Influence of the frequency of external applied voltage on the dielectric anisotropy of liquid crystal

Supported by National Natural Science Foundation of China (No. 11374087 and 11504080); Natural Science Foundation of Hebei Province (No. A2014202123 and A2017202004); Research Project of Hebei Education Department (No. QN2014130); Research and practice project of Hebei Province Higher Education and Teaching Reform(No.2017GJJG018);Project of College Students Innovation and Entrepreneurship Training Program (No.20170080289);Key Subject Construction Project of Hebei Province University. further investigations of the influence of frequency on the dielectric constant of liquid crystal materials, firstly, the thickness of the liquid crystal cells and the thickness of the polymide (PI) orientation layers were measured by UV spectrophotometer (METASH UV-9000S) and Surface Profiler (ContorGK-T) respectively. The precision hot-stage (LTS 350) was used to control the experimental temperature of 20, and the capacitance of parallel and vertical aligned nematic liquid crystal cells were measured by a precision LCR meter (Agilent E4980A) at the frequency of 100 similar to 2 000 Hz for four different liquid crystal materials. Then, the parallel and vertical dielectric constants of liquid crystals at different frequencies were calculated through the capacitance model of LC cell, the curves of dielectric constant versus frequency were plotted as well. Finally, the influence of the frequency of external applied voltage on the dielectric anisotropy of liquid crystal was analyzed. The experimental results showed that the dielectric anisotropy of the positive liquid crystal decreased with the increase of the frequency at a certain temperature first, and then gradually flatten out, while dielectric anisotropy of negative liquid crystal remained unchanged with the change of frequency. This research has a guiding significance for further analysis of the dielectric characteristics of liquid crystal materials.