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

We use the neutron scattering properties to characterize the influence of t he stereoregularity on the nature of the liquid crystalline phase and on th e polymer conformation of a series of stereoregular side-chain liquid cryst alline polynorbornenes (SCLC-polynorbornenes) with various spacer lengths. It is demonstrated that trans-SCLC-polynorbornenes with short spacer length s display a nematic phase and a prolate main-chain conformation whereas lon g spacer lengths favor the smectic phase and an oblate main-chain conformat ion. This smectic structure is no more stable at low temperature, making a spectacular smectic-reentrant nematic transition appear. For identical spac er 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 mai n-chain conformation. An equirepartition of cis and trans is at the origin of a smectic-nematic reentrance. This reentrant nematic phase reveals moreo ver a very peculiar conformation since the main-chain anisotropy is inverte d from small to large scales.