Development of Liquid Crystalline Elastomers as Soft Mechanical Actuators
Liquid crystalline elastomers (LCE) are a class of loosely cross-linked rubbers that combine native liquid crystal phases (e.g., orientational order) and conventional elastomeric properties (e.g., rubber elasticity). Due to their ability to undergo spontaneous and reversible shape change when subjected to external stimuli, there is a continuously growing interest in the scientific community to construct soft mechanical actuators from these materials. However, the development of LCE actuators is currently limited within the academic community and it lacks interest from the industry. Despite being studied for the past 30 years, LCEs have not yet been used in any real-world applications, which can be attributed to several problems: First, the external stimuli to trigger LCE actuation is demanding (i.e., stimuli can be difficult to deliver). Second, less desirable properties of the LCE limit their deployment (i.e., diminishing mechanical properties during their actuation). Third, industries simply need to take time to absorb the exotic features of LCE material. This thesis aims to solve these three major hurdles. Regarding the first problem, this thesis attempted to use light actuation in LCE with the assistance of upconverting nanoparticles and azobenzene dyes. However, due to time and resource limitations, they yielded no successful discovery in the end. Nevertheless, some experimental plans were made in order to inspire the reader to join the further discussion. The second problem is caused by the decline of rubber modulus in LCE (at high temperatures), which causes reduced mechanical properties thus limiting their potential as soft actuators. To solve this, the thesis discussed different strategies to improve the mechanical properties of LCEs during their actuation. Various modification solutions to the material were explored, and the best procedures were selected to construct LCEs that are resistant to temperature change. Moreover, a new type of vitrimer-enabling mesogen and their corresponding LCEs were discovered and synthesised, which added to the existing library of LCE modification and offered extra functionality in LCE reprocessing. For the third problem, this thesis aims to accelerate the technical development of LCEs by discussing various LCE fabrication techniques using the existing methods from industries. The thesis successfully used the known direct-ink-writing technique (widely used in 3D printing) to fabricate LCE with sophisticated actuation patterns. The thesis introduced a scalable fiber spinning process that can cross-link the material on the fly to achieve a massive rate of production. The thesis also demonstrated how the spun LCE fibers are suitable to be made into smart fabrics using a common weaving machine. Finally, the thesis discussed some additional experiments that can be conducted in the future to fill the oversights in this PhD work and to complete our understanding of LCE material. On the sideline, a new project was also proposed to package those failed experiments in this thesis and to explore the exciting unknown territory.
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