STUDY OF FREE-VOLUME AND CRYSTALLINITY IN POLYBITHIOPHENE AND POLY(3-METHYLTHIOPHENE)

Compressed pellets of partly crystalline, chemically synthesized, dope d (Cl- and FeCl4-) polybithiophene (PBTd), poly(3-methylthiophene) (P3 MTd), and their neutral (dedoped) forms (PBTn and P3MTn) were studied by wide-angle x-ray diffraction and positron annihilation lifetime spe ctroscopy. As synthesized, PBTd and P3MTd polymers have a helical syn conformation, they crystallize in the hexagonal system. On dedoping, P BT macromolecules change their helical syn conformation in a rodlike a nti conformation and crystallize in the orthorhombic or monoclinic sys tem, whereas P3MT macromolecules retain their helical syn conformation . Chemical doping-dedoping cycles lead to amorphous PBT and P3MT in ei ther doped or dedoped states. The P3MT helical macromolecule behaves l ike a spiral spring; by doping, it becomes axially compressed. The uni t-cell volume of P3MTd is smaller than that of P3MTn. The positron lif etime spectra for all polymers were resolved, without constraint, into three components. The tau1 lifetime is attributed to free-positron an nihilation events, the tau2 lifetime to positrons annihilating trapped in voids, and the tau3 lifetime to positrons annihilating as o-Ps tra pped in cavities located inside the polymer grains for P3MTn and at th e surface of the grains for PBTd, PBTn, and P3MTd. Most positrons anni hilate when trapped in voids, both in doped and dedoped PBT and P3MT. The doping apparently increases the concentration of the voids and the ir mean diameter in P3MT, and probably also in PBT. Cavities anchored in the bulk are produced by dedoping. (C) 1993 John Wiley & Sons, Inc.