Liquid crystalline elastomers (LCE) have been studied for over 30 years, due to their remarkable actuation characteristics, and also their unique ‘soft elasticity’. Fundamentally, LCE actuation can be induced by any stimulus that affects the underlying nematic order in the polymer; the thermally induced phase change is the most natural phenomenon. These properties make LCE a competitive material in applications ranging from soft robotics, to sensors and smart textiles. Recently the concept of ‘vitrimers’ was brought forward: unlike conventional crosslinked thermosets, vitrimers have a polymer network containing molecular groups capable of bond exchange reactions (BER); initiation of BER under stress can alter their internal topology. This results in the plastic flow under stress above the 'vitrification temperature' to fully reconfigure (and thus re-cycle) the ordinarily thermoset plastic. APRA project aimed to develop ‘exchangeable LCE’ (xLCE) that combine these properties: of LCE and vitrimers, making moulding complex geometries possible, as well as the recycling through subsequent thermal remoulding. APRA aimed to gain fundamental knowledge and produce re-usable LCE actuators in complex configurations required by practical application, thus transforming this field from a niche academic activity into the mainstream applied R&D. The objectives at the beginning of the project were divided into two groups: A - new materials chemistry, and B - new applications enabled by these materials.
All of the group-A objectives were achieved, and the field was expanded much further. Initially only thinking about the BER of hydroxyl transesterification, we have expanded the range of successful BERs (and their corresponding xLCEs) very wide. The thio-ester transesterification, the siloxane exchange, the borolate exchange, the disulfide exchange, and the imine exchange have all been pioneered in our project for xLCE, and thio-urethane exchange has also been used in new materials we produced. This had a lot of international traction, and other groups have added several other BERs to the growing list of possibilities, following our work. The invited core review has cemented our leading position in the field. This was possible, because APRA project has built a uniquely strong materials-chemistry team, which combined the people directly employed by APRA and also research visitors who contributed strongly: this team has delivered a lot of xLCE innovation in these five years.
Many application objectives of group-B were achieved, with high-profile publications and live demonstrators. The active LCE textile woven from a strong aligned LCE fibre, and the sunlight-sensitive heliotracking device, are the highlights. The dynamic Braille pixel has been made, and demonstrated to deliver sufficient motion range, blocking force, and response time to make it viable in tactile applications; we did not manage the 80x80 pixel Braille display, because there was no electronics support to build the addressing system. In 2019, we have made the discovery of anomalous adhesion in LCE, and much of our energy and focus have been diverted there. It is now a focus of many research groups worldwide (and also the most realistic industrial application of LCE among many other promises). We discovered that this adhesion effect is linked to viscoelastic dissipation in LCE, which in turn has a strong application potential in impact and vibration damping. A number of high-profile papers have been published on both of these topics, going increasingly deep into better under understanding these effects, and developing better LCE materials for them. This was not anticipated at the start of APRA, but perhaps is the second strongest result and greatest achievement of this project, with broad international recognition. The planned heat-driven motor has not been made, in spite of several increasingly complex and optimised devices designed and constructed. During APRA, we have realised that LCE can never work in such a motor in the configuration theoreticians have predicted, because of its intrinsic slow relaxation: to spin continuously, the LCE actuator strut must recover its shape very fast after the power stroke. However, our increasing understanding of LCE dynamics says that this is fundamentally impossible, as the anomalous internal dissipation is directly linked to slow relaxation. Therefore, this LCE heat-driven engine concept has been totally changed, and with all the new understanding, we may yet build such a motor based on the ‘old’ crankshaft ideas used in steam engines.
