Melting and crystallization behavior of poly(vinylidene fluoride) produced isothermally in the intercrystalline spaces of poly(ethylene terephthalate) from its blends

Abstract

The melting of isothermally crystallized poly(vinylidene fluoride) (PVF₂), produced in the intercrystalline spaces of poly(ethylene terephthalate) (PET) from its blends, showed a unique behavior: the melting temperature decreased with the increasing crystallinity of PVF₂ (i.e., with increasing crystallization time) for PVF₂ volume fractions of 0.64 and 0.51. The melting temperature of already crystallized PET also decreased as the PVF₂ crystallization progressed and the isothermal crystallization temperature of PVF₂ increased. Separate reasons were proposed to account for these behaviors. The equilibrium melting temperatures of PVF₂ in the blends, measured by the Hoffman-Weeks extrapolation procedure, were used to calculate the polymer-polymer interaction parameter (χ₂₁); only the noncrystallized portion of PET contributing to the mixed amorphous phase was considered. The χ₂₁ value (−1.75) was lower than χ₁₂ (−0.14), calculated from the melting temperature depression of PET. However, when they were normalized to the unit volumes of the respective components, the two values were found to be the same. The crystallization rate of PVF₂ decreased with an increasing volume fraction of PET in the blend. The Avrami exponent increased for the volume fraction of PVF₂ (0.77) and then progressively decreased with an increasing volume fraction of PET. A gradual change in the nature of the regime transition from regime II/regime I to regime III/regime II with increasing PET concentration was observed. The value of the chain-extension factor of PVF₂ significantly increased with an increase in the PET concentration in the blends.