The influence of fluorine atoms introduced into the surface of polyimide films by direct fluorination on the liquid crystal alignment

Abstract

Fluorinated bulk polyimide (PI) films are usually preferred as alignment layers in liquid crystal displays. We performed a novel approach to obtain surface-fluorinated PI films by fluorine/nitrogen (F₂/N₂) direct fluorination. The results of attenuation total reflection Fourier Infrared Spectrum test (ATR-FTIR) and X-Ray photoelectron spectroscopy indicate that fluorine atoms were introduced into the surface of PI films, and the surface energy of the PI films increased from 47.50 to 69.86 mN m⁻¹. At the same time, the volume of the F atom is larger than that of the H atom, which results in in-situ enhancement of the rigidity of the side chains and alkyl spacers relative to the surface of the PI film, which has taken more upstanding conformation after fluorination. Unexpectedly, the pretilt angle of the PI alignment films can be enormously improved by fluorination by more than 20°, and the magnitude of pretilt angle depends on the content of fluorine atoms introduced.

Keywords:

• polyimide alignment film
• direct fluorination
• pretilt angle

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Grant Nos. 50773044 and 50433010).