thermal decomposition temperature and IPDT of cured ECN/BMITB/DDS blends were also improved (>390 and 1080[degrees]C, respectively) in the presence of BMITB. The use of small amounts of BMITB for ECN/DDS blends (5 and 10 wt%) may improve the thermomechanical properties of the composite DMA form.

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Bo Xuan Zhou, (1) Yi Jun Huang, (2) Xing Hong Zhang, (1) Zhi Sheng Fu, (1) Guo Rong Qi (1)

(1) Department of Polymer Science and Engineering, Key Laboratory of Macromolecular Synthesis and Functionalization of the Ministry of Education, Zhejiang University, Hangzhou 310027, China

(2) Department of Science, QianJiang College, Hangzhou Normal University, Hangzhou 310036, China

Correspondence to: Xing Hong Zhang: e-mail: xhzang@zju.edu.cn

Contract grant sponsor: China Postdoctoral Science Foundation: contract grant number: 20060400339.

DOI 10.1002/pen.2l38l

Published online in Wiley InterScience (www.interscience.wiley.com).

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Author:Zhou, Bo Xuan; Huang, Yi Jun; Zhang, Xing Hong; Fu, Zhi Sheng; Qi, Guo Rong
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Date:Aug 1, 2009
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