Molecular dynamics and biaxiality of nematic polymers and elastomers

To gain insight into the origins of phase biaxiality in nematic polymers and elastomers the relation of molecular dynamics and biaxiality is studied. H-2-NMR investigations of various nematic polymers differing in the attachment geometry of the mesogens to the polymer (side-chain side-on, end-on and main-chain attachment) are presented. The mesogens are H-2-labelled at specific positions of their spacers and in the aromatic core. The NMR experiments are supplemented by dielectric relaxation measurements of polymers labelled with dipole groups. Only minor differences are found for the molecular dynamics of polymers and elastomers and comparing the various labelling positions suggests that the mesogenic moiety rotates as a whole. We find broad biaxial nematic phases for all investigated systems and a slowing down of the rotational diffusion around the mesogens' long axes upon approaching the glass transition. It seems that in nematic polymers and elastomers phase biaxiality sets in when the rotational diffusion around the long axis gets hindered enough to allow biaxial nematic order. Comparing different attachment geometries we find that the side-on attachment of the mesogens seems to hinder the rotation of the molecules around their long axes considerably more than the other geometries.