A METHOD FOR STUDYING CONFORMATIONAL RELAXATIONS BY MOLECULAR SIMULATION - CONFORMATIONAL DEFECTS IN ALPHA-PHASE POLY(VINYLIDENE FLUORIDE)

A general method is proposed for the molecular level characterization of relaxation and transformation processes in polymer crystals mediate d by propagation of conformational imperfections. The proposed approac h combines a definition of the defect ensemble, in terms of the extent and boundary conditions of the defect zone, with procedures for searc hing conformation space for stable conformations in the ensemble and f or determining the adiabatic reaction path connecting one symmetry-equ ivalent position of the defect in the lattice to another. The method w as applied to the alpha(c) relaxation that occurs in the crystalline a phase of PVDF (alpha-PVDF). A large number of possible defects were f ound with a wide spectrum of heats of formation. Several of the lowest energy defects could be packed efficiently into the alpha-PVDF crysta l. The adiabatic pathway for defect transport was estimated and vibrat ional contributions to the thermodynamic functions were obtained at al l the stationary states. Two distinct relaxation processes were found. One exhibits a large intermolecular packing energy contribution to it s heat of formation and must traverse free-energy barriers of 6-9 kcal /mol to propagate through the crystal, while the other exhibits a larg e intramolecular conformational energy contribution to its heat of for mation but may propagate with free-energy barriers of about 2.4 kcal/m ol. The intermediate states far this latter process involve short all- trans segments, suggesting that this may also be a mechanism for the a lpha-to-gamma and alpha-to-beta phase transitions.