Structural Characterization of Diselenide-Containing Liquid-Crystal Elastomers Using Anomalous X-Ray Scattering

Liquid-crystal elastomers (LCEs) are a class of stimuli-responsive polymers that exhibit a unique property profile that includes reversible actuation, soft elasticity, and energy dissipation. These properties arise from the interplay between the entropic elasticity of the rubbery network and the anisotropy of the liquid crystalline repeat units. A subclass of LCEs, known as dynamic LCEs, has gained significant attention over the past decade for its ability to facilitate efficient monodomain alignment of mesogens, which is essential for actuation. While numerous studies have explored the incorporation of various dynamic bonds into LCEs, a comprehensive structural understanding, focusing on the distribution of dynamic bonds within the network, remains limited. In this work, anomalous wide-angle X-ray scattering (AWAXS) was employed to probe the structural nature of polydomain and monodomain dynamic LCEs. Importantly, this technique also allowed the determination of the distribution of dynamic bonds within the amorphous regions. The main-chain LCEs were synthesized using aza-Michael chemistry, where diamines containing dynamic diselenide bonds were used as crosslinkers. Notably, the high absorption energy of selenium enabled the selective tracking of the diselenide moieties, allowing investigation of their distribution within bulk LCEs in both polydomain and monodomain states using AWAXS. Analysis of the experimental AWAXS profiles, supported by coarse-grained X-ray scattering simulations based on fast Fourier transform, revealed that the diselenide-containing crosslinks are heterogeneously distributed throughout the LCE networks. Small-molecule model reactions show that the diselenide bond distribution heterogeneity within the LCE networks may result, at least in part, from differences in the reactivities between the chain extender monoamine and the crosslinker diamine in reaction with acrylates. Such insights into the network structure of dynamic LCEs could directly link the molecular organization to their functional performance.