Gradient scaling phenomenon of piezoelectricity in the trapezoid shape non-piezoelectric Polyvinylidene Fluoride (PVDF) homopolymer films
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In this work, we are demonstrating a new phenomenon of increased effective piezoelectric coefficient (d eff ) by exploiting flexoelectric effect in two forms of non-piezoelectric, semi-crystalline polyvinylidene fluoride (PVDF) films by reducing its size proportionally. PVDF has been classified into four major molecular conformation based on its repeated arrangement of [-CH2-CF2-] n unit cells within the amorphous region. Of the four groups, only the alpha phase is non-polar; other three groups are polar in nature. In our experiment, first one is a α-phase PVDF film which is transparent in nature and second one is a mixture of α- and β-phase PVDF film (or simply α ' -phase), with micro pores and micro bubbles are randomly arranged within them. Both the PVDF films are fabricated using free radical polymerization technique under different physical condition. The molecular conformations of both the PVDF films are in minimum energy state because they are neither stretched nor poled using any external method which results in increased life of polymer film. The tensile stretching experiment is performed in trapezoid shape polymer film. Application of uniform load to a non-uniform shape will lead to generation of strain gradient, and the net effect is due to flexoelectricity. The experiment is performed in α-phase and α ' -phase of PVDF films separately. As the size of trapezoid shape PVDF film is proportionally scaled down from its original size to a ratio of 0.4, we observed an increase of 1.8 times and 2.74 times in the effective d eff response of α-phase and α ' -phase respectively. This result gives an insight into the structure of PVDF film, it demonstrates that the scaling phenomenon depends not only on the shape of the film but also on the material property of the film and also suggests that these films can generate an increased effective piezoelectric response if its size is reduced to micrometer range. The fabricated low cost film can be attached to any flexible surface for micro-level sensor or actuator application with an enhanced performance.