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

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 (Delta T) 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-III transition for KY PVF2 and their cocrystals. The product of surface free energies (sigma sigma(e)) is lower for the cocrystal than that of the pure component. Assuming the fold surface energy (sigma(e)) as invariant for the above two PVF2 samples, the lateral surface energy (sigma) values are calculated and have been interpreted according to a recent theory of Hoffman et al. (Macromolecules, 1992, 25, 2221) 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 similar to 0.6 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 Delta T has been presented. (C) 1998 Elsevier Science Ltd. All rights reserved.