Construction of the Coexistence System of Polymer Dispersed and Stabilized Liquid Crystal

Huai Yang*,Yanzi Gao,Cheng Zou, Meina Yu

Peking University

ABSTRACT: In order to meet the development of LCD devices towards light weight, low price, especially flexible thin film, and people's demand for multi-functional intelligent response film, liquid crystal/polymer composite film has become a research hotspot in the field of display and intelligent materials. This kind of film is expected to have both stimuli response and continuous production of large area flexible thin film products by roll to roll processing technology. At present, there are two main systems: polymer dispersed liquid crystal (PDLC) and polymer stabilized liquid crystal (PSLC).

In general, PDLC films are prepared from a mixture of liquid crystalline/non-liquid crystalline photopolymerized monomers (NLCM) sandwiched in two transparent ITO conductive plastic films by UV curing of NLCM, in which the liquid crystals are dispersed in the polymer matrix in the form of droplets. When the electric field is not applied, the liquid crystal molecules are randomly distributed in the polymer matrix interface, and the PDLC film is in a strong light scattering state; after the electric field is applied, the long axis of the liquid crystal molecules is parallel to the direction of the electric field, and the PDLC film is transparent. The polymer matrix content of practical PDLC film is generally between 40~70 wt%, and the peel strength between two layers of ITO film is high, so it could be prepared into large area film by roll to roll processing. It has been widely used in the fields of building and automobile doors and windows, glass curtain wall, large projection screen and touch screen. However, due to the high content of polymer matrix, the anchoring effect of its interface on the liquid crystal molecules is strong, and the driving voltage of PDLC film is high, and its saturation voltage is generally above 60 V. Therefore, the current PDLC membrane has high energy consumption, poor security, and is difficult to be used in the display field.

PSLC film is a liquid crystal/polymer network composite system formed by liquid crystal/liquid crystal photopolymerization monomer (LCM) mixture after the formation of specific molecular orientation. The polymer network has a stable effect on the initial orientation of liquid crystal molecules. PSLC film has the characteristics of low driving voltage and fast response speed, but the polymer network content in the system is generally low, usually about 3~5 wt%, otherwise the liquid crystal molecules will be strongly bound by the polymer network and difficult to drive. Therefore, the peeling strength between the two substrates is very low, so it is difficult to carry out large-area flexible processing, so it can only be used in glass substrate.

Based on the difference of reaction rate between non-liquid crystalline and liquid crystalline monomers in liquid crystal, a coexisting system of a polymer dispersed and a stabilized liquid crystal (PD&SLC) systems with both PDLC and PSLC microstructures was constructed by a second photopolymerization method of first polymerizing the non-liquid crystalline monomers and then the liquid crystalline monomers. Based on the high throughput computation, the photopolymerization monomer which could significantly reduce the driving voltage of the coexisting system could be selected, which could significantly improve the electro-optical properties of the coexisting system, and lay a foundation for the development of novel light transmittance adjustable thin films and liquid crystal display devices.

Keywords: Liquid crystal; Polymer; Coexisting systems

* Corresponding author: yanghuai@pku.edu.cn