CRYSTALLINE POLYIMIDE PARTICLES GENERATED VIA THERMAL IMIDIZATION IN A HETEROGENEOUS MEDIUM

Highly crystalline polyimide powders were prepared from diluted soluti ons of polyamic acid in N-methyl-2-pyrrolidone (NMP) which were cured at 200 degrees C for 4 hr with a high heating rate. The chemical struc tures of the repetitive units were chosen to obtain rigid polymeric ba ckbones precluding any solubility in polar aprotic solvents. Therefore the starting polyamic acids were produced by polycondensation from an equimolar ratio of tile following pairs of monomers: pyromellitic dia nhydride (PMDA) and 4,4'-oxydianiline (ODA); 3,3:4,4'-biphenyl tetraca rboxylic dianhydride and ODA; PMDA and p-phenylenediamine. After optim izing the reaction conditions, the resulting powders were first charac terized by scanning electronic microscopy and granulometric analysis. Well-divided particles with a spherulitic shape and average particle s ize of 5 mu m were observed. The Xray diffraction patterns and the sol id-state C-13 nuclear magnetic resonance spectra together revealed a h ighly organized structure. The degree of imidization of the powder is nearly complete as demonstrated by Fourier transform infrared analysis and the inherent viscosity after dissolution in concentrated sulfuric acid is about 0.8 dl/g. (C) 1998 John Wiley & Sons, Ltd.