Liquid crystal elastomers (LCEs) have emerged as a versatile class of soft actuator materials with applications spanning robotics, micromachines, artificial muscles, and tissue engineering. Their ability to undergo dramatic reversible deformation in response to various stimuli, including thermal, optical, chemical, electric, and magnetic cues, makes them highly attractive. For many applications, thermal actuation, driven by the LC-isotropic phase transition, is particularly relevant. In macroscopically oriented (monodomain) LCEs, cooling below Ti causes elongation, while heating above Ti results in contraction. Therefore, Ti is a critical parameter determining the actuation temperature and needs careful consideration for specific applications. A significant challenge has been the inability to tune Ti after LCE synthesis. Traditional methods like annealing are ineffective because the adjusted Ti reverts to its original value upon heating above Ti. While photoresponsive groups or photo-crosslinking can offer some control, their limited penetration depth and marginal tuning effects limit their practicality. Chemical modification of the structure is an option, but it requires restarting the molecular design and synthesis process. This research addresses this limitation by introducing a novel approach.

Previous research on LCEs has focused on various aspects including their synthesis, characterization, and actuation mechanisms. Studies have explored different strategies to manipulate LCE properties, such as incorporating photoresponsive groups to achieve light-driven actuation or employing chemical crosslinking methods to adjust material stiffness. However, these approaches have limitations such as limited penetration depth, marginal tuning effects, and the inability to adjust the actuation temperature without chemical restructuring. The existing literature highlights a significant gap in the ability to post-synthetically and reversibly tune the actuation temperature of LCEs, which is addressed in the present study.

The authors synthesized a model LCE, termed xLCE-BP, containing dynamic covalent bonds. This synthesis involved reacting diglycidyl ether of 4,4′-dihydroxybiphenyl with sebacic acid in the presence of a transesterification catalyst. The full crosslinking and removal of small molecules were ensured by curing and swelling procedures. The xLCE-BP was characterized using Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC). To investigate the tunability of Ti, the researchers subjected xLCE-BP samples to annealing at different temperatures (above and below the initial Ti). DSC was used to monitor the changes in Ti as a function of annealing time. The reversibility of Ti tuning was tested by subsequent annealing at different temperatures. The stability of the new Ti values was evaluated through repeated heating-cooling cycles using DSC and dynamic mechanical analysis (DMA). X-ray diffraction (XRD) was employed to investigate the structural changes occurring during annealing, examining both polydomain and monodomain samples. Finally, the researchers explored patterning different Ti values on a single LCE film using electrothermal films to demonstrate practical applications. Control experiments with LCEs lacking dynamic covalent bonds and other polymer systems (Vitrimer-BA, Vitrimer-PU, and PDMS) were conducted to validate the key role of dynamic bonds in the observed phenomena.

The study's central finding is the successful reversible tuning of the actuation temperature of fully cross-linked LCEs through annealing. This was achieved by incorporating dynamic covalent bonds into the LCE network. Specifically: 1. **Annealing below the initial Ti (T_i0):** Increased the Ti value of the polydomain xLCE-BP, with Ti increasing gradually with annealing time. Similar behavior was observed in cooling traces. 2. **Annealing above T_i0:** Decreased the Ti value of polydomain xLCE-BP, with a decrease proportional to annealing time and temperature. The T_i peak broadened and overlapped with the glass transition at longer annealing times. 3. **Reversibility:** The changes in Ti induced by annealing were reversible. Samples with increased Ti could have their Ti reduced by annealing at higher temperatures, and vice-versa. 4. **Stability:** While vitrimers inherently exhibit reduced thermal stability, the adjusted Ti values showed comparable stability to conventional LCEs, particularly for lower Ti values. Repeated heating-cooling cycles showed a gradual decrease in Ti, but the adjusted values remained significantly higher than the initial T_i0. 5. **Generality:** The method was shown to be applicable to different LCEs with dynamic covalent bonds, demonstrating its broader relevance. While the magnitude of Ti shift varied based on chemical composition, the overall trend of reversible tuning remained consistent. 6. **Structural Changes (XRD):** XRD measurements confirmed that annealing at temperatures below T_i0 resulted in a more compact layered structure, whereas annealing above T_i0 disrupted the lamellar stacking. This correlates with the observed changes in Ti. 7. **Monodomain LCEs:** Similar Ti tuning was demonstrated for monodomain xLCE-BP samples prepared by annealing under stretching conditions. The actuation stability was comparable to that of polydomain samples, with higher stability for samples with lower Ti. 8. **Patterning:** Successful patterning of different Ti values on a single LCE film was demonstrated using a combination of digital patterning, electrothermal films, and annealing, opening up new possibilities for complex actuation modes and applications such as anti-counterfeiting.

The successful tuning of actuation temperature in fully cross-linked LCEs via annealing demonstrates a significant advance in the field. This capability addresses the long-standing challenge of post-synthetic modification of LCE properties. The reversibility and relative stability of the adjusted Ti values, coupled with the generality of the method across different LCE chemistries, significantly expand the design space for LCE-based actuators. The ability to pattern different Ti values on a single film allows for complex and spatially-controlled actuation, opening exciting avenues for advanced soft robotics and other applications. The findings highlight the importance of dynamic covalent bonds in enabling the structural rearrangements necessary for the observed Ti tuning, contrasting with the behavior of traditional LCEs without such bonds. This study provides a pathway towards more versatile and adaptable LCEs for a wider range of applications.

This research successfully demonstrated the reversible tuning of the actuation temperature in fully cross-linked liquid crystal elastomers (LCEs) using annealing. The key to this achievement was the incorporation of dynamic covalent bonds into the LCE network. This novel approach allows for the creation of actuators with diverse actuation temperatures from a single material, greatly enhancing the design flexibility and potential applications of LCEs. Future research could explore the optimization of dynamic covalent bond chemistry to further enhance stability and investigate the effect of other parameters on the tunability of Ti.

While the study demonstrated the successful tuning of actuation temperature, some limitations should be acknowledged. The stability of the adjusted Ti values, while comparable to traditional LCEs, is still influenced by the inherent nature of vitrimers and the rate of transesterification reactions at high temperatures. Further optimization of the dynamic covalent bond chemistry might be needed to enhance long-term stability at high actuation temperatures. Additionally, a more comprehensive investigation into the long-term effects of repeated heating-cooling cycles is warranted.