Optical and wetting properties of nanostructured fluorinated ethylene propylene changed by mechanical deformation and its application in triboelectric nanogenerators

Abstract

We demonstrate that nanostructured fluorinated ethylene propylene (FEP) polymer films undergo a non-reversible structural transition when exposed to an applied force. While reactive ion etching (RIE) treatment creates FEP films with well-defined nanostructures, applied stress causes a permanent deformation which alters the optical reflectance spectrum. The structural changes of the FEP films also altered the contact angles of water droplets. It was found that the contact angles changed from 109° before to 139° after RIE treatment, and plastic deformation reduced the contact angles to 111°. Scanning electron microscopy images revealed freshly formed homogeneous surfaces, with nanostructures hidden below, which correlated with the macroscopic changes in optical reflectance. Interestingly, the contact electrification between FEP and aluminum did not change when the nanostructures were deformed, and we propose that the nanostructured FEP surface can be used to both optically monitor the state of the nanostructure as well as functioning as a part of an energy harvesting system. At an average power of about 22 μW with an area of 4 cm2, the energy harvester is able to light up a large number of light emitting diodes.