Electronic structures of crystalline phases in poly(vinylidene fluoride) films probed by far- and deep-ultraviolet spectroscopy and quantum chemical calculations

Abstract

titled "Electronic struPoly(vinylidene fluoride) (PVDF) is a widely used polymer well known for its remarkable piezoelectric and pyroelectric properties. Although elucidating the electronic structures of PVDF polymorphs has been challenging due to their exclusive absorption in the far-ultraviolet (FUV, <200 nm) region, the present study successfully characterizes these structures for the first time. Herein, the electronic structures of crystalline phases in PVDF films were investigated by FUV and deep-ultraviolet (DUV, <300 nm) spectroscopy in combination with quantum chemical calculations. By systematically controlling the annealing temperature and spin-coating speed, phase evolution between the α and β crystalline forms was induced. Characteristic absorption bands were observed at approximately 150 and 200 nm in the FUV and DUV regions, respectively. FUV absorption of PVDF was successfully detected for the first time using attenuated total reflectance spectroscopy, which enables measurements of strongly absorbing condensed phases. Transmission spectroscopy clearly resolved the temperature- and rotation-dependent evolution of the 200 nm band in the DUV region, revealing a progressive blue shift and spectral narrowing with increasing annealing temperature and spin-coating speed. Quantum chemical calculations demonstrated that intermolecular interactions are essential for reproducing the experimentally observed spectral features. In particular, a reduced interchain distance leads to significant destabilisation of the highest occupied molecular orbital level and concomitant modulation of the electronic transition energy. These results establish that crystalline phase transitions in PVDF can be directly probed through electronic transitions in the FUV and DUV regions, providing new insight into the phase-dependent electronic structures of semicrystalline polymers. ctures of crystalline phases in poly(vinylidene fluoride) films probed by far-and deep