Molecular order and dynamic mechanical behavior of polyurethanes based on liquid crystalline diol

Synthesis of the liquid crystalline (LC) diol 6,6'-[ethylenebis(1,4-phenylene-oxy)]-dihexanol (I) is described. The structure of polyurethanes prepared from diol I and 4,4'-methylenedi(phenyl isocyanate) (MDI), 4,4'-methylenedi(cyclohexyl isocyanate) (HMDI), or 2(4)-methyl-1,3-phenylene diisocyanate (TDI) at 1:1 molar ratios of isocyanate and hydroxy groups is studied by dynamic mechanical spectroscopy, differential scanning calorimetry (DSC), polarizing microscopy, and x-ray scattering. The polymer prepared from HMDI and the diol (I/HMDI) shows, on cooling, thermal behavior typical of amorphous polymers. A frequency-temperature superposition could be applied to the mechanical data, and the horizontal shift factor satisfied the Williams-Landel-Ferry (WLF) equation. A more-complex thermal behavior was found for I/HMDI polymer during subsequent heating; above 70 degrees C, the formation of an ordered structure takes place, and this structure melts at about 120 degrees C. Complex thermal behavior is exhibited by I/TDI polymer. On cooling its melt, the polymer forms a nematic phase at about 80 degrees C, which freezes into the LC glassy state. On heating, the mesophase melts, and, subsequently, a better-ordered smectic phase is formed at 95 degrees C, which melts at 120 degrees C. This structure buildup is accompanied by a rapid increase in storage modulus G', and the sample shows thermorheologically complex mechanical behavior. The polymer formed from the diol and MDI (I/MDI) exhibits a most-complex thermal behavior. On cooling and heating, four transitions can be detected in its thermal mechanical behavior, and the structure of the polymer is strongly dependent on its thermal history.