Preparation and properties of polybenzoxazine/poly(imide-siloxane) alloys:: In situ ring-opening polymerization of benzoxazine in the presence of soluble poly(imide-siloxane)s

Benzoxazine monomer (Ba) was blended with soluble poly(imide-siloxane)s in various weight ratios. The soluble poly(imide-siloxane)s with and without pendent phenolic groups were prepared from the reaction of 2,2 ' -bis(3,4-dicarboxylphenyl)hexafluoropropane dianhydride with alpha,omega -bis(aminopropyl)dimethylsiloxane oligomer (PDMS; molecular weight = 5000) and 3,3 ' -dihydroxybenzidine (with OH group) or 4,4 ' -diaminodiphenyl ether (without OH group). The onset and maximum of the exotherm due to the ring-opening polymerization for the pristine Ba appeared on differential scanning calorimetry curves around 200 and 240 degreesC, respectively. In the presence of poly(imide-siloxane)s, the exothermic temperatures were lowered: the onset to 130-140 degreesC and the maximum to 210-220 degreesC. The exotherm due to the benzoxazine polymerization disappeared after curing at 240 degreesC for 1 h. Viscoelastic measurements of the cured blends containing poly(imide-siloxane) with OH functionality showed two glass-transition temperatures (T-g's), at a low temperature around -55 degreesC and at a high temperature around 250-300 degreesC, displaying phase separation between PDMS and the combined phase consisting of polyimide and polybenzoxazine (PBa) components due to the formation of AB-crosslinked polymer. For the blends containing poly(imide-siloxane) without OH functionalities, however, in addition to the T-g due to PDMS, two T-g's were observed in high-temperature ranges, 230-260 and 300-350 degreesC, indicating further phase separation between the polyimide and PBa components due to the formation of semi-interpenetrating networks. In both eases, T-g increased with increasing poly(imide-siloxane) content. Tensile measurements showed that the toughness of PBa was enhanced by the addition of poly(imide-siloxane). Thermogravimetric analysis showed that the thermal stability of PBa also was enhanced by the addition of poly(imidesiloxane). (C) 2001 John Wiley & Sons, Inc.