Morphology of poly(p-oxybenzoyl) crystals prepared by direct polycondensation of p-hydroxybenzoic acid with diphenyl chlorophosphate

We have studied the morphology of poly(p-oxybenzoyl) (POB) crystals prepared by the direct polycondensation of p-hydroxybenzoic acid (HBA) using diphenyl chlorophosphate (DPCP) in the presence of N,N-dimethylformamide. Lozenge-shaped sheaf-like POB crystals were obtained of which the longer diagonal was 16.0-16.5 μm. Favorable polymerization conditions to prepare the crystal exhibiting clear crystal characteristic were the molar ratio of DPCP to HBA of 1.5, ther molar ratio of DMF to HBA of 2.6 and the polymerization concentration of 3.0%. The crystals were comprised of low molecular weight polymer of which the degree of polymerization (DPn) was 14 and possessed extremely high crystallinity. The formation mechanism of the crystals was considered by the polymerization time dependences of morphology, yield, density, DPn, crystallite size, and lattice constant as follows. When the DPn of the oligomers exceeded a critical value of about 3, they were precipitated to form the hexagonal lamellae. The lamellae grew very quickly to sheaf-like lozenge-shaped crystal through the screw dislocation with the continuous precipitation of the oligomer from the solution. Finally, further polymerization and the reorganization of polymer chains occurred in the precipitated crystals.