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Liquid crystal elastomers
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Part 1

Overview of liquid crystal elastomers

Liquid crystal elastomer (LCEs) is a kind of polymer with liquid crystal phase and soft elasticity formed by the liquid crystal polymer after proper crosslinking. The synthesized LCEs is usually multi-domain state, and the multi-domain state is transformed into single domain state (m-LCEs) after the macro-orientation of the liquid crystal elementary by external force. When m-LCEs is stimulated by external elements, the phase transformation of liquid crystal makes the shape change of m-LCEs reversible, which has great application potential in flexible actuators, medical devices, bionic materials and other fields.

Liquid crystal elastomer development process

In 1975, De Gennes et al. proposed the idea of liquid crystal elastomers, and the liquid crystal polymer flexible materials with three-dimensional network structure can be obtained by moderately crosslinking small molecular liquid crystals. The first LCE was synthesized by Finkelmann et al. by a two-step crosslinking method. The liquid crystal elements of the LCE exist in the cross-linked network in the form of side chains. After the polymer network is stretched, the liquid crystal elements produce macro orientation, and the elastomer changes from white to transparent, with thermal response characteristics. The LCE contracts along the orientation direction when heated and extends along the orientation direction when cooled, with the ability to reverse shape change. The mechanism of the reversible shrinkage elongation of the thermal response LCEs is as follows: when the oriented LCEs are heated, the liquid crystal elementary changes from anisotropy to isotropy, which causes the film to shrink along the orientation direction. When cooled, the liquid crystal primitive exhibits the opposite phase transition process, which makes the size of the film change reversibly. This reversible deformation can produce internal stress, the size of the internal stress is comparable to the human muscle, so LCEs has received a lot of attention in the field of artificial muscles. At present, there are many types of LCEs, and different types of LCEs are suitable for different needs. According to different stimulus response modes, LCEs can be divided into thermal response LCEs, optical response LCEs and magnetic response LCEs, etc., according to the type of liquid crystal primitive can be divided into nematic LCEs, smectic LCEs and cholesteric LCEs, and the commonly used liquid crystal primitive is mostly nematic phase. According to the position of the liquid crystal in the polymer cross-linking network, the liquid crystal can be divided into main chain LCEs and side chain LCE. Compared with side-chain LCEs, main-chain LCEs have better mechanical properties, deformation response and driving stress than side-chain LCEs. Therefore, the application potential of main-chain LCEs is greater, and it has become a research hotspot.


Preparation of single domain liquid crystal elastomers

At present, there are several methods for preparing single-domain liquid crystal elastomers:

(a) One-step method, that is, all reaction monomers are directly mixed to form a film. As shown in Figure 1-1a, suitable for reaction mixtures with low viscosity, the liquid crystal is oriented by means of surface friction, electricity or magnetism, etc., and then the cross-linked network is fixed by optical or thermal polymerization. LCEs prepared by one-step method can be oriented before polymerization, but due to the shape of the liquid crystal box, the size of LCEs is small and the driving ability is limited.

Liquid crystal elastomers

(b) Two-step method, that is, the liquid crystal polymer prepolymer is first prepared and then reacted with the crosslinker to form a cross-linked network. As shown in Figure 1-1b, it is suitable for the preparation of siloxane single domain liquid crystal elastomer. The liquid crystal monomer and the crosslinking agent are connected to the siloxane main chain in the form of side chain by hydrosilylation addition reaction, and then the single domain liquid crystal elastomer is obtained through the second cross-linking step. The advantage of the two-step method is that large single-domain liquid crystal elastomers can be prepared, and the preparation method is relatively simple. However, the orientation of LCEs is completed by external tensile force, and it is difficult to prepare LCEs with complex shapes.

(c) Post-crosslinking method, the prepared loosely crosslinked liquid crystal elastomer is stretched by external force to achieve the orientation of the liquid crystal unit, and then the orientation network is cured by secondary crosslinking through optical crosslinking and crosslinking agent reaction. By introducing liquid crystal metal-bound 2, 6-bibenzimidazopyridine (Bip) monomer into the polymer network through a thiol-ene click chemical reaction, LCEs with multiple stimulus responses (light, heat, and metal) were prepared through a second photocrosslinking orientation.

(d) Dynamic covalent bond crosslinking, using dynamic covalent bonds to cross-link liquid crystal polymers, the prepared LCEs have repeatable editing, shape memory and self-healing functions. The dynamic covalent bonds suitable for LCEs include allyl sulfur, transesterification and disulfide bonds. A programmable, surface weldable and degradable LCE was synthesized by transesterification. The synthesized LCE films were programmed under creep deformation induced by transesterification reaction, and 31% free-standing bidirectional drive was achieved. However, the problem of dynamic covalent bond is that it breaks the bond and reconnects at high temperature, resulting in continuous decline in drive performance during long-term thermal drive.

(e) Physical cross-linking, using a quadruple hydrogen bond as a cross-linking agent, introduces a liquid crystal unit containing azophenyl group into the polymer network in the form of a side chain, and develops a side chain type LCEs with a light response. The electrostatic adsorption of quadruple hydrogen bond provides crosslinking and self-healing ability for the material, and the cis-trans isomerization of azobenzene makes the film exhibit reversible photodeformation under different light sources


Stimulus response and principle of liquid crystal elastomers

The LCEs response behavior originates from the reversible phase transformation of the liquid crystal primitive after orientation, which transforms into a single domain state. When subjected to external stimulation, the conformation of the liquid crystal primitive changes, resulting in changes in the overall structure. The type of the liquid crystal primitive, the structure of the liquid crystal monomer and the structure of the cross-linked network have an important influence on the deformation ability of the material.

Thermal response

Thermal response is one of the most common response modes in liquid crystal elastomers. Figure 1-2 shows the stimulus response mechanism of thermal response LCEs. Reversible liquid crystal phase transformation occurs in the oriented liquid crystal primitive when the temperature changes, resulting in reversible contraction and elongation of the liquid crystal primitive with the orientation direction, and thus reversible shape change of the material. Under the action of mechanical stretching, the orientation change of the liquid crystal elementary is manifested as the bending, curling and spiraling of the material. By controlling the phase transition temperature and cross-linking strategy of liquid crystal elastomer, a liquid crystal elastomer ink for 4D printing was designed. The resulting deformed three-dimensional structure has a programmable shape change when heated. The second is to prepare a heat-responsive liquid crystal elastomer covered with directional microcolumns. At room temperature, the LCE sample is filled with a large number of microcolumns with an average length of 8.76 μm, forming a superhydrophobic surface with a water contact Angle (WCA) of 135°. When the temperature rises above the clarification point, all the microcolumns disappear and the LCE surface becomes completely flat, showing a hydrophilic state with a WCA of 64°. The surface of the microstructured LCE has good circulability in multiple heating/cooling cycles.

Liquid crystal elastomers

Photo response

As a kind of clean energy, visible light has the characteristics of accurate operation, remote control, portability and fast response. The liquid crystal unit of the photoresponsive LCEs contains photosensitive groups. Under specific illumination, the structure of the photosensitive groups changes, and the shape of the LCEs changes. Azophenyl groups are one of the most commonly used photosensitive groups in LCEs. Figure 1-3 shows the reverse photoisomization mechanism of azophenyl groups. When the arrangement direction of azophenyl groups is consistent with the orientation direction of the film, under the irradiation of 365 nm ultraviolet light, the stable trans-structure of azophenyl groups changes into cis-structure, driving the whole material to bend towards the light source. When the visible light is irradiated or heated at 460 nm, the opposite change occurs. When the orientation of the azophenyl group is perpendicular to the orientation direction of the film, the bending change of the film is opposite.