EQUILIBRATION OF MIXTURES OF POLYAMIC ACIDS STUDIED USING SIZE-EXCLUSION CHROMATOGRAPHY AND LIGHT-SCATTERING

The transamidation reaction in a polyamic acid solution has been investigated using size-exclusion chromatography and low-angle light scattering. Mixtures of a high-molecular weight (DPBAR = 150) and a low-molecular weight (DPBAR = 10) polymer and of the high-molecular weight polymer with monomer were studied. Mixtures were made at high and low concentrations. The polyamic acid studied is the product of the polycondensation of 3,3',4,4'-biphenyltetracarboxylic acid dianhydride (BPDA) with oxydianiline (ODA). In all cases the molecular weight distribution equilibrated with time to a most-probable distribution with a DPBAR consistent with the stoichiometry of the mixture. Equilibration required about 2 weeks for mixtures of 10% by weight at ambient temperatures. The effect of addition of a small amount (5%) of low-molecular weight material to sample of high-molecular weight is dramatic; for DPBAR = 150 the molecular weight is decreased by more than one-half. In an entangled solution or melt, this would reduce the viscosity by an order of magnitude.