Toward supramolecular side-chain liquid crystal polymers .5. The template receptor approach

Three series of copolymers have been prepared by the reaction of a mesogenic methacrylate, 6-((4-((4-methoxyphenyl)azo)phenyl)oxy)hexyl methacrylate, with either 4-vinylpyridine, 2-vinylpyridine or styrene. These copolymers were blended with either a mesogenic acid, 6-((4-((4-methoxyphenyl)azo)phenyl)oxy)hexanoic acid, or a nonpolar low molar mass liquid crystal, 4-(hexyloxy)-4'-methoxyazohenzene. The blends were prepared such that the low molar mass component was in an equimolar ratio to the nonmesogenic units in the copolymer. The acid was essentially immiscible in the styrene-based copolymers, while the nonpolar mesogen was miscible providing the mole fraction of mesogenic side chains exceeded 0.27. This miscibility was attributed to the mesogen-mesogen interactions and resulted in a linear dependence of the nematic-isotropic transition temperature on composition. By contrast the miscibility of the mesogenic acid is greater than that of the nonpolar mesogen with the vinyl pyridine-based copolymers and for the acid mixtures results in the observation of either induced or enhanced liquid crystalline behavior; these effects were more pronounced for the 4-vinylpyridine-based copolymers. This nonlinear dependence of the nematic-isotropic transition temperatures on composition was attributed to the formation of hydrogen bonds between the carboxylic acid and pyridyl units. Thus the vinylpyridine-based copolymers are described in terms of template polymers, i.e., preformed receptors, for the low molar mass acid substrates.