Photopolymerization Kinetics And Electro-optic Properties Of The Polyacrylate Dispersed Liquid Crystal Composites

With the development of holographic technology, holographic polymer dispersed liquid crystals (PDLCs) emerge as the times require. These materials can be prepared into thick films through photopolymeriztion inducing phase separation, and the properties of PDLCs are closely related to their photopolymerization kinetics and phase separation. These materials can be used in a wide array of applications such as display, swiching, smart window, and so on.In this dissertation, polyacrylate dispersed liquid crystal composites were prepared through photopolymeriztion inducing phase separation in mixture systems including 2-ethylhexyl acrylate (EHA), trimethylolpropane trimethacrylate (TMPTA) and nematic liquid crystal (LC, P0616A) in the presence of 2, 4, 6-trimethylbenzoyldiphenyl phosphine oxide (TPO) as photo-initiator. The photodifferential scanning calorimetry (P-DSC) and differential scanning calorimetry (DSC) were used to characterize the photopolymerization kinetics and phase separation behaviors of polyacrylate dispersed liquid crystal composites, respectively. And the influence of liquid crystals contents and surfactant, average functionality on electro-optic properties of the composites were investigated. Then, the polyhedral oligomeric silsesquioxane (POSS) was added to the polyacrylate dispersed liquid crystal composite systems in order to improve their thermal stability. The effects of POSS on photopolymerization kinetics, electro-optic properties, microstructure, and thermal stability of the composites were studied. The main results are as follows:1. The rate of photopolymerization is faster than that of traditional free radical polymerization, the conversion of the above mixture system reaches the plateau in about 50s, and there are maxima in photopolymerization rate and apparent kinetic constant. All final conversion and maximum photopolymerization rate increase with the reaction temperature. When the reaction temperature rises to 20oC, the above values increase slightly. In addition, Rpmax decreases with LC content, and the conversion increases slowly with reaction time. Noticeably, the final conversions for the above mixtures have little difference at different LC content, and all of them are closed to 80%. With UV intensity strengthening, both Rpmax and the final conversion increase obviously, and the threshold value of UV intensity is 4 mW/cm2. However, Rpmax and the final conversion tend to decrease with the average functionality of monomers. The final conversion increase obviously with the reaction time, and it can be indicated some traces of dark reaction after the illumination turned off when the reaction time between 60s and 180s.2. The phase diagram of the acrylate monomer/LC mixture before cured and the degree of phase separation after cured was studied by differential scanning calorimetry (DSC). The results showed that the phase separation temperatures of the mixture increase with the LC content. And the LC solubility limitβin the polyacrylate is 0.29 at room temperature.3. With the content of liquid crystal increasing, both the threshold voltage and saturation voltage of the composite film tend to decrease. However, the two type of voltage increase with the average functionality. Surfactant reduces the anchoring energy and the phase separation of the composites, which results in the threshold voltage being lowered slightly.4. With POSS content increasing, the Rpmax of polyacrylate dispersed liquid crystal composites tends to decrease and liquid crystals droplet becomes small. The POSS enhances the thermal stability of the composites, while the rigid cage-like structure of POSS increases their switching voltages.