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 exam ined included ring size, spacer group length, and type and composition of pendant mesogenic groups. Both pentamethylhydro-and tetramethylhyd rosiloxane rings were reacted with mesogens based on cholesterol, biph enyl, or equimolar mixtures of both. Four different length spacer grou ps containing terminal vinyl groups were used to attach the mesogens t o 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 re sulted in lower crystallization temperatures for the biphenyl-based co mpounds and lower glass transition temperatures for the cholesterol- a nd mixed mesogen-based compounds. The tetramethyl ring derivatives exh ibited higher glass transition or crystallization temperatures than th eir pentamethyl counterparts. Biphenyl-based compounds exhibited low t emperature crystalline phases while the cholesterol-based compounds ex hibited low temperature glassy phases. (C) 1994 John Wiley & Sons, Inc .