Effect of Ions on the Flow-Induced Crystallization of Poly(vinylidene fluoride)

The flow-induced crystallization of poly(vinylidene fluoride) (PVDF)–ion composites in the presence of externally added ions was investigated by in situ wide-angle X-ray diffraction (WAXD) and ex situ Fourier transform infrared (FTIR) spectroscopy. In this work, two types of hexadecyl trimethyl ammonium bromide (CTAB) and tetrabutylammonium hydrogen sulfate (TBAHS) ions with distinct electrostatic interactions were chosen at a weight concentration of 0.5% to design different influences on crystallization. For quiescent crystallization, TBAHS ions induced the appearance of the polar γ-phase with accelerated kinetics, but CTAB ions exhibited no detectable influence in the crystallization of the nonpolar α-phase as in neat PVDF. Interestingly, flow induced the formation of β-phase with prominent piezo/ferroelectricity and the presence of ions could increase the amount of this flow-generated β-phase. With a strain of 2.7 applied at 1 s–1, the CTAB and TBAHS ions increased the crystallinity index of the β-phase obtained just after the flow to 2.4 and 3.6%, respectively, with respect to that of 2.0% in neat PVDF. In addition, the ions could increase the enhancement amplitude of the flow-generated β-phase by increasing the strain rate over a broad range from 0.01 to 10 s–1. Moreover, both CTAB and TBAHS ions induced the crystallization of the γ-phase during the isothermal process at a high temperature of 160 °C, where the α-phase appearing in neat PVDF was completely suppressed.