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Active Polymers for Renewable Functional Actuators

Periodic Reporting for period 2 - APRA (Active Polymers for Renewable Functional Actuators)

Reporting period: 2020-04-01 to 2021-09-30

Liquid crystalline elastomers (LCE) have been on the front of research and development for over 30 years, due to their remarkable actuation characteristics, but also due to their unique ‘soft elasticity’. The linear actuation of LCE can achieve a maximum strain of 500% and is fully reversible: the equilibrium length of an aligned LCE sample directly reflecting the degree of the internal nematic order. Fundamentally, LCE actuation can be induced by any stimulus that affects the underlying nematic order in the polymer, and although the thermally induced phase change is the most natural phenomenon, the change in order can be triggered by other stimuli, such as light and magnetic field, when incorporating photoabsorbers or magnetic nano-particles. These properties make LCE a competitive material in practical applications ranging from soft robotics, to sensors and smart textiles.
Recently the concept of ‘vitrimers’ was introduced by Leibler et al. Unlike conventional covalently crosslinked thermosets, vitrimers have a polymer network containing molecular groups capable of bond exchange reactions (BER), and the initiation of BER under stress can alter their internal topology. Today, there are many examples of such generic dynamic covalent chemistry, which in all cases enables programming after full cross-linking by activating the BER. In the context of ‘exchangeable LCE’ (xLCE) this mitigates the competition between alignment and crosslinking and makes moulding complex geometries possible – as well as the recycling through thermal remoulding.
The objectives are:
A1: Optimise the chemical structure and synthetic procedures to make nematic xLCE materials in quantities sufficient to realise the target applications.
A2: Select the most suitable photo-absorbing dopants and optimise their dispersion in xLCE matrix.
A3: Develop and optimise procedures for the medium-large scale xLCE synthesis.
A4: Verify chemical safety and biocompatibility of xLCE for use in human contact.
B1: Develop and demonstrate the actuating textile-grade fibre in continuous production; develop and demonstrate ‘active fabric’ by combining the xLCE fibre with standard yarns in simple weaves. Side-track applications in reversibly self-tightening strings and ropes.
B2: Develop and demonstrate the continuous-spin rotary motor using photo-actuating xLCE as the driving unit. Side-track study of motor efficiency, and of electric generation by a dynamo.
B3: Develop and demonstrate single-pixel and 80x80 matrix dynamic tactile display, using the xLCE pin as the vertical actuator and no other moving parts.
B4: Develop and demonstrate three active elements of microfluidic circuitry: peristaltic pump, membrane pump and relay flow switch between channels, all achieved by the photo-actuating xLCE elements of the capillary tubing.
B5: Develop and demonstrate the heliotracking device, based on a solid xLCE filament optimised such that it bends to always point directly at the light source (sunlight).
Two postdoctoral researchers (appointed staggered by a year: Dr Mohand Saed started in 2018, and Dr Goutam Kar in 2019) and two PhD students (also staggered by a year: Alexandra Gablier started 2018, and Xueyan Lin in 2019) formed the core of the research team. In addition, we have hosted two self-funded long-term postdoctoral researchers (Dr. Takuya Ohzono from Japan, and Dr. Weike Zou from China). Dr. Saed and Ms Gablier are full-time organic chemists, and took the lead on developing new xLCE materials and characterising their properties - eventually optimising the material chemistry. Dr. Kar and Mr Lin are more focused on physical characterisation and developing application systems, the first focussing on melt processing the other on 3D printing. Dr Kar has resigned in 2021, and their place was offered to Dr. Hsin-Lin Liang, who is presently the second PDRA on the project.

There has been a lot of progress on new xLCE materials, developing and critically comparing the alternative concepts:
1. Epoxy-thiol elastomers, where the mesogens were epoxy-terminated (which required their synthesis in the lab) - connected and crosslinked by thiol spacers. The BER is the hydroxyl transesterification, which we found to be the 'hard' type that requires high temperature and long time to flow.
2. Thiol-acrylate elastomers, where the mesogens were commercially available acrylate-terminated units - but the thiol-terminated spacers contained borolate groups (which required their synthesis in the lab). The BER is the boronic-transesterification, which was the 'easy' type with low temperature and fast flow rates.
3. Thiol-acrylate/ene elastomers, with the commercial acrylate-terminated mesogens, linked by thiol spacers - but the crosslinkers were based on siloxane (or in addition - some or all siloxane spacers used). The BER is the siloxane exchange, which we found to be the 'hard' type that requires high temperature and long time to flow.
4. Thiol-acrylate elastomers, with the commercial acrylate-terminated mesogens, linked by thiol spacers and crosslinkers. The thiol-ester BER was used here, which was the 'easy' type with low temperature and fast flow rates
5. Amine-acrylate elastomers, with the commercial acrylate-terminated mesogens, linked by amine spacers and crosslinkers, which additionally had hydrogen-bonding between parallel chains. The BER was not applicable here: the re-moulding and alignment were achieved via two-step process using the large difference in reacting rates of acrylate with primary and secondary hydrogen of amine.
6. Imine-based mesogens producing a different kind of xLCE.
7. Dual networks, combining two of the above elastomer chemistries in one interpenetrating network, to improve its mechanical properties.

An invited review article was produced for ACS Chemical Reviews.

Several fundamental physical properties of LCE were investigated, with new progress achieved in:
1. Viscoelasticity and relaxation mechanisms of vitrimer networks, and partial vitrimers where a fraction of crosslinks remains permanent.
2. Internal constraints introduced by crosslinks and entanglements, affecting relaxation rates in LCE.
3. The diffuse nature of LC phase transition, affected by random disorder and polydomain structure in LCE.
All these aspects of work will continue, with greater emphasis on physical properties and xLCE application design.
The practical applications that were completed, and published, are:
1. The dynamic adhesion of LCE surface, which switches between being very 'sticky' in the LC phase - to 'not sticky' in isotropic phase, suggesting applications in grippers and mechanical couplers.
2. Dynamic manipulation of friction in LCE-textile composite was demonstrated, with the friction coefficient switching between high in the LC phase and low in isotropic phase, suggesting applications in smart textiles.
3. Heliotracking device that uses light-induced LCE actuation to track the face of the Sun with the payload platform, to increase the daily flux on the solar cell.
4. The work had developed a side track, with our group exploring the vitrimers produced from commodity thermoplastic polymers, adding novel and practically attractive properties.
Towards the end of the project, we will aim to cover all planned objectives about xLCE properties and applications, and additionally expand the work on vitrimers from commodity thermoplastics.
part 2 of the Public Reporting of the project by European Dissemination Agency
part 1 of the Public Reporting of the project by European Dissemination Agency
part 3 of the Public Reporting of the project by European Dissemination Agency