Devesh Mistry

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 / c m 3 to 1.19 g / c m 3 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 the mesogenic content in side-chain acrylate LCEs, suggesting design approaches that allow for the potential customization of the material properties for applications.

Using reactive atomistic molecular dynamics, we simulate the network formation and bulk properties of chemically identical liquid crystal elastomers (LCEs) and isotropic elastomers. The nematic elastomer is from a family of materials that have been shown to be auxetic at a molecular level. The network orientational order parameters and glass transition temperatures measured from our simulations are in strong agreement with experimental data. We reproduce, in silico, the magnitude and onset of strain-induced nematic order in isotropic simulations. Application of uniaxial strain to nematic LCE simulations causes biaxial order to emerge, as has been seen experimentally for these auxetic LCEs. At strains of ~1.0, the director reorients to be parallel to the applied strain, again as seen experimentally. The simulations shed light on the strain-induced order at a molecular level and allow insight into the individual contributions of the side-groups and crosslinker. Further, the agreement between our simulations and experimental data opens new possibilities in the computational design of high-molecular-weight liquid crystals, especially where an understanding of the properties under mechanical actuation is desired. Moreover, the simulation methodology we describe will be applicable to other combinations of orientational and/or positional order (e.g., smectics, cubics).

Determining the tunability of the optical coefficients, order parameter, and transition temperatures in optically transparent auxetic liquid crystal elastomers (LCEs) is vital for applications, including impact-resistant glass laminates. Here, we report measurements of the refractive indices, order parameters, and transition temperatures in a family of acrylate-based LCEs in which the mesogenic content varies from ∼50 to ∼85%. Modifications in the precursor mixture allow the order parameter, ⟨P₂⟩, of the LCE to be adjusted from 0.46 to 0.73. The extraordinary refractive index changes most significantly with composition, from ∼1.66 to ∼1.69, in moving from a low to high mesogenic content. We demonstrate that all LCE refractive indices decrease with increasing temperature, with temperature coefficients of ∼10^-4 K^-1, comparable to optical plastics. In these LCEs, the average refractive index and the refractive index anisotropy are tunable via both chemical composition and order parameter control; we report design rules for both.

We develop a mathematical model that builds on the surprising nonlinear mechanical response observed in recent experiments on nematic liquid crystal elastomers. Namely, under uniaxial tensile loads, the material, rather than thinning in the perpendicular directions, becomes thicker in one direction for a sufficiently large strain, while its volume remains unchanged. Motivated by this unusual large-strain auxetic behaviour, we model the material using an Ogden-type strain-energy function and calibrate its parameters to available datasets. We show that Ogden strain-energy functions are particularly suitable for modelling nematic elastomers because of their mathematical simplicity and their clear formulation in terms of the principal stretches, which have a direct kinematic interpretation. This article is part of the theme issue ‘The Ogden model of rubber mechanics: Fifty years of impact on nonlinear elasticity’.

Soft-elasticity in monodomain liquid crystal elastomers (LCEs) is promising for impact-absorbing applications where strain energy is ideally absorbed at constant stress. Conventionally, compressive and impact studies on LCEs have not been performed given the notorious difficulty synthesizing sufficiently large monodomain devices. Here, we use direct-ink writing 3D printing to fabricate bulk (>cm³) monodomain LCE devices and study their compressive soft-elasticity over 8 decades of strain rate. At quasi-static rates, the monodomain soft-elastic LCE dissipated 45% of strain energy while comparator materials dissipated less than 20%. At strain rates up to 3000 s⁻¹, our soft-elastic monodomain LCE consistently performed closest to an ideal-impact absorber. Drop testing reveals soft-elasticity as a likely mechanism for effectively reducing the severity of impacts – with soft elastic LCEs offering a Gadd Severity Index 40% lower than a comparable isotropic elastomer. Lastly, we demonstrate tailoring deformation and buckling behavior in monodomain LCEs via the printed director orientation.

In recent years, the field of liquid crystal elastomers (LCEs) has grown considerably in size. Applications for these materials promise to be imminent, and yet many unknowns still surround some of their fundamental behaviours. In addition, new thermal and mechanical properties are continually being discovered. In this article, I review and summarise the key properties of an acrylate LCE developed during my PhD and compare them to the material from which it was derived – developed by the Uryama group. I highlight our material’s surprising negative Poission’s ratio and templateable properties and explore the possible structural roots of these behaviours.

Liquid crystal elastomers (LCEs) have long been celebrated for their exceptional shape actuation and mechanical properties. For much of the last half century, a major focus for the field has been the development of LCE chemistries and how to process the so-called “monodomain” configurations. This foundation work has now led to a plethora of materials and processes that are enabling the demonstration of devices that are close to real-world applicability as responsive and reprocessable actuators. In this Perspective, we review and discuss the key recent developments in the processing of actuating LCE devices. We consider how processing has been used to increase the practicality of electrical, thermal, and photo stimulation of LCE shape actuation; how dynamic chemistries are enhancing the functionality and sustainability of LCE devices; and how new additive manufacturing technologies are overcoming the processing barriers that once confined LCE actuators to thin film devices. In our outlook, we consider all these factors together and discuss what developments over the coming years will finally lead to the realization of commercial shape actuating LCE technologies.

Digital Light Processing (DLP) 3D printing enables the creation of hierarchical complex structures with specific micro‐ and macroscopic architectures that are impossible to achieve through traditional manufacturing methods. Here, this hierarchy is extended to the mesoscopic length scale for optimized devices that dissipate mechanical energy. A photocurable, thus DLP‐printable main‐chain liquid crystal elastomer (LCE) resin is reported and used to print a variety of complex, high‐resolution energy‐dissipative devices. Using compressive mechanical testing, the stress–strain responses of 3D‐printed LCE lattice structures are shown to have 12 times greater rate‐dependence and up to 27 times greater strain–energy dissipation compared to those printed from a commercially available photocurable elastomer resin. The reported behaviors of these structures provide further insight into the much‐overlooked energy‐dissipation properties of LCEs and can inspire the development of high‐energy‐absorbing device applications.