Mesogen-jacketed liquid crystalline polymers and elastomers bearing polynorbornene backbone

Abstract

This manuscript describes a series of mesogen-jacketed liquid crystalline polymers (MJLCPs) and elastomers (MJLCEs) bearing a novel polynorbornene backbone, onto which the liquid crystalline mesogen is laterally attached by a single covalent bond without spacers. The side-on mesogenic monomers are synthesized via palladium-catalyzed exo-hydroarylation of norbornadiene, and then polymerized using a Grubbs' catalyst initiated ring opening metathesis polymerization (ROMP) method. MJLCEs are prepared by mixing the MJLCPs with bifunctional benzophenone cross-linkers, pulling fibers to achieve a fine alignment, and crosslinking under UV illumination. Their mesomorphic, thermo-actuation and mechanical properties are studied in detail by a combination of ¹H NMR, gel permeation chromatography, thermogravimetric analysis, differential scanning calorimetry, polarized optical microscopy, X-ray diffraction and dynamic mechanical analysis. These newly designed polymers can decouple the dynamics of the very bulky and rigid polynorbornene main chain and the mesogenic side groups to achieve liquid crystallinity, only if the length of alkoxy terminal chains of the mesogens is long enough (the number of the carbons in the alkoxy group, n > 9). Moreover, interesting external force induced reversible thermal-actuation behaviors of smectic MJLCEs are observed and analyzed as well.