Characterization of Polyimides and Polyamic Acids in Dilute Solution

Abstract

A fundamental understanding of the structure of the cured polyimide and the precursor polyamic acid from the condensation of pyromellitic dianhydride and 4,4′-diaminodiphenylether has been obtained through the use of several dilute solution techniques on samples prepared and cured under controlled conditions. The results obtained by these methods are compared with similar measurements on commercially available materials. Measurement of the concentrated solution viscosity as a means to identify changes in the molecular weight of the precursor polyamic acid is found to be inadequate because of its sensitivity to changes unrelated to molecular weight, i.e., strong polymer-polymer interactions. In this study we have augmented the concentrated solution viscosities with both light scattering and dilute solution viscometry to separate molecular weight changes from other effects. Unusual behavior in both dilute solution viscosity and light scattering measurements is attributable to polyelectrolyte effects arising from base impurities. We have also been able to measure directly the weight average molecular weight of the cured polyimide by low angle light scattering in concentrated sulfuric acid, and compare these molecular weights with those of the uncured precursor. These results have demonstrated that little change in molecular weight or chain dimensions occurs on curing, i.e., no evidence of branching or cross-linking is observed for this system.