Curing studies of the meta, para and 50/50 mixed isomers of the ethyl ester of 4,4′-oxydianiline/pyromellitic dianhydride polyamic acid

Abstract

The cure behaviour of the ethyl ester derivatives of 4,4′-oxydianiline (ODA)/pyromellitic dianhydride (PMDA) poly(amic acid) to the corresponding imide was investigated using isothermal and dynamic mass spectroscopy, FT-Raman spectroscopy and density gradient column analysis. Both the rate of cure and per cent conversion were dependent on the isomer composition, occurring in the order para > meta > 50/50 mixed isomer at temperatures below 275°C. This order holds both for amorphous solid samples and samples cast from N-methylpyrrolidone (NMP) solution. With increasing isothermal temperatures or ramp speeds of the dynamic mass spectroscopy scans, the imidization rates and per cent conversions of all three isomers converged. The cure kinetics were also found to be dependent on the presence of solvent. The temperature of the maximum rate of cure in the dynamic mass spectroscopy data decreased with increasing solvent content and the temperature range over which imidization occurred narrowed by 300% for samples cast from NMP solution compared with solid samples precipitated from solution. The presence of NMP directly affected the cure by plasticizing the films. In addition, the narrowing of the imidization temperature range occurred even if there was complete removal of NMP before the onset of imidization, suggesting that the morphology of the films or the orientational alignment of the chains was changed by prior annealing in NMP, perhaps due to mesophase formation. © 1997 Elsevier Science Ltd.