PHASE-BEHAVIOR OF CYCLIC SILOXANE-BASED LIQUID-CRYSTALLINE COMPOUNDS

A series of 24 cyclic siloxane-based liquid crystalline compounds was synthesized using conventional hydrosilation chemistry. Variables examined included ring size, spacer group length, and type and composition of pendant mesogenic groups. Both pentamethylhydro-and tetramethylhydrosiloxane rings were reacted with mesogens based on cholesterol, biphenyl, or equimolar mixtures of both. Four different length spacer groups containing terminal vinyl groups were used to attach the mesogens to the cyclic siloxane core. The thermotropic liquid crystalline phase behavior was studied using differential scanning calorimetry (DSC) and polarized optical microscopy (POM). A lengthening of spacer groups resulted in lower crystallization temperatures for the biphenyl-based compounds and lower glass transition temperatures for the cholesterol- and mixed mesogen-based compounds. The tetramethyl ring derivatives exhibited higher glass transition or crystallization temperatures than their pentamethyl counterparts. Biphenyl-based compounds exhibited low temperature crystalline phases while the cholesterol-based compounds exhibited low temperature glassy phases. (C) 1994 John Wiley & Sons, Inc.