Fluoroethylenepropylene ferroelectrets with superimposed multi-layer tubular void channels

Stacked ferroelectret film systems have been proposed as a simple solution to increase the piezoelectricity beyond that of a single ferroelectret layer. Recent developments in ferroelectret research have also shown that organized void structures can be produced by thermal lamination of polymer films. Here, the two approaches are combined to produce multi-layer ferroelectrets with controlled voids. The multi-layer ferroelectrets are produced by arranging Teflon^® fluoroethylenepropylene (FEP) films alternately with Teflon^® polytetrafluoroethylene (PTFE) templates. The five- or seven-layer stacks are laminated at an elevated temperature between the melting temperatures of the two fluoropolymers in order to fuse the FEP films, but not the templates. After removing the templates, a polymer system with superimposed open channels is formed. In this work different multi-channel-layer structures are investigated under several charging voltages. The results indicate that ferroelectrets with two layers of tubular channels show enhanced piezoelectricity. However, for three channel layers, the piezoelectric d₃₃ decreased in comparison with only two channel layers. As an explanation, we propose that an insufficient increase in the charge density does not allow to fully compensate the higher elastic modulus of the polymer system that contains more films.