Synthesis and Liquid Crystalline Behavior of Laterally Substituted Polyimides with Siloxane Linkages

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Abstract

Novel siloxane-containing liquid crystalline (LC) polyimides with methyl, chloro, and fluoro substituents on mesogenic units have been developed from siloxane-containing diamines with pyromellitic dianhydride (PMDA) or 3,3′,4,4′-tetracarboxybiphenyl dianhydride (BPDA), and their thermotropic LC behavior was examined. Among these, chloro and fluoro substituents are effective for the formation of LC phases, particularly when those are substituted away from the center of the mesogenic unit: the isotropization temperature is not much affected, but the crystal−LC transition temperatures are significantly decreased. On the other hand, the methyl substituent tends to interrupt liquid crystallization as well as crystallization. Thus, the fluoro-substituted polyimide derived from BPDA exhibited the lowest crystalline−LC transition temperature (Tcr−lc = 134 °C) among all polyimides, showing a wide liquid crystal temperature up to 238 °C. From the X-ray diffraction measurement conducted for the oriented mesophases of fibrous polyimides, they were found to form SmA and SmC as high- and low-temperature mesophases, respectively.

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