Polymorphism of poly(vinylidene fluoride): potential energy calculations of the effects of head-to-head units on the chain conformation and packing of poly(vinylidene fluoride)

Potential-energy calculations have been used to investigate the effects of head-to-head defects on the chain conformation and packing of poly(vinylidene fluoride). The calculations have demonstrated that such positional isomerism can contribute to the polymorphism exhibited by PVF2. We have shown that increased concentration of head-to-head defects or tetrafluoroethylene comonomer can cause the phase-I form to become the favored structure for poly(vinylidene fluoride). The calculations have also proven capable of reflecting some of the subtle changes arising from the insertion of such defects. Packing calculations have shown that isomorphous replacement significantly affects the way in which PVF2 chains pack into a crystalline array. The differences in the structures proposed by Tadokoro and by Doll and Lando for phase II of poly(vinylidene fluoride) have been attributed to the inclusion of head-to-head defects in the polymer chains. In general, the potential-energy functions have proven useful qualitatively, if not semiquantitatively, in investigating the relation between chains in a crystal. Finally, these calculations have shown that both conformation and packing must be considered in determining the minimum energy structure for a polymer, and that these factors cannot be treated independently.