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

Abstract

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 smectic A (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.

Keywords:

• polysilane
• phase behaviour
• liquid crystalline polymer (LCP)
• smectic phase
• reentrant isotropic phase

Acknowledgements

This research was supported by a Grant-in-Aid for Creative Research from the Ministry of Education, Science, and Culture in Japan.