Studies on the odd-even effect of methylene spacers in poly(pyromellitimide-ester)s

Three series of poly(pyromellitimide-ester)s were synthesized from various N,N'-bis(w-hydroxyalkyl)pyromellitimides (HAPMIs) by melt condensation with dicarboxylic acids, including terephthalic acid (TPA), 4,4'-biphenyldicarboxylic acid (BPDA), and 4,4'-azobenzenedicarboxylic acid (ABDA). Polymers were characterized by elemental analysis, solubility, inherent viscosity, spectra (IR, IH-NMR, C-13-NMR), and X-ray diffraction (XRD). Thermal stability and phase transition behaviour were evaluated by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and hot-stage optical polarized microscopy (HOPM). The d-spacings, calculated from XRD data, showed an odd-even effect with varying numbers of methylene spacers. Crystallinity of polymers decreased in the following order: azobenzene > biphenyl > phenyl polymers. Similarly, DSC-obtained melting temperatures (T-m's) showed an odd-even effect, and glass transition temperatures (T-g's) decreased with increasing numbers of methylene spacers. Thermal stability decreased as methylene chain length increased. Thermal stability of polymers occurred in the following order: phenyl > biphenyl > azobenzene polymers. (C) 1999 John Wiley & Sons, Inc.