Spectroscopic Studies of Dyes in Liquid Crystal Impregnated Microporous Polypropylene Films
Microporous polypropylene films such as Celgard 2400 (Celanese Plastics Co., Summit, N. J.) contain an ultra-fine pore structure resulting in a high film surface area on the order of 50m2/g. The pores are about 0.2µ long and 0.02µ wide, forming slightly tortuous channels extending from one surface to the other. We have found that when this film is impregnated with a nematic liquid crystal containing a dissolved dye, the absorption of polarized light by the dye is strongly dependent upon the orientation of the film with respect to the plane of polarization. The liquid crystal is therefore spontaneously oriented by the micropores, and this orientation is not affected by subsequent directional application of a static charge or mechanical deformation. Further, the results are consistent with the slit-like nature of the voids since a cylindrical pore would not be expected to exhibit polarization effects. A potentially important application of this observation lies in the simplicity of the technique. Quantitative as well as qualitative results are readily obtained, and the matrix can be rendered reversibly isotropic by heat or vapor absorption. The results of these and other measurements are discussed.
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