An Analysis of the Photo-Polymerization Induced Phase Separation Process in Liquid Crystal/Polymer Composite Films
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the relation between the phase separation process during photo-polymerization and factors on materials including molecular structure, solubility, and curing condition. EXPERIMENT To prepare liquid crystal/polymer composite films, a homogeneous solution containing the liquid crystal mixture of RO-571 manufactured by RODIC Inc. as shown in Figure 1 and various diacrylates having the different alkyl main chain as shown in Figure 2 were used as a starting material. A 4 mol% of photoinitiator (Irg-651 manufactured by Chiba-Gaigy) was added to the total amount of diacrylate. After filling materials in a LC cell with a IOjim or a 50rtm cell gap, 10 to 400 w/m 2 of UV light at 365nm was irradiated through UV-35 optical sharp-cut edge filter from HOYA Co., Ltd using a high pressure mercury lamp at a 2 *C above isotropic-nematic transition temperature to induce the polymerization. The phase transition temperatures in the homogeneous solution were observed by crosspolarized and phase-contrast optical microscopy. The homogeneous solution was placed on a hot stage (ME'ITLER, FP-82HT) controlling a temperature between 80 'C and -60 'C with a cooling rate of -2*C/min. The cross-polarized mode was employed to observe the isotropic-nematic transition of the liquid crystal domains below Tm, and the phase-contrast mode was used to observe the phase-separation above Tm, since the difference of refractive indices for the two domains can be 85
Mat. Res. Soc. Symp. Proc. Vol. 559 01999 Materials Research Society
observed. The phase transition temperatures in the cured films with various UV-exposure time is also observed. The light-scattering measurement was carried out to observe the phase separation process during photo-polymerization. The detailed setup is shown in Figure 3.A He-Ne laser of 5mW was used as the light source with the wavelength of 632.8nm for the detection of light-scattering profile. A sample injected to LC cell with a 6im cell gap was placed on the hot stage of the apparatus at a constant temperature. The change in the light-scattering profile was recorded at 0.05 sec interval using a CCD camera during UV light-irradiation (100 or 400 W/m2 ). An observation by scanning electron microscopy (SEM) was performed to identify morphology in the films, and to analyze a phase-separation process. A homogeneous solution placed in LC cell with a 50 gm cell gap was cured with a 400W/m2 of UV-light for several seconds. A 20W/ m2 of UV-light intensity was selected for an analysis of phase-separation process to suppress a rate of phase separation. After curing, a sample was braked under the liquid-nitrogen, and then extract liquid crystals from the films with methanol. A surface tension, an contact angle, and a solubility parameter were evaluated to estimate the affinity for liquid crystals and monomers. A surface tension is measured with the platinum plate method using the high sensitive surface tension meter. An contact angle in a drop of liquid crystals placed on the polymer thin films is observed.
CN
45 wt%
C7H15"=O-GCCN
29
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