Development of applications of liquid crystalline elastomers from Materials to Applications

Abstract

Liquid crystalline elastomer (LCE) is a class of "smart polymer" with constituent molecules called mesogens, exhibiting orientational ordering. These mesogens can self-assemble into many types of ordering and transition to being isotropic reversibly subjected to external stimuli. In the last four decades, this field has seen many advancements: the first generation of polysiloxane LCEs successfully introduced this wonderful material to the world; the second generation of LCEs deployed much more robust and easier synthesis procedure, bringing in researcher from many other disciplines; the third generation of LCE utilised the bond-exchange behaviour in vitrimers to produce exchangeable LCE (xLCE), which enabled post-polymerisation alignment, welding and recycling of LCE. Stemming from such unique network structures and a wide range of synthetic chemistries, LCEs have a variety of remarkable properties. LCEs are renowned for the "soft elasticity", which is the ability to be elongated without increasing the stress; as well as the enormous viscoelastic dissipation originating from the rotation of internal nematic orders. LCEs are also famous for the reversible actuation properties which can be triggered by a wide range of external stimuli. LCEs have promising practical applications such as artificial muscles, smart textiles, sensors and soft robotics. This thesis aims to contribute to the field of LCE in the following three key aspects: (1) developing new LCE materials with novel synthesis routes and network structures, in order to find new or improved material properties; (2) investigating new fundamental physical effects associated with LCE in order to find new application areas; and (3) Designing and constructing demonstrator devices to illustrate the great potential of LCE materials. New materials First, novel side-chain LCEs based on the robust thiol-acrylate click reaction were developed. The effects of mesogen structures on the LCEs’ phase transition behaviours and network structures were investigated. The unusual features were the semi-crystalline nature of elastomers with non-polar mesogens, and the clear role of side-by-side rod dimerisation of polar mesogens leading to the higher smectic layer spacing. When stretching beyond the full alignment, the smectic structures were found to evolve in two ways: forming the coarsened Helfrich-Hurault zig-zag layer texture, or the large-scale stripe domains of uniform layer rotation in the systems with lower order parameter and the associated layer constraints. This then led to a new composite liquid crystalline elastomer, combining main-chain nematic and side-chain smectic together, which resulted in micro-phase separated regions of nematic and smectic ordering in the macroscopically homogeneous elastomer. Thermal phase transitions of both phases coexisting in the material were detected by calorimetry, and the nematic/smectic structure investigated by X-Ray scattering. The tensile stress–strain data revealed the key effect of such a multi-phase composite, where the nematic fraction adds ductility while the smectic fraction increases the modulus and mechanical stiffness. Thus, mechanical properties of this material type can be optimised by varying the composition. New physical effects Next, the effect of LCE viscoelastic impact damping was investigated, in comparison with a reference silicone rubber. This project focused not only on the energy dissipation, but also on the momentum conservation and transfer during the collision, because the latter determines the maximum force exerted, which is responsible for damaging the target and/or the impactor. To better assess the momentum transfer, I compared the collision with a very heavy object and the collision with a comparable mass, when some of the impact momentum is retained in the target receding away from the collision. A method to estimate the optimal thickness of an elastomer damping pad was proposed for minimising the energy in impactor rebound. It has been found that thicker pads introduce a large elastic rebound and the optimal thickness is therefore the thinnest possible pad that does not suffer from mechanical failure. This anomalously high vibration/impact damping in nematic poly-domain LCEs has also been assumed to be the cause of their anomalously high pressure-sensitive adhesion (PSA). The mechanism behind enhanced PSA was investigated by manufacturing generic LCE coated adhesive tapes with varying cross-linking densities. Industrial standard adhesion tests were performed to characterise LCE adhesion behaviours, which helped to reveal the strong dependence of adhesion strength on contact time. The long saturation time (sometimes exceeding 24 hours) is caused by the slow relaxation of local stress and director orientation in nematic domains after pressing against the surface. This mechanism was further confirmed by a freshly pressed and annealed tape reaching the same maximum bonding strength on cooling, when the returning nematic order is forming in its optimal configuration in the pressed film. Based on these findings, a class of much stronger, multi-use amine-acrylate LCE adhesive materials was developed. These adhesives exhibit low tackiness at room temperature; however, upon heating and annealing, they can be activated, enabling effective deployment. The LCEs were formulated to have tunable glass transition temperatures which is crucial for the low tackiness at ambient temperature. All formulations showed high adhesion strength (peel force) in the nematic region (1.0 to 1.6 N/mm) and low peel force in the isotropic region. Furthermore, the adhesive materials demonstrated the capability for reuse in more than five heating and cooling peeling cycles and have shown remarkable contamination tolerance to sand, oil, and dirt. Moreover, these adhesive materials displayed adhesion strengths (lap shear) that comparable to those of traditional PSAs, reaching up to 3 MPa, with a clean detachment. New device Finally, a spontaneous heliotracking prototype device was designed and constructed based on the differential light-induced actuation of LCE. The design drew inspiration from nature itself with many living organisms responding to light stimulus and track the light source. The synthesis of the actuator material involves a robust thiol-acrylate "click" polymerisation, while the addition of indocyanine green (ICG) dye imparts the sensitivity to broad-spectrum and near-infrared light. Highly reproducible thermal and photo-induced linear actuation was demonstrated. The device is based on a freely pivoting payload platform held in place by several linear LCE actuators around the 360° circumference. The side of the device, when exposed to light, has the actuators contracting and tilting the platform towards the light source. As the light source was moving around the device, the platform tilt followed, always exposing the payload face to the light; in the dark, the device recovers its neutral position.