In this study, we report the melting behavior of poly(vinylidene fluoride) (PVF2) annealed in a differential scanning calorimeter. PVF2 annealed under isothermal conditions often shows double or triple melting endotherms depending on the annealing temperature (Ta) and the heating rate. The lower melting peak temperature increases as Ta increases. When the annealing time is varied, there is a systematic increase in the size of the lower endotherms. This suggests that a portion of the main endothermic response is due to reorganization during the scan. Annealing PVF2 not only increases the degree of crystallinity, but also improves the crystal perfection. The ability of an annealing sample to reorganize decreases as the annealing time increases. However, an additional third melting peak appears when PVF2 is annealed at 140°C for a sufficiently long time. The existence of this peak suggests that more than one kind of distribution of crystal perfection may occur when PVF2 is quenched from the melt into liquid nitrogen and subsequently annealed. 相似文献
An increase in the dielectric constant (800 ± 15; 100 ≤ f ≤ 104 Hz) of PVDF without a coresponding increase in the dielectric loss is reported. This is realized by preparing sandwich structures composed of CNF/PVDF composite layers and a pure PVDF interlayer. The influence of the interlayer thickness on the dielectric properties is investigated. It is shown that a 30 µm interlayer is sufficient to prevent formation of the conductive network in the sandwich structures that would result in lower loss. It is demonstrated that with better CNF dispersion, a further increase of the dielectric constant and a lower loss can be obtained simultaneously. The sandwich approach thus leads to nanocomposites with enhanced dielectric constants while maintaining low loss.
