Varied Alignment Methods and Versatile Actuations for Liquid Crystal Elastomers: A Review

Abstract

Liquid crystal elastomers (LCEs) are a class of polymer materials that can produce reversible strain responses under heat, [1,2] light, [3][4][5][6] electric field [7] (or electric induction heat), [8,9] magnetic field (or magnetic induction heat), [10,11] moisture, [12] or solvent, [13] etc. They have attracted intense attention due to their enormous potential applications in soft robots, [1,9,[14][15][16][17][18] soft actuators, [4,8,14,[19][20][21][22][23] sensors, [22,[24][25][26] artificial muscles, [27][28][29] biomedicine, [30,31] flexible electronics, [32,33] light energy collection, [34] etc.LCEs are composed of rigid mesogens and flexible polymer networks, with the result that the anisotropy of liquid crystal and the rubber elasticity of polymer network can be both shown in LCEs. [35][36][37] Induced by mechanical stress, external field, or surface effect, the long axes of the mesogens can be aligned into specific directions (expressed by the director n), and this process is called alignment. Liquid crystal polymers after alignment are chemically [38] or physically [39] crosslinked to form anisotropic monodomain (including nematic, smectic, or cholesteric) LCEs. On the contrary, LCEs are in a polydomain state with isotropy when the directors of liquid crystal domains are arranged disorderly. Triggered by the external physical or chemical stimuli, the response and deformation originated from the phase transition from a liquid crystalline phase to an isotropic phase is significantly enhanced in the monodomain LCEs. This process is macroscopically manifested as the size shrinkage of LCEs in the orientation direction. [40,41] Therefore, the orientation degree of LCEs affects the maximum deformation actuated by the external stimuli to a large extent, whereas the orientation direction of LCEs determines their macroscopic deformation direction. The programmability of the mesogen orientation lays a foundation for achieving versatile deformation actuations of LCEs. [42] Aligning the mesogen orientation and achieving versatile actuations of LCEs are always the central research objects in the field of LCEs, and have developed rapidly in recent years. However, few reviews are addressing these two important issues simultaneously.Herein, in Section 1, we briefly elucidate the relationships between the microscopic orientation structure of LCEs, external stimuli, and macroscopic deformation characteristics of LCEs. In Section 2, we introduce three types of LCE alignment methods and their basic principles, and focus on comparing their programmability toward the mesogen orientation. In Section 3, we summarize two types of actuations for LCEs on the basis of the latest development, and focus on comparing their deformation controllability. In Section 4, we propose an outlook for the future key technologies to develop versatile, precise, and fastresponsive deformation actuations for LCEs. This review aims to help people to design and prepare LCEs with programmable orientation structure and excellent actuation characteristic...

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