Insight into the composition-dependent transition from auxetic nematic to frustrated smectic in liquid crystal elastomers

The ability to optimize the properties of a material is particularly advantageous for applications. Here the tuneable properties of side-chain liquid crystal elastomers (LCEs) are investigated. LCEs were fabricated using the same acrylate monomers, varying the proportion of mesogenic groups to influence the phase, density, and mechanical properties. The density increases with mesogenic content, varying from 1.11 g/cm³ to 1.19 g/cm³ as the mesogenic content is increased from 51 mol% to 99 mol%. The 7% increase in density was attributed to closer packing of the nanoscale network, observed directly through Small- and Wide-Angle X-ray Scattering. A 22% reduction in the network end-to-end spacing was measured for LCEs as the mesogenic content was changed from 56 mol% to 75 mol%. LCEs with above 70 mol% mesogenic content were revealed to exhibit a frustrated smectic phase, with nematic phases confirmed for the lower mesogenic concentrations. The frustrated smectic materials exhibit a higher correlation length parallel to the director (>125 Å) than the nematic LCEs (<100 Å) and demonstrate significantly higher Young’s moduli, with the greatest moduli perpendicular to the director measured at 21 °C as 8.3 MPa for the smectic LCEs and as a constant value of 2.1 MPa for the nematic LCEs. Crucially, this demonstrates that only a small change in the mesogenic content can induce a transition from a nematic LCE to a frustrated smectic LCE. This series of materials reveals how the nanoscale packing, the macroscopic material properties, and the phase can be controlled via the mesogenic content in side-chain acrylate LCEs, suggesting design approaches that allow for the potential customization of the material properties for applications.