CURING STUDIES OF THE META, PARA AND 50 50 MIXED ISOMERS OF THE ETHYL-ESTER OF 4,4'-OXYDIANILINE PYROMELLITIC DIANHYDRIDE POLYAMIC ACID/

The cure behaviour of the ethyl ester derivatives of 4,4'-oxydianiline (ODA)/pyromellitic dianhydride (PMDA) poly(amic acid) to the correspo nding imide was investigated using isothermal and dynamic mass spectro scopy, FT-Raman spectroscopy and density gradient column analysis. Bot h the rate of cure and per cent conversion were dependent on the isome r composition, occurring in the order para > meta > 50/50 mixed isomer at temperatures below 275 degrees C. This order holds both for amorph ous solid samples and samples cast from N-methylpyrrolidone (NMP) solu tion. With increasing isothermal temperatures or ramp speeds of the dy namic mass spectroscopy scans, the imidization rates and per cent conv ersions of all three isomers converged. The cure kinetics were also fo und 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 i midization occurred narrowed by 300% for samples cast from NMP solutio n compared with solid samples precipitated from solution. The presence of NMP directly affected the cure by plasticizing the films. In addit ion, 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 align ment of the chains was changed by prior annealing in NMP, perhaps due to mesophase formation. (C) 1997 Elsevier Science Ltd.