Cocrystallization mechanism of vinylidene fluoride-tetrafluoroethylene copolymers with different copolymer composition

Abstract

The crystallization rate of two vinylidene fluoride(VF₂)-tetrafluoroethylene(VF4) copolymers (VF₂/VF₄ 90/10) (Cop-1), (VF₂/VF₄ 86/14) (Cop-2) and their cocrystals of different compositions was measured by differential scanning calorimetry (DSC). At same undercooling (ΔT) the crystallization rate of cocrystals is faster than that of the pure components. The Avrami exponent varies from 1.8-0.4. Analysis of the crystallization rate using the Lauritzen-Hoffman (L-H) growth rate theory indicates that the crystallization is occurring in a single regime and the product of surface free energies (σσ_e) is lower for the cocrystals than those of the pure components. Assuming σ_e as invariant for the samples, σ values are calculated and are interpreted according to a recent theory of Hoffman et al. The chain characteristic ratio (C_∝) calculated from the kinetic data indicates chain extension in the melt of the blend. The entropies of cocrystallization are also calculated from the kinetic data and amount to ca. 0.4 cal · mol⁻¹ . K⁻¹.