Synthesis of Thermotropic Polyurethanes Containing Imide Units and Their Mesophase Behavior

Thermotropic polyurethanes were synthesized from 1,6-hexane diisocyanate (HDI) as a diisocyanate, 1,6-hexane diol (HD), and rigid diols containing imide unit such as N,N＇-bis(4-hydroxyphenyl)-3,4,3＇,4＇-biphenyl-dicarboxyimide (BPDI) or bis-N-(4-hydroxyphenyl)-4,4＇-oxydiphthalimide (ODPI). The effects of structure difference between BPDI and ODPI and composition of HD/BPDl (ODPI) on the thermal and liquid crystalline behavior were studied. Thermotropic polyurethanes with an inherent viscosity of 0.59~0.70 were obtained. The melting temperature of BPDI-based polyurethanes were in the range of 150~$290^{\circ}C$, however, those of ODPI-based polyurethanes were in the range of 150~$190^{\circ}C$. All the polyurethanes based on ODPI (25~100 mole %) clearly exhibited a stable liquid crystalline phase, and BPDI-based polyurethane having 5-25% of BPDT showed a mesophase. The melting and isotropization temperatures ($T_m$, $T_i$) and ΔT($T_i$ - $T_m$) increased with increasing BPDI and ODPI content. The polyurethanes based on BPDI has higher melting points and thermal stability compared to ODPI-based polyurethanes.