Solvent and isomer effects on the imidization of pyromellitic dianhydride-oxydianiline-based poly(amic ethyl ester)s

Abstract

The imidization behaviour of the isomeric poly(amic ethyl ester) (PAE) precursor molecules of the polyimide pyromellitic dianhydride—oxydianiline was followed using forward recoil spectrometry. The degree of imidization after 1 h at various temperatures was found to depend strongly on the spin-casting solvent. Films cast from dimethyl sulfoxide (DMSO) showed an imide fraction, f, which was independent of depth and of isomeric precursor (i.e. whether the precursor was para-PAE, meta-PAE or a random mixture of meta and para (mixed-PAE).) In films spun cast from N-methylpyrrolidone (NMP), however, fs for the para- and mixed-PAE were larger than that for the meta-PAE. In addition the fs for the para-PAE and mixed-PAE were found to be greater for films cast from NMP than from DMSO for any given thermal treatment. For the meta-PAE, f was independent of the casting solvent. Over an imidization temperature range of 180–280°C f was found to vary with depth, being lower than the bulk value over the first 3000 Å from the film surface. These observations appear to be related to a retention of the NMP with f being larger where NMP is retained.