Effect of biaxial orientation on dielectric and breakdown properties of poly(ethylene terephthalate)/poly(vinylidene fluoride-co-tetrafluoroethylene) multilayer films

Polymer films with enhanced dielectric and breakdown properties are essential for the production of high energy density polymer film capacitors. By capitalizing on the synergistic effects of forced assembly nanolayer coextrusion and biaxial orientation, polymer multilayer films using poly(ethylene terephthalate) (PET) and a poly(vinylidene fluoride-co-tetrafluoroethylene) [P(VDF-TFE)] copolymer were produced. These films exhibited breakdown fields, under a divergent field using needle/plane electrodes, as high as 1000 kV mm1. The energy densities of these same materials, under a uniform electric field measured using plane/plane electrodes, were as high as 16 J cm3. The confined morphologies of both PET and P(VDF-TFE) were correlated to the observed breakdown properties and damage zones. On-edge P(VDF-TFE) crystals induced from solid-state biaxial stretching enhanced the effective P(VDF-TFE) layer dielectric constant and therefore increased the dielectric contrast between the PET and P(VDF-TFE) layers. This resulted in additional charge buildup at the layer interface producing larger tree diameters and branches and ultimately increasing the breakdown and energy storage properties. In addition to energy storage and breakdown properties, the hysteresis behavior of these materials was also evaluated. By varying the morphology of the P(VDF-TFE) layer, the low-field dielectric loss (or ion migration behavior) could be manipulated, which in turn also changed the observed hysteresis behavior. (c) 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 882896