Development of oriented structure and properties on drawing of poly(vinylidene fluoride) by solid-state coextrusion

Gel films of poly(vinylidene fluoride) (PVDF) consisting of alpha -form crystals were drawn uniaxially by solid-state coextrusion to extrusion draw ratios (EDR) up to 9 at an optimum extrusion temperature of 160 degreesC, about 10 degreesC below the melting temperature (T-m). The development of an oriented structure and mechanical and electrical properties on coextrusion drawing were studied as a function of EDR. Wide-angle X-ray diffraction patterns showed that the a crystals in the original gel films were progressively transformed into oriented beta -form crystals with increasing EDR. At the highest EDR of 9 achieved, the drawn product consisted of a highly oriented fibrous morphology with only beta crystals even for the draw near the T-m. The dynamic Young's modulus along the draw direction also increased with EDR up to 10.5 GPa at the maximum EDR of 9. The electrical properties of ferroelectricity and piezoelectricity were also markedly enhanced on solid-state coextrusion. The D-E square hysteresis loop became significantly sharper with EDR, and a remanent polarization P-r of 100 mC/m(2) and electromechanical coupling factor along the thickness direction k(t) of 0.27 were achieved at the maximum EDR of 9. The crystallinity value of 73-80% for the EDR 9 film, estimated from these electrical properties, compares well with that calculated by the ratio of the crystallite size along the chain axis to the meridional small-angle X-ray scattering (SAXS) long period, showing the average thickness of the lamellae within the drawn beta film. These results, as well as the appearance of a strong SAXS maximum, suggest that the oriented structure and properties of the beta -PVDF are better explained in terms of a crystal/amorphous series arrangement along the draw axis. Further, the mechanical and electrical properties obtained in this work are the highest among those ever reported for a beta -PVDF, and the latter approaches those observed for the vinylidene fluoride and trifluoroethylene copolymers. (C) 2001 John Wiley & Sons, Inc.