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

Oriented poly(vinylidene fluoride) (PVDF) films with beta-form crystals hav e been commonly prepared by cold drawing of a melt-quenched film consisting of alpha-form crystals. In this study, we have successfully produced highl y oriented PVDF thin films (20 mu m thick) with beta-crystals and a high cr ystallinity (55-76%), by solid-state coextrusion of a gel film to eight tim es the original length at an established optimum extrusion temperature of 1 60 degrees C, some 10 degrees C below the melting temperature. The resultan t drawn films had a highly oriented (orientation function f(c) = 0.993) fib rous structure, showing high mechanical properties of an extensional elasti c modulus of 8.3 GPa and tensile strength of 0.84 GPa, along the draw direc tion. Such highly oriented and crystalline films exhibited excellent ferroe lectric and piezoelectric properties. The square hysteresis loop was signif icantly sharper than that of a conventional sample. The sharp switching tra nsient yielded the remnant polarization P-r of 90 mC/m(2), and the electrom echanical coupling factor k(t) was 0.24 at room temperature. These values a re about 1.5 times greater than those of a conventional beta-PVDF film. Thu s, solid-state coextrusion near the melting point was found to be a useful technique for the preparation of highly oriented and highly crystalline bet a-PVDF films with superior mechanical and electrical properties. The morpho logy of the extrudate relevant to such properties is discussed. (C) 1999 Jo hn Wiley & Sons, Inc.