Fractionation and crystal morphology of rigid polymer, poly(p-phenylene benzobisthiazole)

Fractionation of the rigid polymer, poly(p-phenylene benzobisthiazole) (PBZT), was carried out in dilute solution in concentrated methane sulfuric acid (MSA) using silica gels as packing material of a column. Several combinations of the average chain length of the fractionating materials and the average pore diameter of the gels were examined to improve fractionation resolution. The gels with average pore diameter near the average chain length resulted in high fractionation resolution. Single crystals of the fractionated and unfractionated PBZTs were observed by transmission electron microscopy (TEM). Both single crystals were fundamentally composed of rod crystals with the chain orienting normal to the rods. The unfractionated PBZT made a cluster of parallel rod crystals, where longer chains penetrated a few rod crystals leaving their chain ends within the crystalline core. On the contrary, with the fractionated polymer, extended-chain rod-like crystals were dispersed, isolated from each other. This morphology enables us to estimate the chain length visibly by TEM, for which a few milligrams of the material is enough for the observation.