A new series of thermotropic polyurethanes was synthesized by reacting such para-substituted diisocyanate monomers as 2,5-tolylene diisocyanate (2,5-TDI) and 1,4-phenylene diisocyanate (1,4-PDI), with 4,4'-bis(oi-hydroxyalkoxy)biphenyls (BPm: HOCmH2mOC6H4C6H4OCmH2mOH; m is the carbon number of the hydroxyalkoxy group) in DMF. Intrinsic viscosities of the polymers were in the range of 0.41-0.99 dL/g. DSC thermograms for these polymers exhibited two endothermic peaks corresponding to phase transitions of melting and isotropization. Mesomorphic behavior of the polyurethanes was also observed under a polarizing microscope. For example, polyurethane 1,4-PDI/BP6 with [tj] = 0.77 prepared from 1,4-PDI and 4,4'-bis-[(6-hydroxyhexyl)oxy] biphenyl (BP6) exhibited a liquid crystalline phase from 247 to 259 °C. Infrared study indicated that the hydrogen bonding between urethane linkages affected the mesomorphism. Thermogravimetric analysis of 1,4-PDI/BPm showed that no weight loss of the polymer occurred up to 300 °C under an air atmosphere.

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ABSTRACT:A series of thermotropic polyurethanes containing no mesogenic unit were synthesized by polyaddition of a para-type diisocyanate such as 2,5-tolylene diisocyanate (2,5-TDI) with 2,6-bis(w-hydroxyalkoxy)naphthalenes (BHNm: HO(CH2)mOC10H 6 O(CH2)mOH; m is the carbon number in the hydroxyalkoxy group). Intrinsic viscosities of the polymers were in the range of 0.27---0.48 dL g-1 . The thermal properties of these polymers were studied by differential scanning calorimetry, thermogravimetric analysis, polarizing microscopy, and X-ray diffractometry. Polyurethane 2,5-TDI/BHNm's (m=5, 6, 8, 11) prepared from BHNm and 2,5-TDI having methyl substituent on the phenylene unit exhibited monotropic liquid crystallinity. For example, polyurethane 2,5-TDI/BHN8 with [IJ]=0.30 exhibited a mesophase from 139 to lll°C on cooling. However, in the series of polyurethanes prepared by polycondensation of 1,4-phenylene diisocyanate (1,4-PDI) with BHNm, no explicit mesomorphic behavior was observed by DSC measurement and polarizing microscopic observation.