Synthesis and properties of poly(amide-imide)s based on N,N′-bis(4-carboxyphenyl)-4,4′-oxydiphthalimide, p-aminobenzoic acid and various aromatic diamines

A new diimide-diacid monomer, N,N′ -bis(4-carboxyphenyl)-4,4′-oxydiphthalimide (I), was prepared by azeotropic condensation of 4,4′-oxydiphthalic anhydride (ODPA) and p-aminobenzoic acid (p-ABA) at a 1:2 molar ratio in a polar solvent mixed with toluene. A series of poly (amide-imide)s (PAI, III_a-m) was synthesized from the diimide-diacid I (or I′, diacid chloride of I) and various aromatic diamines by direct polycondensation (or low temperature polycondensation) using triphenyl phosphite and pyridine as condensing agents. It was found that only III_k-m having a meta-structure at two terminals of the diamine could afford good quality, creasable films by solution-casting; other PAIs III using diamine with para-linkage at terminals were insoluble and crystalline; though III_g-i contained the soluble group of the diamine moieties, their solvent-cast films were brittle. In order to improve their to solubility and film quality, copoly(amide-imide)s (Co-PAIs) based on I and mixtures of p-ABA and aromatic diamines were synthesized. When on equimolar of p-ABA (m=1) was mixed, most of Co-PAIs IV had improved solubility and high inherent viscosities in the range 0.9-1.5 dl g^-1; however, their films were still brittle. With m=3, series V was obtained, and all members exhibited high toughness. The solubility, film-forming ability, crystallinity, and thermal properties of the resultant poly(amide-imide)s were investigated.