A Method for Studying Conformational Relaxations by Molecular Simulation: Conformational Defects in α-Phase Poly(vinylidene fluoride)

Abstract

A general method is proposed for the molecular level characterization of relaxation and transformation processes in polymer crystals mediated by propagation of conformational imperfections. The proposed approach combines a definition of the defect ensemble, in terms of the extent and boundary conditions of the defect zone, with procedures for searching conformation space for stable conformations in the ensemble and for determining the adiabatic reaction path connecting one symmetry-equivalent position of the defect in the lattice to another. The method was applied to the αc relaxation that occurs in the crystalline α phase of PVDF (α-PVDF). A large number of possible defects were found with a wide spectrum of heats of formation. Several of the lowest energy defects could be packed efficiently into the α-PVDF crystal. The adiabatic pathway for defect transport was estimated and vibrational contributions to the thermodynamic functions were obtained at all the stationary states. Two distinct relaxation processes were found. One exhibits a large intermolecular packing energy contribution to its heat of formation and must traverse free-energy barriers of 6−9 kcal/mol to propagate through the crystal, while the other exhibits a large intramolecular conformational energy contribution to its heat of formation but may propagate with free-energy barriers of about 2.4 kcal/mol. The intermediate states for this latter process involve short all-trans segments, suggesting that this may also be a mechanism for the α-to-γ and α-to-β phase transitions.