Structure–property relations of polyimide films

A series of well‐characterized polyamic acids from pyromellitic dianhydride and 4,4′‐diaminodiphenyl ether of systematically varied molecular weight and known molecular weight distribution were chemically converted to polyimide films under regulated conditions. The films were essentially noncrystalline, of moderate orientation, and soluble in concentrated sulfuric acid. The mechanical properties of the films varied sigmoidally with the solution properties of the precursor. Molecular weights were derived for the onset of mechanical strength and limiting property values are given. The relations between film properties and molecular structural parameters suggest that cyclization probably occurs here with little contribution from crosslinking, degradation, or other side reactions which would appreciably alter molecular weight and molecular weight distribution.