Introduction to Special Topic: Programmable liquid crystal elastomers

Liquid crystal elastomers (LCEs) are a class of materials where liquid crystalline order and rubber elasticity combine to yield interesting properties.¹ These elastomers can exhibit a variety of liquid crystalline phases, which results in materials with distinct functional properties. For example, nematic elastomers with an oriented director (molecular orientation) are thermally responsive and undergo a large, reversible shape change when heated and cooled through the nematic–isotropic transition.² By patterning the nematic director within a material, this shape transformation can be programmed.^3–5 Chiral nematic elastomers can have structural color, which can be tuned in response to mechanical deformation and temperature.^6,7 The key functional properties in these materials rely on strategies to tune the phase behavior and to control the alignment of the liquid crystalline order of the material. A number of challenges remain regarding experimental techniques to control phase behavior, alignment, and other physical properties of the elastomers. Related challenges exist in modeling the resulting functional material properties. In this Guest Editorial, we will briefly review important fundamental concepts related to LCEs and introduce the articles found in the Special Topic. These articles present advances in (i) models of shape transformation in LCEs, (ii) experimental approaches to program shape transformation, and (iii) strategies to achieve dynamic optical properties in LCEs.

LCEs form a class of stimuli-responsive polymers where molecular order engenders properties not found in typical amorphous elastomers. These materials have been the subject of comprehensive reviews and books.^1,8,9 In this Guest Editorial, we aim to introduce some of the most important concepts in the area of LCEs as a means to provide context to the papers found in this Special Issue.

Liquid crystals are widely used in displays for the anisotropic and electrically responsive properties of these ordered liquids. Similarly, polymer networks that exhibit liquid crystalline order exhibit anisotropic and spatially varied optical and/or mechanical properties and can be induced to respond to stimuli.^10,11 The key distinction between low molar mass liquid crystals and LCEs is that in elastomers, the interplay between the responsive nature of the liquid crystal and the polymer network governs stimulus-response. De Gennes first proposed that this coupling of liquid crystal order and elasticity would result in surprising properties, including polymer networks that undergo large and reversible changes in form in response to temperature.¹² Most commonly, this property has been observed in cross-linked nematic polymers, where the polymer chain conformation transitions reversibly from more prolate (ordered) to more spherical (disordered) on heating through the nematic to isotropic transition. We should note that similarly ordered polymer networks with different network structures respond to stimuli differently.⁸ Historically, two classifications have been used in the field to describe what is arguably, a single broader class of polymers. Lightly cross-linked polymer networks were frequently described as LCEs, while densely cross-linked materials were described as liquid crystal networks (LCNs). While the initial experimental realizations of each class of material were quite different, LCEs were lightly cross-linked siloxanes, and LCNs were densely cross-linked acrylates,^2,10,13 the field is now characterized by a range of polymer chemistries and cross-link densities. In all of these materials, the salient feature is that molecular order and orientation dictate the optical and mechanical properties of the material and how these respond to changes in environmental conditions.

The programming of the shape transformation of LCEs requires a coupled synthesis and processing approach. Programming of shape transformation occurs by orienting the liquid crystal phase prior to ultimate cross-linking.² This process encodes the stimulus-response of the material and ensures that the original form will be recovered on removal of the stimulus. Mechanical stretching of a partially cross-linked gel or elastomer followed by additional cross-linking to fix the alignment is the most widely employed method to synthesize aligned LCEs.^14,15 This technique can be applied to a large range of synthetic approaches. LCEs can also be oriented by first aligning a nematic monomer solution using directed self-assembly to surfaces, magnetic fields, or shear associated with fiber spinning or 3D printing.^16–20 Directed self-assembly techniques and 3D printing are particularly well suited to the fabrication of LCEs with spatially varied molecular orientation.^21–24 Understanding of the fundamental design concepts that can be used to design the molecular orientation to achieve a particular shape transformation has been an area of major focus with both finite element and analytical approaches described.^25–27 Many of the efforts focused on achieving uniaxial or patterned reversible deformation of LCEs have been motivated by the use of these materials as components of soft machines. Soft robots and self-cleaning coatings are among the many proposed applications that utilize the mechanical response of LCEs.^28,29

Finally, we note that LCEs, like low molar mass liquid crystals, are birefringent when oriented.¹⁰ Periodic changes of the birefringence through the elastomer thickness, as is observed in chiral nematic elastomers, lead to LCEs with structural color.⁷ This structural color can be modulated by stimuli that change the order of the material, such as heat, or through mechanical loading. The resulting materials have also been proposed for a wide range of applications, from smart windows to sensors.^30–33 

To probe the complex relationship between actuation and patterning in programmable liquid crystal elastomers (LCEs) with an eye toward concepts of morphogenesis, Warner and co-workers design a comprehensive experimental–theoretical–numerical framework, focusing on flat LCE sheets encoded with curved ridges and highlighting a trade-off between isometric stretching and curvature profiles.³⁴ Pedrini and Virga utilize a theoretical approach to relate the bending energy of a nematic polymer network to a specific shape change via energy minimization of ridge approximation, demonstrating applicability to the design of soft grippers with potential applicability to non-symmetric objects.³⁵ Nikzad and co-workers compare various force fields in the analysis of the responsive behavior of LCEs using molecular dynamics (MD) simulations of LCEs, highlighting the utility of these approaches in the prediction of shape memory response and the potential to accelerate material discovery.³⁶ Giudici and Biggins expand concepts of curvature (e.g., twisting and bending) to describe the spontaneous, large strain actuation of nematic LCE fibers via application of rubber elasticity principles.³⁷ Annabattula and co-workers develop a model to probe the triggered photo-chemical or thermomechanical response of LC films doped with azobenzene, incorporating structural parameters (e.g., thickness, exposure intensity, temperature) on the observed actuation.³⁸ Expanding the schemes of actuation in nematic LCEs, Lee and Bhattacharya detail a new theoretical treatment to describe large-scale deformation of thin structures, specifically the impact of specific stimuli and imposed mechanical forces.³⁹ Liu et al. provide an analytical solution to describe the actuation of nematic LCE/substrate assemblies, combining aspects of constitutive models and material parameters to inform bio-inspired design of soft robotics.⁴⁰ 

Yakacki and co-workers provide a perspective of the evolving processing strategies for LCE actuators, discussing new formulation development, the accessibility of various stimuli to drive actuation, and the utility of additive manufacturing approaches with an eye toward commercialization.⁴¹ Schenning and co-workers fabricate LC networks derived from flexible oxetane precursors that exhibit responsive mechanics, including one-way shape memory behavior and two-way actuation, for potential deployment in compact, actuating environments.⁴² Sharma et al. explore a microfluidic approach in the design of spherical LCEs, focusing on decoupling shell fabrication and polymerization and realizing complex, curved actuation driven by the ability to encode precise topological defects.⁴³ Lavrentovich and co-workers provide a framework to arrange disclination lines in LCE coatings, which generate microchannels when the coating when heated.⁴⁴ 

Shishido and co-workers extend the programmability of cholesteric LCEs by utilizing bending instead of extension, achieving wavelength tuning across almost the entire visible spectrum without high loading.⁴⁵ Using photonic crystal (PC) films assembled on a liquid crystal polymer (LCP) substrate as models, Zhang and Huo expand current bilayer theory to address the influence of spontaneous bending and contraction of the subtraction on wavelength tuning and showcase key parameters for photo- and thermo-responsive PC/LCP constructs.⁴⁶ By incorporating randomly metal nano-spheres within an LCE, Reyes and Reyes explore the role of twist defects for the construction of photonic materials, outlining key features, such as filler composition, that impact polarization.⁴⁷