Polyimide solution properties

Solution characterization measurements indicate that cylization of the polyamic acid from 2,4‐diaminoisopropylbenzene and pyromellitic dianhydride (PMDA) to the polyimide can be effected with small change in number‐average degree of polymerization. An increase in weight‐average molecular weight on conversion to polyimide is attributed to branching side reactions. Inherent viscosities of polyimides from 4,4′‐diaminodiphenyl ether and PMDA decreased slowly with time in concentrated (97%) sulfuric acid, probably as a result of hydrolytic cleavage. The zero‐time viscosity intercepts were related to both the precursor and the estimated polyimide molecular weight. The latter gave ηinh (c = 0.5% solids) = 2.76 × 10−3M̄0.53w for 60,000 < Mw < 300,000. The exponent is near the lower limit of 0.5 found in theta solvents. The characteristic ratio of the root‐mean‐square end‐to‐end distance of a chain to the square root of its molecular weight is 1.1 Å and the steric factor is 1.1; these results suggest that the unperturbed polyimide chain is almost as extended as an equivalent freely rotating chain, a general feature of long‐bonded polymers with p‐phenylene rings in the chain backbone.