Synthesis and Properties of Thermotropic Main-Chain Type Liquid Crystalline Polyurethanes Containing Biphenyl Mesogen

A new type of thermotropic main-chain liquid crystalline polyurethanes containing biphenyl units was synthesized by polyaddition reaction of diisocyanates such as 2,6-tolylene diisocyanate, 2,5-tolylene diisocyanate, 2,4-tolylene diisocyanate, and 1,4-phenylene diisocyanate, with 4,4′-Bis(8-hydroxyoctoxy)biphenyl (BP8). The structure of the monomer and the corresponding polymers were confirmed using FT-IR and 1H-NMR spectroscopic methods. BP8 exhibited a smectic type mesophase, however, nematic phases were found for all synthesized liquid crystalline polyurethanes except for 1,4-phenylene diisocyanate/BP8 based polyurethane. For example, polyurethane 2,5-TDI/BP8 exhibited monotropic liquid crystallinity in the temperature ranging from 172 to 160 °C on the cooling stage. Properties of these polyurethanes were studied by differential scanning calorimetry (DSC), and optical polarizing microscopy. The FT-IR study indicated that the hydrogen bonding among urethane linkages attributed to the mesomorphism.