Thermal and i.r.-dichroic properties of side-chain type liquid-crystalline elastomers

Abstract

Several liquid-crystalline elastomers (LCEs) have been prepared by crosslinking side-chain type liquid-crystalline polymers (precursors) in which mesogens are dangling from flexible main chains through spacers. Thermal properties and phase behaviour of the precursors and LCEs have been investigated by means of differential scanning calorimetry (d.s.c.) and polarizing microscopy. The glass transition and meso-to-isotropic phase transition temperatures are slightly affected by an introduction of crosslinks to precursors. Orientation of the mesogens in the LCEs under uniaxial elongation has been evaluated in terms of order parameter for orientation ( f) obtained by i.r.-dichroism measurements. All the LCEs under deformation show negative values of f, meaning the orientation direction of mesogens perpendicular to the axis of elongation. The value of f changes greatly in the small deformation region, and reaches equilibrium at ca. 70% elongation, suggesting that a high degree of orientation for the mesogens in LCEs is achieved by a relatively small elongation. Dependence of f on elongation ratio is not influenced by spacer length nor by crosslinking density, implying that the interaction between mesogens is the most dominant factor for the orientational order of mesogens in the LCE system in this study.