Increasing pretilt angle by grafting hexafluorobutyl acrylate into the surface of polyimide alignment films via electron beam irradiation

In this study, the polyimide (PI) alignment film whose surface contained fluorinated side chain was prepared by a new method. The fluorine compound, hexafluorobutyl acrylate (HFBA), was grafted into the surface of main-chained PI film as side chain via electron beam irradiation. The pretilt angle of liquid crystal (LC) was significantly improved from 2° to 20° or above. It was indicated by X-ray photoelectron spectroscopy spectra and attenuated total reflection Fourier transform infrared spectroscopy spectra that HFBA was successfully grafted into the thin surface layer of PI alignment films in the form of covalent bond. Scanning electron microscopy images showed that the irradiation process had no obvious influence on the surface morphology of the PI film. The surface energy of the PI film decreased from 49.85 to 31.50 mN/m after surface modification. Besides, the reason for the increase of pretilt angles was discussed.