Reversible actuation in main-chain liquid crystalline elastomers with varying crosslink densities

Main-chain smectic-C liquid crystalline elastomers (LCEs) with varying crosslink densities were prepared using a two stage hydrosilylation reaction scheme, in which diene mesogens were first polymerized with hydride-terminated poly(dimethylsiloxane) and subsequently crosslinked using a tetravinyl molecule. Adjustment of prepolymer molecular weight afforded control over network crosslink density. The LCEs exhibit two thermal transitions that are determined by the mesogen composition and are independent of crosslink density: the mesogen glass transition (∼−30 °C) and isotropization (∼40 °C). Thermal cycling around the isotropization transition under tensile load leads to reversible extension (cooling) and contraction (heating) of the sample, a phenomenon called two-way shape memory or “actuation”. Actuation strains reached up to 30%, a substantial amount for a polydomain LCE. Creep experiments revealed different dynamics between the smectic and isotropic phases, and comparison to actuation results indicated that actuation involves more than simply a transition between isotropic and smectic rheological behavior. Instead, the phase transition itself plays an important role. Wide-angle X-ray scattering analysis revealed that samples strained to the same level, whether by creep or actuation, showed different orientation levels. This indicates that microstructure is not a unique property of the deformed state and supports the hypothesis that cooling to the liquid crystalline phase under stress is important to achieving the large strains associated with actuation.