Crystal-structure dependence of electroactive properties in differently prepared poly(vinylidene fluoride/hexafluoropropylene) copolymer films

The electroactive properties of two random copolymers of vinylidene fluoride (VDF) and hexafluoropropylene (HFP) were studied. The compositions were 95/5 and 85/15 mol % P(VDF/HFP). For each composition, three different film-preparation methods were used-solvent casting, melt-pressed quenched, and melt-pressed slow-cooled. The ferroelectric properties observed were strongly dependent on the preparation methods of the films as well as the HFP molar content of the samples. The highest remanent polarizations (P-r) obtained from electric displacement versus electric field (D-E) hysteresis data are 80 and 50 mC/m(2) for the 5 and 15 % HFP solvent-cast samples, respectively. The slow-cooled samples do not exhibit any ferroelectric behavior for either the 5 or 15% HFP copolymers. It was also observed that both the 5 and 15% HFP slow-cooled samples have a smaller electrostrictive response relative to the other two types of samples. Wide-angle X-ray diffraction and DSC results suggest that the 5% HFP sample has a higher crystallinity relative to the 15% HFP sample for each preparation method. In addition, different crystal phases form in the samples resulting from the different preparation methods. Fourier transform infrared results suggest that the slow-cooled samples are in the nonpolar alpha phase, whereas the quenched and solvent-cast samples are more likely in the polar beta phase. The slow-cooled samples do not show a switching peak in their nonpolar alpha -phase crystalline state. (C) 2001 John Wiley & Sons, Inc.