The role of molecular defects on the structure and phase transitions of poly(vinylidene fluoride).

Having prepared poly(vinylidene fluoride) (PVF(2)) with 0. 2-23.5 mol % head-head/tail-tail (HHTT) defects (as compared to the commonly obtainable 3.5-6.0 mol %), we examined the influence of such regiodefects on crystal structure, polymorphism, and Curie transitions. The ambient-temperature structure changes from alpha to beta as the HHTT content is increased beyond ~ 11.4 mol %. At that composition, either of these two phases may predominate depending upon temperature of crystallization (higher ones favoring beta). For defect contents > 15.5 mol %, the polymer chains show progressive intramolecular disorder, yet are packed with increasing intermolecular order in a pseudohexagonal lattice. When PVF(2) containing 13.5-15.5 mol % regiodefects is heated, clear ferro-electric-to-paraelectric phase transformations are obtained; these are reversible (with thermal hysteresis) upon subsequent cooling. This confirms our previous findings of Curie transitions in PVF(2) copolymers with tri- or tetra- fluoroethylene, and shows that such Curie behavior is an inherent property of PVF(2) itself.