Anomalous thermotropic liquid crystalline phase behaviour in poly[n-decyl-(RS)-2-methylbutylsilane]s with narrow molecular weight distributions

The thermotropic liquid crystalline (LC) phase behaviour of a rod-like polymer, poly[n-decyl-(RS)-2-methylbutylsilane], with narrow molecular weight distributions, was studied by optical microscopic observations and X-ray diffraction measurements. This polymer exhibited a very unusual LC phase sequence of smecticA(SmA)-reentrant isotropic (I(re))-nematic (N). Upon heating, the I(re) phase prevails in the SmA phase, and then the N phase goes over into the I(re) phase, while upon cooling, the I(re) phase enantiotropically grows in the N phase. Moreover, the orientation of the polymer and smectic layer reflection simultaneously disappeared during the transition to the I(re) phase, so that the anchoring transition to the pseudo-isotropic homeotropic alignment can be ruled out. To the best of our knowledge, this is the first example of a rod-like LC polymer exhibiting a reentrant isotropic phase without a drastic conformational change, although the exact molecular origin of the I(re) phase is still unclear.