Cocrystallisation in blends of poly(vinylidene fluoride) samples: 4. Kinetics study

Abstract

The overall crystallisation rates of poly(vinylidene fluoride) (PVF₂) samples [KF PVF₂; head to head defect = 3.5 mol%, KY PVF₂ head to head defect = 5.3 mol%] and their cocrystals are measured. At the same undercooling (ΔT) the crystallisation rate of cocrystals is faster than that of the pure component with identical head to head defect concentration. The isothermal temperature range and the Avrami exponent vary linearly with composition. Analysis of the crystallisation rate using Lauritzen-Hoffman growth rate theory indicates regime I-II transition of KF PVF₂ sample and regime II-Ill transition for KY PVF₂ and their cocrystals. The product of surface free energies (σσ_e) is lower for the cocrystal than that of the pure component. Assuming the fold surface energy (σ_e) as invariant for the above two PVF₂ samples, the lateral surface energy (σ) values are calculated and have been interpreted according to a recent theory of Hoffman et al. The results indicate a small extension of the PVF₂ chains in the melt of the blend. The entropy of cocrystallisation has been calculated from the above theory and has been found to be 10.6 cal deg⁻¹ mol⁻¹. A model for explaining the higher crystallisation rate of the cocrystal compared with that of the pure components at the same ΔT has been presented.