Stress-strain and thermomechanical characterization of nematic to smectic A transition in a strongly-crosslinked bimesogenic liquid crystal elastomer

Bimesogenic liquid crystal elastomers, composed with controlled concentrations of two mesogenic constituents, specifically a nematogen and a smectogen, exhibit either enhanced nematic or smectic structure. Here we present a thorough examination of thermomechanical and elastic properties of such a system at high crosslinking density, with ensuing disordered smectic structure which prevents the observation of smectic-nematic phase transition by means of thermomechanical response measurements. Yet, the onset of local smectic order is clearly observable in stress-strain measurements via an anomaly in the elastic modulus. Breakdown of the local smectic structure is detected at higher mechanical loads. Bimesogen composition - temperature phase diagram is as well constructed, consistent with the one obtained by complementary experimental techniques. Stress-strain temperature dependence measurements can thus serve as efficient and simple phase diagram characterization method that is able to detect formation of smectic clusters in strongly crosslinked and thus highly disordered smecto-nematic liquid crystal elastomers.