Irradiation polymerization of polypropylenegly coldiacrylates/Liquid crystal e7 systems

Abstract

This contribution focuses on a detailed investigation of the relationship between the methods of polymerization/cross-linking, such as slow and rapid UV radiation, and high voltage accelerated electron beam (EB), and the resulting physical properties including phase diagrams, polymerization and phase separation kinetics, morphologies, thermal properties and electro-optical responses of polypropyleneglycoldiacrylate (PPGDA800) and ripropyleneglycoldiacrylate (TPGDA) monomers, in the presence of the nematic liquid crystal E7, used to prepare a polymer dispersed liquid crystal (PDLC). Based on the phase diagrams obtained by polarized optical microscopy, 60wt.% E7 was found to be the limit of solubility of the LC in PPGDA800. The longer the spacing between the double bonds in PPGDA800, the more rapid was the photopolymerization under both UV systems; nevertheless, the opposite was proved under EB. More homogenous and regular morphologies were obtained under the more homogeneous EB curing; whereas, irregular ones resulted under the heterogeneous photopolymerization. The electro-optical responses of various polymer dispersed liquid crystals (PDLCs) systems exhibited remarkable differences between the UV-cured samples and those cured by the EB technique. It was found that the threshold and saturation voltages considerably increased in the case of the UV-cured systems. Other results involving the contrast ratio, which is higher for EB-cured systems, confirm their higher quality, although the rapid photopolymerization UV source was employed, which slightly improved the electro-optical responses. Moreover, EB curing leads to high enough conversions without a photoinitiator, which may act as an impurity that might have a strong impact on the electro-optical performance and also lead to the discoloration of the obtained PDLCs.