Enhanced electrical properties of highly oriented poly(vinylidene fluoride) films prepared by solid-state coextrusion

Abstract

Oriented poly(vinylidene fluoride) (PVDF) films with β-form crystals have been commonly prepared by cold drawing of a melt-quenched film consisting of α-form crystals. In this study, we have successfully produced highly oriented PVDF thin films (20 µm thick) with β-crystals and a high crystallinity (55–76%), by solid-state coextrusion of a gel film to eight times the original length at an established optimum extrusion temperature of 160°C, some 10°C below the melting temperature. The resultant drawn films had a highly oriented (orientation function fc = 0.993) fibrous structure, showing high mechanical properties of an extensional elastic modulus of 8.3 GPa and tensile strength of 0.84 GPa, along the draw direction. Such highly oriented and crystalline films exhibited excellent ferroelectric and piezoelectric properties. The square hysteresis loop was significantly sharper than that of a conventional sample. The sharp switching transient yielded the remnant polarization Pr of 90 mC/m2, and the electromechanical coupling factor kt was 0.24 at room temperature. These values are about 1.5 times greater than those of a conventional β-PVDF film. Thus, solid-state coextrusion near the melting point was found to be a useful technique for the preparation of highly oriented and highly crystalline β-PVDF films with superior mechanical and electrical properties. The morphology of the extrudate relevant to such properties is discussed. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2549–2556, 1999