Main-chain liquid crystalline networks were prepared from mesogenic dienes using two different synthetic routes. First, main-chain liquid crystalline copolymers were synthesized by polymerizing a mesogen with a nonmesogenic comonomer using acyclic diene metathesis (ADMET) chemistry. The resulting polymers form nematic phases, with composition dictating the glass transition and isotropization temperatures. Free-radical crosslinking through the unsaturated bonds in the polymer was demonstrated for a selected composition to lead to an elastomeric network. This two step process was employed to control the polymer properties before crosslinking and serves as a viable route to tailored nematic networks for applications as anisotropic adhesives.
Liquid crystalline elastomers (LCEs) were prepared using a second synthetic route that employed hydrosilylation chemistry to react the mesogens with hydride-terminated poly(dimethylsiloxane) and a vinyl crosslinker. The resulting LCEs formed a smectic-C phase with transition temperatures that depend on mesogen composition. The mesogens impart two distinct active behaviors to the elastomers. The first of these is actuation, the reversible extension and contraction of the polymer when cooled and heated, respectively, through the mesogen isotropization transition. Actuation is dependent on the crosslink density of the material and can cause the samples to elongate as much as 30 % under tensile load. The second active behavior is shape memory, the ability to fix a temporary deformation and later recover the equilibrium shape by heating. The LCEs have excellent shape memory fixing and recovery ratios, both of which generally exceeded 95 %. The ability of a soft network to fix strains above room temperature is unusual and was investigated using a combination of thermal analysis, mechanical testing, and wide angle x-ray scattering, where it was found that strain is fixed by freezing the mesogens within the smectic layers. The LCE’s low modulus was exploited by reversible embossing, the localization of a temporary topography onto the LCE using shape memory. A microscale embossed topography was stable until erased by heating to recover the LCE’s flat, permanent shape. Possible applications of these LCEs include artificial muscles, smart shear-based actuators, and active substrates.