Dependence of the stereoregularity on the structure and the conformation of side-chain liquid crystalline polynorbornenes of various spacer lengths

International audienceWe use the neutron scattering properties to characterize the influence of the stereoregularity on the nature of the liquid crystalline phase and on the polymer conformation of a series of stereoregular side-chain liquid crystalline polynorbornenes (SCLC-polynorbornenes) with various spacer lengths. It is demonstrated that trans-SCLC-polynorbornenes with short spacer lengths display a nematic phase and a prolate main-chain conformation whereas long spacer lengths favor the smectic phase and an oblate main-chain conformation. This smectic structure is no more stable at low temperature, making a spectacular smectic-reentrant nematic transition appear. For identical spacer lengths, a high population of trans configurations favors the emergence of the nematic phase and a prolate main-chain conformation. In contrast, a majority of cis configurations produces the smectic phase and an oblate main-chain conformation. An equirepartition of cis and trans is at the origin of a smectic-nematic reentrance. This reentrant nematic phase reveals moreover a very peculiar conformation since the main-chain anisotropy is inverted from small to large scales