FT-IR studies of side chain liquid crystalline thermoplastic elastomers

Abstract

Side chain liquid crystalline polyurethanes are a new class of materials that show promise for mechanooptic applications. The rich morphology afforded by these materials also provides a chance to understand the interplay between polyurethane morphology and liquid crystalline ordering. In this paper, we study the response of a polyurethane with liquid crystals pendant to the soft segments to an applied strain using fourier transform infrared (FT-IR) linear dichroism. We find that this complex material follows the trend established in the literature for both side chain liquid crystalline homopolymers and segmented polyurethanes. At low strains, the soft segments align with strain inducing an orientation in “lone” hard segments. Up to strains of 40%, the LC mesogens align with the strain field and the hard segments in hydrogen bonded domains align perpendicular to the field. At strains above 40%, we find a rearrangement of the ordering that result in smectic layers and the hard segments aligning parallel to the field. A model is proposed to represent these findings, and reflections on the cooperative movement of the different macromolecular components of the polyurethane are offered.