Ordered phases in mesogen-containing polyurethanes

Polyurethanes containing the biphenyl mesogen were synthesized and characterized by differential scanning calorimetry, wide-angle X-ray scattering and polarizing optical microscopy. One polyurethane (D6T24), which has an asymmetric structure, displays a monotropic smectic mesophase. The crystallization and mesophase-forming behavior of D6T24 is highly dependent on molecular weight. Another polyurethane (D6T26), which has a symmetrical structure, also displays a monotropic smectic mesophase. Because the crystalline unit cells of D6T24 and D6T26 are significantly different, combining the two polyurethanes in copolymers and blends suppresses crystallization, and in some cases produces enantiotropic systems. Blends of D6T24 and D6T26 containing more than 30% D6T24 by weight, and a random copolymer containing 47% D6T24 repeat units and 53% D6T26 repeat units, form enantiotropic smectic mesophases which do not crystallize. Annealing at high temperatures appears to transform the blends into copolymers, probably by means of transurethanification reactions