Novel liquid crystalline polymers. Formation of mesogenic units through hydrogen bonding with polysiloxanes containing carboxylic acid groups

In this work, novel polysiloxanes containing side-chains terminated by carboxylic acid groups are shown to possess liquid crystalline behavior due to the formation of hydrogen-bonded dimer-type structures. Despite the occurrence of hydrogen-bonding that effectively crosslinks the polymers, the polysiloxanes still remain processible and soluble as the hydrogen-bonds can be broken thermally or through interactions with appropriate solvents. In these structures, dimer formation increases the length to diameter ratio of the side chain of polymer thereby inducing liquid crystallinity. The phase transition temperatures of the various carboxy ended polysiloxanes are presented. The phase transition temperatures especially the crystalline to smectic transitions, were found to be dependent on the thermal history of the sample, presumably due to the occurrence of hydrogen-bond reorganization processes between the pendant carboxylic acid groups in the melt state. The phase transition temperatures, were determined by optical microscopy during a second heating.