Synthesis of Enantiotropic Liquid Crystal Copolyurethanes Containing Para- and Meta-Type Diisocyanate Units in the Backbone

Enantiotropic liquid crystal (LC) polyurethanes were synthesized for the first time. Thermotropic LC copolyurethanes PU n T( m / p )s ( n, the number of methylene unit, 6, 8, and 11; m, meta-type unit; p, para-type unit) were synthesized by polyaddition of 4,4‘-bis(ω-hydroxyalkoxy)biphenyls (BPn) with two kinds of diisocyanates, that is, meta-type 2,4- and para-type 2,5-tolylene diisocyanates (2,4- and 2,5-TDI). The liquid crystallinity was examined by differential scanning calorimetry (DSC), polarized optical microscopy, and X-ray diffractometry. All copolyurethanes consisting of PU6T( m / p )s and PU8T( m / p )s with all m / p ratios showed liquid crystallinity in the cooling stage under shear. Of them, six PU n T( m / p )s ( n = 6 and 8; m / p = 80/20, 60/40, and 50/50) showed enantiotropic liquid crystallinity on both heating and cooling stages. However, copolyurethane PU11T( m / p )s having the undecamethylene spacer did not exhibit any liquid crystallinity at the m / p ratio from 100/0 to 20/80, although PU11T(0/100) was monotropic. For example, a copolyurethane PU6T(80/20) exhibited enantiotropic liquid crystallinity in the temperature ranges from 156 to 164 °C on the heating stage and from 147 to 138 °C on the cooling stage.