Enabling liquid crystal elastomers with tunable actuation temperature

Liquid crystal elastomers (LCEs) can undergo large, reversible deformations when exposed to thermal, optical, chemical, electric, or magnetic stimuli, making them promising for robotics, micromachines, artificial muscles, and biomedical devices. For thermally driven actuation, the isotropization temperature (Ti) determines the temperature at which aligned (monodomain) LCEs contract upon heating and elongate upon cooling. Post-synthesis tuning of Ti has historically been difficult because thermal properties are closely tied to chemical structure and are essentially fixed after crosslinking. Strategies using photoresponsive mesogens can disrupt LC order via trans–cis isomerization, and photo-crosslinking can slightly alter network density, but light penetration is limited and shifts in Ti are marginal. Conventional annealing of LCEs can temporarily alter Ti via microstructural changes, but the effect vanishes upon heating above Ti, which is required for actuation. Therefore, there has been no effective post-synthesis strategy to tune actuation temperature without changing composition. The authors propose that introducing dynamic covalent bonds (vitrimer-type associative exchange) into LCE networks enables topology rearrangement during annealing, preserving the annealed microstructure and thereby stabilizing new Ti values even after heating above Ti. This allows reversible and programmatic tuning of actuation temperature in fully cross-linked LCEs.

Prior work established LCE applications and stimulus-responsive behavior (e.g., photomechanics, electrically controlled actuators, light-driven soft robots). Photochromic mesogens and photo-crosslinkable groups can modulate LC phases, but penetration depth and small Ti shifts limit utility. Annealing near Ti in thermotropic LC polymers can alter phase behavior transiently, with effects disappearing upon reheating above Ti. Dynamic covalent networks (covalently adaptable networks, vitrimers) enable associative exchange reactions (e.g., transesterification, transcarbamoylation) leading to reprocessability, healing, welding, and reprogramming without changing crosslink density. Prior LCE vitrimers demonstrated reprogrammability and reshaping, but systematic post-synthesis, reversible tuning of Ti (and thus actuation temperature) in fully cross-linked LCEs had not been established. The current work builds on these concepts to provide a robust, reversible annealing-based method to set Ti via vitrimer topology rearrangement.

Materials and synthesis: The model exchangeable LCE (xLCE-BP) was synthesized by reacting diglycidyl ether of 4,4′-dihydroxybiphenyl with sebacic acid (1:1 epoxy:carboxyl stoichiometry) using 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) as a transesterification catalyst (0.25 mol% relative to COOH) at 170 °C. Samples were cured for 6 h (unless otherwise noted), swollen to remove small molecules, and dried 24 h. FTIR confirmed epoxy consumption (909 cm−1 peak disappearance after 3 h). TGA showed ~5% weight loss at ~350 °C before/after swelling. The material is polydomain as-prepared. Annealing protocols: Annealing temperatures (Ta) were chosen relative to the original isotropization temperature Ti0 and glass transition Tg to control exchange rate and phase state. Below Ti0 (e.g., 110 °C for xLCE-BP with Ti0 ≈ 114 °C) promotes LC growth with slower exchange; above Ti0 (e.g., 140–180 °C) accelerates exchange in isotropic state. Long-term anneals (hours to 50 days) were performed to program Ti. Re-annealing experiments probed reversibility (switching Ti upward at Ta < new Ti or downward at Ta > new Ti). Characterization: Thermal transitions were measured by DSC (second heating/cooling cycles; 5 °C/min). Mechanical actuation stability was tested via DMA/thermal cycling and hot-plate cycling for monodomain samples. X-ray diffraction (2D and integrated 1D profiles) characterized smectic layer spacing and order; measurements used a SAXSLAB Ganesha system (Cu Kα source; Pilatus 300K detector). Comparative controls included vitrimers without LC order (Vitrimer-BA via bisphenol A diglycidyl ether; Vitrimer-PU via transcarbamoylation) and PDMS elastomer; all were swollen and dried before DSC. Monodomain preparation: Polydomain xLCE-BP samples were uniaxially stretched during annealing to fix alignment through dynamic bond exchange, yielding aligned monodomain films with tunable Ti. Patterning: Electrothermal polyimide (PI) heaters with CAD-defined patterns were laminated to xLCE films to locally anneal areas at controlled temperatures/times, producing films with spatially varying Ti values. Data handling: Multiple annealing times/temperatures were systematically explored (110 °C up to 50 days; 140 °C for days; 180 °C up to 4 days), with Ti/Tg tracked on heating and cooling. Thermal cycling stability was assessed over tens of hours in DSC and up to 200 cycles on a hot plate. XRD peak positions/intensities were monitored to infer smectic A layer spacing and order changes.

- Post-synthesis tuning of Ti in fully cross-linked LC vitrimer networks: For polydomain xLCE-BP (Ti0 ≈ 114 °C; Tg ≈ 58 °C), annealing below Ti0 (Ta = 110 °C) increases Ti substantially and progressively: to 131 °C (3 d), 145 °C (5 d), 155 °C (10 d), 165 °C (15 d), reaching nearly 180 °C after 50 d. Ti peaks often split with extended annealing, indicating polymorphism/polydispersity. - Annealing above Ti0 decreases Ti: At Ta = 180 °C, Ti decreased from 114 °C to 103 °C (3 h), 95 °C (6 h), 88 °C (12 h), 81 °C (1 d), 80 °C (2 d), 77 °C (4 d); peaks broaden, approaching Tg. At Ta = 140 °C for 5 d, the decrease is slower than at 180 °C. - Reversibility: Ti programmed high at Ta < Ti0 can be reduced by re-annealing at Ta > Ti (e.g., 178 °C → 167 °C after 1 d at 180 °C; → 97 °C after 6 d). Conversely, Ti lowered by high-temperature annealing can be raised by re-annealing at Ta below the new Ti (e.g., after 180 °C for 3–6 h yielding Ti = 103–95 °C, re-annealing at 80 °C for 30 d raised Ti to 122–105 °C; after longer high-T anneals producing Ti = 88–77 °C, re-annealing at 65 °C for 50 d raised Ti modestly to 92–79 °C). - Stability: Despite vitrimer exchange at high T, programmed Ti values show practical stability under cycling. Unannealed sample’s Ti dropped from 114 °C to 83 °C after ~35 h DSC cycling. A high-Ti sample (178 °C) decreased to 147 °C after 35 h cycling, remaining well above the original Ti. A low-Ti sample (77 °C) decreased slightly to 72 °C after 83 h DSC cycling between −10 and 180 °C (3 °C drop over final 61 h). - Generality: Similar Ti tuning trends observed in other LC vitrimers: xLCE-DHMS (smectic) and xLCE-PU and xLCE-RM257 (nematic). At Ta < Ti0 (e.g., 80 °C), Ti increased modestly: xLCE-DHMS from 84 °C to 102 °C (heating) and from 81 °C to 95 °C (cooling) after 50 d; xLCE-PU from 102 °C to 119 °C (heating) and 96 °C to 111 °C (cooling). At Ta > Ti0, Ti decreased, faster at higher Ta (e.g., xLCE-DHMS to 66 °C at 140 °C for 5 d; to 55 °C at 180 °C for 2 d). Non-LC vitrimers (Vitrimer-BA, Vitrimer-PU) and PDMS showed negligible Tg/Ti changes after 50 d annealing, underscoring the role of LC order plus dynamic bonds. - Monodomain actuation tuning: Stretch-annealing produced aligned films with tunable Ti: at Ta = 110 °C, Ti increased over time to 133 °C (3 d), 137 °C (5 d), 153 °C (10 d), 161 °C (15 d) on heating; cooling Ti values were 122, 124, 134, 140 °C, respectively. At Ta = 180 °C for 24 h, Ti ≈ 90 °C. Actuation stability: A Ti ≈ 90 °C sample retained performance over 100 h (100 heating-cooling cycles). A Ti ≈ 150 °C sample showed minimal actuation strain change after 200 rapid cycles between 140–160 °C (≈7 min per cycle). - Multi-temperature actuator set: From one material (Ti0 ≈ 114 °C), five monodomain actuators were programmed with Ti ≈ 90, 112, 122, 135, 152 °C, each contracting at its programmed threshold. - Spatial patterning of Ti: Using patterned electrothermal PI heaters, different regions of a single polydomain film were annealed to distinct Ti values (e.g., unheated 113 °C; patterned 125–152 °C). As temperature increased, regions transitioned to isotropic and became transparent at different thresholds, revealing/hiding patterns stepwise. - Structural insights (XRD): All samples display smectic A lamellar order. Annealing at Ta < Ti0 strengthened order and reduced layer spacing (peaks at 2θ ≈ 2.99° and 5.93° shift right; d-spacings decrease). Annealing at Ta > Ti0 weakened/suppressed lamellar reflections (e.g., d ≈ 28–30 Å peak nearly disappeared after 180 °C, 4 d), indicating reduced lamellar regularity. In monodomains, meridional reflections confirm alignment; additional peaks appear and primary peaks split upon Ta < Ti0 annealing, consistent with coexistence of smectic layers and DSC peak splitting.

The study demonstrates that incorporating dynamic covalent bonds (associative exchange, e.g., transesterification) into liquid crystal elastomer networks allows annealing-induced microstructural reorganization to be topologically fixed, enabling stable, reversible tuning of the isotropization temperature Ti post-synthesis. This directly addresses the long-standing challenge of setting actuation temperature without re-synthesizing LCEs. The mechanism is supported by XRD: annealing below Ti0 compacts and enhances smectic layering (raising Ti), while annealing above Ti0 disrupts lamellar regularity (lowering Ti). Because the network topology rearranges during annealing and then is preserved, the new Ti persists even after heating above Ti, unlike conventional LCEs. The approach is general across different LCE chemistries and phases (smectic and nematic), though the magnitude and rate of Ti change depend on molecular structure and annealing conditions. Actuation performance is stable for low to moderate Ti values due to slower exchange at operating temperatures; at higher Ti, exchange accelerates and stability decreases but remains acceptable with low catalyst loading. The ability to fabricate multiple monodomain actuators from one bulk material and to pattern spatially varying Ti on a single film broadens design space for complex, multi-threshold actuation and functional devices (e.g., anti-counterfeiting patterns, distributed actuation in soft robots).

This work introduces a reversible, post-synthesis method to tune the isotropization temperature and thus actuation temperature of fully cross-linked liquid crystal elastomers by leveraging dynamic covalent bond exchange during controlled annealing. Ti can be increased up to ~180 °C or decreased to ~77 °C from an initial ~114 °C in xLCE-BP, with the programmed values remaining stable through thermal cycling. The strategy extends to diverse LCE chemistries and enables fabrication of monodomain actuators with tailored actuation temperatures and spatially patterned Ti within a single film. These advances decouple actuator design from initial synthesis, simplifying materials selection and enabling sustainable reuse and reprogramming. Future research should systematically dissect how exchange kinetics, catalyst content, LC phase type, and molecular architecture affect the rate and extent of Ti tuning, optimize stability at high operating temperatures (e.g., catalyst reduction or alternative exchange chemistries), and expand patterning/control methods for complex multi-zone actuation.

- Thermal stability at high operating temperatures is intrinsically limited in vitrimers due to faster exchange reactions, leading to gradual Ti drift during extended cycling; lowering catalyst content mitigates but does not eliminate this. - Large upward shifts in Ti at Ta < Ti0 require long annealing times (days to weeks), potentially limiting throughput. - The magnitude of Ti change varies with molecular structure; some systems (e.g., xLCE-PU, xLCE-DHMS) exhibit smaller shifts under identical conditions. - DSC peaks split and broaden with annealing, indicating polymorphism that may complicate precise Ti definition and narrow operating windows. - Stability improvements at high Ti may require additional strategies (e.g., catalyst removal), not fully explored here. - The study primarily uses lab-scale samples; scalability and uniformity of bulk annealing/patterning need further validation.