Polarization characteristics of polymer dispersed liquid crystal films and their effects on electro-optical properties

Abstract

The electro-optical properties of polymer dispersed liquid crystal film vary with liquid crystal content and externally applied electric field, but the analysis of the film morphology cannot directly reflect the mechanism of electro-optical properties. Therefore, the polymer dispersed liquid crystal film prepared by blending liquid crystal material E7 and photopolymer NOA65 is used. Herein, the dielectric polarization regulated electro-optical properties and their related mechanisms under different liquid crystal content and electric fields are revealed. The results show that in a frequency range of 10^–1–10⁶ Hz, the film exhibits three relaxation processes respectively at low frequency, medium frequency and high frequency, which are generated by thermionic polarization, interfacial polarization and orientation polarization. According to the Arrhenius equation, the activation energy values of such polarization processes are calculated. It is found that with the increase of liquid crystal content, the activation energy of orientation polarization decreases from 0.88 eV to 0.83 eV, leading the threshold field strength and the saturation field strength of the diversion of liquid crystal molecule to decrease. Thermionic polarization under DC electric field forms an internal electric field, which causes the threshold field strength and saturation field strength to increase greatly, as compared with the scenarios under AC electric field. Such a thermionic polarization also leads the polarization relaxation time to increase, resulting in the extension of response time. This study is of guiding significance in further analyzing and improving the electro-optical properties of polymer dispersed liquid crystal films.