Use of Intermolecular Hydrogen Bonding for the Induction of Liquid Crystallinity in the Side Chain of Polysiloxanes

A new type of liquid crystalline side chain polysiloxane has been built through self-assembly via intermolecular hydrogen bonding between H-bond donor and acceptor moieties. Poly(methylsiloxanes) and poly(methyl-co-dimethylsiloxanes) with side chains containing 4-alkoxybenzoic acid pendant groups attached through aliphatic spacers were synthesized for use as H-bond donor polymers. trans-4-[(4-Methoxybenzoyl)oxy]-4′-stilbazole and trans-4-ethoxy-4′-stilbazole were prepared as representative mesogenic or nonmesogenic H-bond acceptors, respectively. Formation of the liquid crystalline polymeric complexes occurs by self-assembly of the carboxylic acid group of the polysiloxanes with the stilbazoles through H-bonding as confirmed by spectroscopic techniques. New and extended mesogens result from self-assembly and formation of a single H-bond between the carboxylic acid side chain and the pyridyl group of the stilbazoles. Formation of a side-chain liquid crystalline polymer complex does not require that either or both of the H-bonded fragments display liquid crystallinity. The phase diagrams of a variety of mixtures of the polysiloxanes and the stilbazoles have been established using synchrotron X-ray data, differential scanning calorimetry, and optical microscopy. They show complete miscibility and unusually high thermal stability of the liquid crystals over the whole composition range. X-ray diffraction studies on unoriented samples point to smectic C or smectic A phases for the various complexes. X-ray, DSC, and optical microscopy also show that several of the starting polysiloxanes themselves have smectic C phases with their alkyl chains in an extended conformation. This liquid crystallinity results from dimerization of the benzoic acid moieties.