Liquid Crystalline Elastomers as Renewable Functional Materials: From Chemistry to Application
Liquid Crystalline Elastomers (LCEs) are thermosets that belong to the family of “smart plastics”. They combine the softness and elasticity of elastomers, with the orientational order properties of liquid crystals (LCs), resulting in the ability for these materials to undergo large reversible deformation when subjected to external stimuli. This unique ability to actuate and perform work without mechanical parts has positioned LCEs as attractive materials for applications in fields such as soft robotics, sensors, surface coatings, and tissue engineering. The mechanical performance of LCEs and their capacity for large-amplitude, reversible actuation depend on the underlying chemistry and the alignment of the LC components in the network. However, achieving specific, complex, macroscopic-scale 3D structures with a predetermined actuation behaviour remains a challenge with conventional alignment techniques. The introduction of dynamic covalent chemistry into the networks of LCEs (to form xLCEs) was a significant breakthrough in the field, promising enhanced material processing and sample alignment. However, at the outset of this PhD, the understanding and control over the exchange dynamics of xLCEs was still lacking, stemming in part from a need for a broader library of materials with a greater variety of dynamic properties. Additionally, the field suffered from a lack of LCE applications geared towards addressing real-world problems. This thesis hence aims to contribute to the advancement of the field of LCEs in three key areas: (1) investigating the mechanics of xLCEs to establish fundamental principles, (2) exploring novel network chemistries, and (3) applying the knowledge gained to develop new and practical applications. First, I build on existing bond-exchange reactions to establish wider knowledge about factors controlling the material flow on a macroscopic scale in dynamic covalent polymer systems (vitrimers). I notably demonstrate that the bond exchange reaction activation energy is a poor predictor of material flow at high temperatures, with the network elastic modulus and the concentration of reactive functions for the bond exchange having a dominant impact on flow behaviours. This enhanced understanding provides design principles for controlling material dynamic properties in xLCEs. Second, I expand the library of exchange and network chemistries available for xLCE materials. Through the use of an epoxy-thiol reaction, I introduce a new network chemistry for an established covalent exchange reaction (transesterification). The reaction is simple, utilises mild conditions, cheap starting materials, and results in true elastomer xLCE materials with a wide range of material properties accessible through the system’s modular character. I show that the LC isotropic transition temperature, the material flow at high temperature from bond exchange, and the LC mesophase can all be controlled and tailored through a simple variation of the network composition. The expansion of material properties available broadens the range of possible outcomes for transesterification-based xLCE. Another new type of network with dynamic covalent properties that is introduced in this work is a poly(thiourethane) xLCE system. Such an xLCE thermoset network, containing dynamic covalent thiourethane bonds, is strengthened by physical crosslinks (H-bonding), resulting in a unique set of material properties such as enhanced strength and a remarkably high ductility at room temperature. The material obtained is the first example of an xLCE that can be reprocessed using industrially ubiquitous methods such as injection moulding and extrusion. Lastly, an example of use of LCEs towards a real world problem is investigated, namely through the use of LCEs as a to-scale Braille soft continuum interface for dynamic Braille devices. I demonstrate that the complex and numerous moving parts of a dynamic Braille device could be replaced by a single sheet of LCEs embossed with small actuating bumps. A simple moulding procedure produces a surface patterned with at-scale Braille bumps, as a result of a precise and complex internal organisation within the elastomer sample that emerges during polymerisation (as is evidenced by theoretical modelling). Unlike in previous attempts to use LCEs for Braille technology, the millimetre-scale protruding features are generated out of the bulk of the material, resulting in structural integrity, and high resistance to compression force. The localised bump-to-flat reversible actuation occurs on a timescale of seconds. The potential of this development for application into a complete Braille dynamic display are discussed. These findings open new lines of research in multiple directions for the field, in the hopes of advancing knowledge and bringing LCEs one step closer to commercialisation.
Braille, Dynamic Covalent Chemistry, Liquid Crystalline Elastomers, Polymers, Vitrimers
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