PHASE-TRANSITIONS AND MELTING OF POLY(4-HYDROXYBENZOATE)

Slab-like crystals and needle-like crystals (whiskers) of poly(4-hydroxybenzoate) (poly(4-Hybe)) prepared from 4-acetoxybenzoic acid were studied by means of differential scanning calorimetry and wide-angle X-ray scattering (WAXS) measurements and by microscopy with polarized light. At a heating rate of 20°C min-1 the slab-like crystals display phase transitions at 332, 430-440, 510 and 529°C. The whiskers exhibit these transitions at 353, 430-440, 510 and 532°C. The transition of the whiskers at 353°C is not thermodynamically stable and changes upon repeated heating and cooling to the 332°C transition. The endotherm at 510°C seems to indicate the beginning of the melting process reinforced by thermal rearrangements and degradation. Infra-red spectra indicate a nearly complete change of the chemical structure upon heating above 530°C. WAXS measurements conducted with synchrotron radiation up to 500°C confirm that no melting occurs below this temperature. Yet the WAXS patterns indicate melting (at 405°C) of a 1:1 copolyester of 4-Hybe and 4′-hydroxybiphenylcarboxylic acid. Microscopic observation of the poly(4-Hybe) whiskers revealed that these crystals fade away above 500°C in air without forming a melt. © 1990.