Preparation of Poly(p-oxybenzoyl) Microspheres Having Needlelike Crystals on the Surface

Polymerization of 4-acetoxybenzoic acid (ABA) with 3,5-diacetoxybenzoic acid (DABA) was examined to create a novel morphology of poly(p-oxybenzoyl) (POB) by means of the phase separation of oligomers during polymerization. Polymerizations were carried out at a concentration of 1.0% in liquid paraffin at 320 °C. Polymerization of ABA yielded the POB whiskers. On the other hand, the polymerization of ABA with DABA of which the concentration in the feed (χf) was 0.05−0.20 yielded the microspheres having needlelike crystals on the surface. The average diameter of the microspheres was in the range of 3.4−1.6 μm and the average length of the needlelike crystals was 3.2−0.3 μm. The diameter and length decreased with χf. DABA acted as a liquid−liquid phase separation inducer and the liquid−liquid phase separation of co-oligomers comprising 4-oxybenzoyl units and 3,5-dioxybenzoyl units was induced in the beginning of polymerization to form the core microspheres. Then the phase separation mode was changed to the crystallization of the homooligomers of the 4-oxybenzoyl unit and the homooligomers were crystallized as needlelike crystals on the surface of microspheres already precipitated. Solid-state polymerization occurred in the precipitates. The microspheres having needlelike crystals were prepared by the combination of liquid−liquid phase separation and the crystallization of oligomers during solution polymerization. The obtained microspheres having needlelike crystals possessed very high crystallinity and exhibited good thermal stability.