Polymer International 39 (1996) 289-293 Preparation and Properties of Bismaleimide Resins of Aromatic Sulfone Ether Diamine Lianlai Zhang,* Qitai Jiang, Luxia Jiang & Xingxian Cai Department of Polymer Science and Materials, Sichuan Union University, Chengdu 610065, People's Republic of China (Received 15 September 1995; accepted 14 October 1995) Abstract: Aromatic sulfone ether diamine, bis[4-(4-aminophenoxy)phenyl]sulfone (SED), was prepared by the nucleophilic aromatic substitution of 44'dichlorodiphenylsulphone by p-aminophenolate. The reaction was conducted in the presence of excess potassium carbonate as a weak base, toluene as the dehydrating agent and N-methylpyrrolidone as the dipolar aprotic solvent. SED showed good solubility in common organic solvents, such as dioxan, tetrahydrofuran, butanone and acetone. SED was reacted with maleic anhydride to obtain aromatic sulfone ether bismaleimide, bis[4-(4-maleimidophenoxy)phenyl]sulfone (SEM). The compounds were characterized by FTIR and 'H NMR analysis. Furthermore, copolymer resins of SED with 4,4'-bismaleimidodiphenyl methane (BMI) and SEM were prepared. After curing, crosslinked resins with better thermal stability resulted. The temperature at maximum rate of weight loss (T,,J and the heat-resistant temperature index (TJ in air were found to be 426"C, 208°C and 579"C, 221°C for BMI/SED and SEM/SED resins, respectively. Compared with the corresponding 4,4'-diaminodiphenyl methane (DDM) system, BMI/SED and SEM/SED showed a slight decrease in T,,, and q. SEDmodified BMI/amine resin based glass cloth laminates for printed circuit boards showed higher mechanical properties than those of the corresponding unmodified system. With SED instead of the original amine component in 3-5% weight fraction, the tensile strength, flexural strength and impact strength of the laminates increased markedly. Meanwhile, the stripping strength and weld resistance were also improved by the addition of SED. K e y words: aromatic sulfone ether diamine, copolymer resins, thermal stability, glass cloth laminates, mechanical properties, modifier. INTRO DUCTlON However, the brittle nature of these resins remains a weakness. Moreover, the low basicity of the amine due to the electron withdrawing sulfone group, and its low activity limits its use for thermoplastics. For example, when DDS was reacted with dianhydride to prepare polyimides, only low molecular weight product was obtained because of the hydrolysis of the polyamic acid intermediate.' Aromatic sulfone ether diamine (SED), bis[4-(4aminophenoxy)phenyl]sulfone, is another sulfonecontaining aromatic diamine, whose activity is comparable with that of diaminodiphenylether. SED has found application in the modification of epoxy resins.2 The aromatic ether bond offers advantages in improving fracture toughness and hot-wet properties of the Aromatic diamines are very important as monomers for the synthesis of various thermoplastic polymers, such as polyamides, polyimides and polyureas. They are also widely used in thermosets as the curing agent for epoxies, or chain extender for bismaleimides. In order to improve the mechanical and thermal properties of these resins, aromatic diamines bearing rigid groups, such as sulfone, e.g. diaminodiphenylsulfone (DDS), are often used to modify epoxy and bismaleimide resins. * To whom correspondence should be addressed at: Chengdu Institute of Organic Chemistry, Academia Sinica, Chengdu 610041, People's Republic of China. 289 Polymer International 0959-8103/96/$09.00 01996 SCI. Printed in Great Britain 290 resulting cured resins. SED can also be used in the synthesis of polyamides, polyimides, and poly(amideimide)s and in the modification of bismaleimides. The last named has received less attention. In this paper, we describe the synthesis of SED by a new, convenient procedure. The SED was then introduced into bismaleimide resin systems, and the properties of the modified resins were investigated. EX PER IM ENTAL Materials p-Aminophenol was recrystallized from anhydrous ethyl alcohol under nitrogen, then vacuum dried. 4,4'Dichlorodiphenylsulphone (DCDPS) was recrystallized from toluene, and dried under vacuum. N-Methylpyrrolidone (NMP) was treated with calcium hydride for 24 h and vacuum distilled. Anhydrous potassium carbonate, maleic anhydride (MA), 4,4'-bismaleimidodiphenyl methane (BMI), 4,4diaminodiphenyl methane (DDM), toluene, N,N'-dimethylacetamide (DMAC) and other reagents were used as received. Synthesis of SED SED was synthesized by nucleophilic aromatic substitution of DCDPS by the phenolate of p-aminophenol. The reaction was conducted in the presence of excess potassium carbonate as a weak base, toluene as the dehydrating agent and N M P as the dipolar aprotic solvent. Thus, to a 500 ml four-necked round-bottom flask, fitted with a condenser, Dean-Stark trap, nitrogen inlet, a thermometer and a mechanical stirrer, DCDPS (44.5 g, 0.155 mol), p-aminophenol (38.73 g, 0.355 mol, 14% excess to DCDPS) and NMP (250 ml) were added. Then, anhydrous potassium carbonate (27.0 g, 0.195mol) and toluene (120ml) were charged to the flask. The reaction mixture was stirred under purge of nitrogen and first heated to reflux (about 140"C), and maintained at this temperature for 3 h. Then the toluene was removed and the temperature was raised to 175°C for another 4h. After cooling to 100°C, the reaction mixture was coagulated in 2 litre water, and neutralized with dilute HCl. The precipitate was filtered, washed with water, finally washed with methanol and dried. The crude product was dissolved in DMAC, reprecipitated to remove unreacted p-aminophenol and any trapped salts, filtered and dried in a vacuum oven at 100°C. The yield was above 85%, with m.p. of 192°C. L-L. Zhang et al. mixture was stirred for 15min, then cooled with ice to O"C, and MA (3-2g, 0.0326mol) was added. The reaction was maintained at 15-20°C for l h to form the amic acid intermediate. Then by adding acetic anhydride and triethylamine, the cyclization of the amic acid was carried out immediately. The temperature of the reaction was maintained at 20°C for 4 h, and then raised to 60°C for another 2 h. After cooling, the solution was poured into 250 ml cold water. The precipitate was collected by filtration, repeatedly washed with methanol and dried at 50°C in a vacuum oven. The product was a light grey powder, the yield about 87.8%. Preparation of modified B MI resins SED (4.3 g, 0.00995 mol) and DMAC (25ml) were added to a 100ml three-necked flask equipped with a condenser, a thermometer, and a mechanical stirrer. The mixture was heated to 90-95"C7 and BMI (13.3g, 0.0364mol in 50ml DMAC solution) was added through a funnel over 15min. After another 30min, the solution was cooled with ice to below 30"C, and poured into water. A dark red precipitate resulted, which was filtered off. Using a similar procedure, other copolymer resins were prepared. They were cured at 180°C for 2 h, 200°C for 4 h, and postcured at 250°C for 40 min. The SEDmodified BMI/amine resins were prepolymerized in acetone, then glass cloths were immersed in the solution, dried, and processed to printed circuit board, which was covered with copper foil on one side. The laminates were about 1.5 mm thick. Instrumentation Melting point was measured with a CDR-1 differential thermoanalyser in air at a heating rate of lO"C/min. FTIR spectra were recorded on a Nicolet 20 SXB-IR spectrophotometer, using KBr disc samples. 'H NMR was performed on a Varian FT-80 NMR instrument using CD,COCD, as solvent, and TMS as internal standard. Thermal stability of the cured resins was studied using a Perkin-Elmer TG-7 thermogravimetric analyser at a heating rate of 10"C/min in air, or nitrogen. Mechanical properties were measured using a laboratory mechanical tester. RESULTS AND DISCUSSION Synthesis of SED Reaction of SED with MA SED (6.0g, 0.0139mol) and DMAC (50ml) were charged to a 100ml three-necked flask, equipped with a condenser, a thermometer, and mechanical stirrer. The The synthesis of SED was previously studied by Kawakarni,,q4 and this was followed by several Japanese patents.'-' In summary, these reactions were carried out in dimethylsulfoxide (DMSO)/strong base (NaOH, POLYMER INTERNATIONAL VOL. 39, NO. 4, 1996 291 Bismaleimide resins of aromatic surfone ether diamine aqueous or solid) using usually a two-step procedure to prevent hydrolysis of DCDPS. Firstly, a dehydration reaction was carried out to remove a large amount of water and form the metal salt of p-aminophenol. Secondly, DCDPS was added to the system, and the nucleophilic aromatic substitution reaction was carried out. Use of N-methylpyrrolidone (NMP)/anhydrous potassium carbonate is an alternative method. This convenient procedure has been used to synthesize amineterminated poly(ary1ene ether sulfone) oligomers.' Using solid potassium carbonate as a weak base instead of aqueous alkali solution or alkali solid essentially prevents the hydrolysis side reaction of the halide monomer. The reaction scheme can be simplified to a one-step procedure as shown in Scheme 1. The structure of SED was confirmed by FTIR and 'H NMR spectra. The FTIR spectrum of SED is shown in Fig. 1. The absorption at 3455cm-' was assigned to the stretching of the N-H band in the NH, group. The two resonance peaks at 1500cm-' and 1486cm- ' were due to the resonances of terminal amino-bearing benzene rings and sulfone-bearing benzene rings. The resonance peaks at 1290 cm- ' and 1145cm-' were attributed to asymmetric and symmetric vibrations of SO, groups. Figure 2 presents the 'H NMR spectrum of SED. The resonances at 7.85 ppm and 7-75ppm were assigned to protons in SO,-bearing benzene rings, while the resonances at 6.75 ppm and 6.70 ppm were assigned to protons in terminal amino-bearing benzene rings. 8 I 6 5 4 6 (ppm) 3 Fig. 2. 'H NMR spectrum of SED. Reaction of SED with MA and BMI SED-modified bismaleimide resins may be obtained by synthesis of bismaleimides of SED, or by reaction of BMI with SED. Aromatic sulfone ether bismaleimide (SEM), bis[4-(4-maleimidophenoxy)phenyl]sulfone was synthesized from SED and MA. The reaction route is shown in Scheme 2. The reaction was confirmed by FTIR and 'H NMR spectra. The FTIR spectrum of SEM is shown in Fig. 3 (lower). The presence of absorptions at 1775 cm- and 1710cm-' due to the formation of imide rings, and the absence of N--H absorptions at 3455 cm-' can be seen. The 'H NMR spectrum of SEM also presents similar results. SED and SEM showed good solubility in common organic solvents, such as tetrahydrofuran (THF), butanone, dioxan, and acetone, as shown in Table 1. BMI/SED copolymer resin was obtained by the Michael addition reaction of BMI and SED. Figure 3 (upper) gives the FTIR spectrum of uncured BMI/SED and copolymer. The absorptions at 3455 cm3367 cm- for SED decreased significantly. Copolymer resins of BMI/SED and SEM/SED and the corresponding DDM systems, BMI/DDM and SEM/DDM, were prepared. After curing, crosslinked resins were obtained. ' ' I 4000 I 3600 3200 2800 2400 2000 Wavenumber 1600 1200 800 400 Properties of modified bismaleimide resins (cm-') Fig. 1. FTIR spectrum of aromatic sulfone ether diamine (SED). SED is a good modifier for bismaleimide resins. Table 2 shows the results of thermogravimetric analysis for 3. Solvent/Base Scheme 1. Synthesis of aromatic sulfone ether diamine (SED). POLYMER INTERNATIONAL VOL. 39, NO. 4, 1996 292 L-L. Zhang et al. TABLE 1. Solubility of SED and SEM SED SEM THF Butanone Dioxan Acetone Chloroform Butanol Toluene 0 0 0 0 0 0 0 @ @ @ @ 0 8 0 0 Easily soluble, @ soluble, 8 partly soluble, 0 insoluble. cured copolymer resins in air. The sulfone ether based bismaleimide (SEM)/diamine copolymer showed initial weight loss in the range 340-370"C, and a temperature at maximum rate of weight loss (T,,,) of about 580°C. For the BMI systems, initial weight loss was in the range 27O-29O0C, and T,,, 426-465°C. Compared with the data for the corresponding BMI copolymers, BMI/DDM and BMI/SED, SEM copolymers, SEM/DDM and SEM/SED, showed a higher heat-resistant temperature index (T),220-230°C. A relatively lower thermal stability and T of the SED systems BMI/SED and SEM/SED than the corresponding DDM systems BMI/DDM and SEM/DDM were found. Thermogravimetric curves of SEM/SED copolymer resins are shown in Fig. 4. Below 540°C (36% weight loss), the two curves in air and N2 are almost identical, indicating the resins to have a high level of thermooxidative stability. TABLE 2. Results of TG for cured copolymers (in air) 4000 2533 950 1633 400 Wavenumber (cm-') Fig. 3. FTIR spectra of aromatic sulfone ether bismaleimide (SEM) (lower) and uncured SEM/SED copolymer (upper). 0 Copolymer' T6' ("C) T30C ("C) SED/B M I DDM/BMI SED/SEM DDM/SEM 291 273 340 363 514 540 527 531 ("C) 426 46 5 579 584 T,' ("C) 208 21 2 221 227 BMI, 4,4'-bismaleimidodiphenyl methane; DDM, 4,4diaminodiphenyl methane. 5% weight loss temperature. 30% weight loss temperature. dTemperature at maximum rate of weight loss. ' Heat-resistant temperature index, T, = 0.49[T5 + 0.6(T3, -Tdl. 0 0 0 T,,: 0 0 Scheme 2. Synthesis of aromatic sulfone ether bismaleimide (SEM). POLYMER INTERNATIONAL VOL. 39, NO. 4, 1996 Bismaleimide resins of aromatic sulfone ether diamine 293 4.5 strength and weld resistance were also improved when the content of SED was 5%. CONCLUSION t . 100 200 300 500 400 600 700 Ternpersture('C ) Fig. 4. Thermogravimetric curves for cured SEM/SED copolymer in air (-) and in N, (-----). SED-modified BMI resin based glass cloth laminates for printed circuit boards, covered with copper foil on one side, showed higher mechanical properties than those of the corresponding unmodified system, as shown in Table 3. With SED instead of the original amine component in 3-5% weight fraction, the tensile strength, flexural strength and impact strength of the laminates increased markedly. Meanwhile, the stripping TABLE 3. Properties of glass cloth laminates of SED-modified B MI/amine system SED content (%) Properties Tensile strength, MPa Flexural strength, MPa Impact strength, J/cmZ Stripping strength, kgfcm Weld resistance (300"C), s 0 3 5 184.4 388.1 7.55 1.75 5 232.6 449.2 9.75 1.60 10 231.6 398.1 8.01 1.90 47 POLYMER INTERNATIONAL VOL. 39, NO. 4, 1996 Aromatic sulfone ether diamine (SED) has been prepared by a new convenient procedure. The synthesis reaction was conducted in a N-methylpyrrolidone (NMP)/anhydrous potassium carbonate system. The structure of SED was confirmed by FTIR and 'H NMR spectrometry. SED was reacted with maleic anhydride and bismaleimide to obtain aromatic sulfone ether bismaleimide and copolymer resins, respectively. Curing resulted in crosslinked resins with better thermal stability. Compared with the corresponding 4,4'diaminodiphenyl methane (DDM) systems, copolymers of bismaleimide and SED showed a slight decrease in the heat-resistant temperature index. 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