Unusual Stretching-Induced Molecular Orientation Behavior of a Side-Chain Liquid Crystal Elastomer and Its Reorientation-Enabled Auxeticity

Abstract

A liquid crystal polymer (LCP) is synthesized by the functionalization of polybutadiene with side-group mesogens and methacrylate cross-linker and is used to investigate the stretching-induced orientation of mesogens and chain backbone. Unusual orientation behaviors of the two constituents coupled through the spacer are revealed by X-ray diffraction (XRD) and polarized infrared spectroscopic measurements. When a polydomain LCP film is stretched in the nematic phase, the chain backbone and mesogenic side groups are oriented along and perpendicular to the stretching direction (SD), respectively; at a large strain (500%), the oriented mesogens are tilted in the plane. By contrast, when the polydomain LCP is stretched in the isotropic phase and cooled under strain into the nematic phase, a peculiar state is observed where both the mesogenic side groups and chain backbone are aligned perpendicular to the SD. By photo-cross-linking oriented LCP films under strain, the different macroscopic orientation states are locked in the resulting monodomain liquid crystal elastomer (LCE) films. Restretching such monodomain LCE films in the orientation direction of mesogens to induce their reorientation, an auxetic response is observed, with the thickness of the film increasing upon restretching over a small strain range. Polarized optical microscopic (POM) observation and UV–vis absorption measurement show unambiguously that the auxetic behavior is associated with a mechanically induced reorientation of the mesogens from in-plane homogeneous to out-of-plane homeotropic alignment. The onset and magnitude of the auxeticity as well as the strain range for its manifestation (increased film thickness with respect to the initial thickness before restretching) are affected by the relative orientational states of the side-group mesogens and chain backbone in the monodomain LCE film.