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

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. © 1993.