Synthesis and properties of liquid crystalline polyurethane elastomers

Novel type of mesogenic chain extenders used in this study are N,N′-bis(4-hydroxyphenyl)-3,4,3′,4′-biphenyldicarboxyimide (BPDI) and N,N′-bis[4-(6-hydroxyhexyloxy) phenyl]-3,4,3′,4′-biphenyldicarboxyimide (BHDI). BHDI has a flexible spacer of 6-methylene units but BPDI does not. The liquid crystalline polyurethane elastomers were synthesized from BPDI or BHDI as a mesogenic chain extender, 4,4′-diphenylmethane diisocyanate, and poly(oxytetramethylene)glycol (MW 1000) as a soft segment. Polyurethane based on BHDI exhibited two melting transitions. However, any melting behavior was not shown in the BPDI-based polyurethanes because of higher melting temperature than decomposition temperature. The composition of polyurethanes was varied as a means of manipulating liquid crystalline behavior and physical properties. The BHDI-based polyurethanes containing above 50 wt % of hard segment content exhibited nematic liquid crystal behaviors. As the hard segment content of the BHDI-based polyurethanes increased, the glass transition temperature (Tg), strength, modulus, and the amount of hydrogen bonding increased.