Tough, Shape-Changing Materials: Crystallized Liquid Crystal Elastomers

Abstract

Approaches for the synthesis and processing of responsive materials that combine robust mechanical properties and the ability to undergo shape change in response to a stimulus are of intense interest. Here, we report an approach to integrate these properties by synthesizing liquid crystal elastomers (LCEs) that can be aligned and subsequently crystallized. We polymerize LCEs in the isotropic and nematic states and characterize the resulting actuation and mechanical properties. After polymerization, each of these materials can be reversibly crystallized. By crystallizing LCEs, we demonstrate stiffer and tougher shape changing materials. Notably, crystallized samples exhibit moduli 2 orders of magnitude higher and toughness 5 times higher than nematic elastomers. Heating melts the crystallinity and then induces shape change via melting of the liquid crystalline phase. These LCEs are capable of high load bearing during actuation, up to 1.3 MPa, and high work capacity, up to 730 kJ/m 3 . These aligned and crystallized LCEs offer promising benefits as dynamic smart materials with robust mechanical properties.

Effect of Isomeric Amine Chain Extenders and Crosslink Density on the Properties of Liquid Crystal Elastomers

“…At comparable crosslink densities, LCE-I2 generated a higher blocking stress and exhibited a higher breaking temperature, which imply that the rigid nature of the isobutyl group makes LCE-I2 mechanically more robust than LCE-N2; this is in good agreement with the stress-strain test results. Meanwhile, the higher blocking stress of our LCEs compared with previously reported LCEs [34,37,45] is possibly because of the relatively high crosslink density (i.e., high rubbery modulus) of our LCEs.…”

“…For example, a high crosslink density increases the glass transition temperature and mechanical stiffness of LCEs; however, the work capacity decreases due to a low actuation strain [34][35][36]. As separately demonstrated by the research groups of Ware [37] and Yakacki [38], crystallizable LCEs not only increase the mechanical properties, including stiffness and toughness, but also enhance the blocking stress and work capacity of thiol-acrylate LCEs. Recently, our group also reported that the glass transition temperature and mechanical properties of main-chain LCEs can be effectively manipulated by adjusting the length of the alkyl groups in the primary amine chain extenders, which can systematically alter the actuation temperature [30].…”

Desynchronized Liquid Crystalline Network Actuators with Deformation Reversal Capability

“…Liquid crystal networks including elastomers (LCNs) have become a particularly promising material system for intelligent actuators and soft robots, thanks to their ability to reversibly and macroscopically deform upon the order-disorder phase transition, when properly processed [1][2][3][4][5][6][7][8][9][10] . By adjusting the alignment of mesogens and/or the distribution of crosslinking domains, monolithic LCNs can display a wide range of predesignated deformations, including those based on contraction/extension, bending, twisting and their various possible combinations [11][12][13][14][15][16][17][18][19] . Accordingly, a myriad of complex shapes (e.g., wave, accordion, helix, saddle shapes, periodical patterns and 3D pro les of Gaussian curvatures) [20][21][22][23][24][25][26][27][28][29] as well as robotic and bionic motions (e.g., gripping, rolling, walking, swimming and oscillating) [30][31][32][33][34][35][36][37][38][39][40] have been achieved, making the eld ourish.…”

Dynamically Crystalizing Liquid‐Crystal Elastomers for an Expandable Endplate‐Conforming Interbody Fusion Cage

“…LCEs are traditionally considered as soft and rubbery materials, yet recent studies have shown by changing the composition of the polymer backbone, LCEs can experience dynamic and transient crystallization over short and long timescales -a process accompanied by a large increase in elastic modulus. 20,35 This property was initially explored by Saed et al who demonstrated an LCE vascular stent which was initially soft and flexible but became rigid and structural after time. 35 In our proposed methodology, the significant increase in modulus of the device between implantation (soft) and the functional state (rigid) allows insertion of a relatively large device in a minimally-invasive manner.…”