Improved Energy Storage Density of Polypropylene Film via Surface Functionalization

The increasing demands for integration and miniaturization of capacitors have placed requirements on enhancing the energy storage density of biaxially oriented polypropylene (BOPP) films. This study proposed two surface functionalization methods of ultraviolet (UV) irradiation and surface grafting of acrylic acid (AA) respectively. By adjusting the treatment time, the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">${\text{BOPP}}_{\text{uv}}$</tex> films and BOPPuv-AA films treated for 10 s both achieved the highest energy storage density, with values of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$4.25\ \mathrm{J} / \text{cm}^{3}$</tex> and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$4.88\ \mathrm{J} / \text{cm}^{3}$</tex> at <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$700\ \text{kV} / \text{mm}$</tex>. The microstructure of the modified films was analyzed using Fourier transform infrared spectrometer and water contact angles measurement, and the molecular simulation was further implemented. Results showed that the surface functionalization could introduce polar groups and more deep traps were brought by grafted AA groups compared with UV irradiation, which suppressed the transportation of charge carriers and thus enhanced the energy storage density.