Inverse Elastocaloric Output in Supramolecular Liquid Crystalline Elastomers

L iquid crystal elastomers (LCEs) are an important class of stimuli-responsive polymeric materials that combine the elastic properties of polymer networks with the anisotropic properties of liquid crystals (LCs). [1][2][3][4][5] This combination imparts LCEs with remarkable mechanical properties that can be accessed by external stimuli such as temperature, light, magnetic field, and electric field. 6,7 LCEs combine the molecular order of LCs and the entropic elasticity of the polymer matrix. The structural anisotropy is retained from liquid crystalline (e.g., mesogenic) monomer precursors. [8][9][10] The coupling between the molecular alignment of LCs and the macroscopic behavior of the polymer matrix allows LCEs to undergo significant, reversible deformations in response to small changes in external conditions. [11][12][13][14][15][16] The stimuli-responsive deformation makes LCEs highly attractive for applications in soft robotics, 17,18 actuators, [19][20][21][22][23] sensors, [24][25][26][27] and artificial muscles. [28][29][30] The elastocaloric exect, a phenomenon where materials undergo heating or cooling due to mechanical deformation, has been observed in various systems, most notably in natural rubber and shape memory alloys (SMAs). [31][32][33][34] In natural rubber, stretching aligns polymer chains (strain-induced crystallization), reducing entropy and causing a temporary increase in temperature. Upon releasing the strain, the chains return to a disordered state, and the material cools. [34][35][36] In SMAs, the elastocaloric response is associated with stress-induced martensitic phase transitions, where structural rearrangements under load result in significant entropy changes and corresponding thermal exects. These materials have demonstrated promising applications in solid-state refrigeration technologies, oxering energy-e,cient alternatives to conventional vaporcompression cooling. 37,38 Recently, attention has turned to exploring other classes of materials such as shape memory polymers (SMPs) as well as LCEs. 38,39 These materials exhibit unique coupling between mechanical deformation and molecular ordering, providing exciting opportunities to explore and optimize elastocaloric performance for responsive, low-hysteresis, and cyclable cooling applications.

The elastocaloric exect in LCEs has been the focus of a few recent reports. 39,40 For LCE, the application of mechanical stress induces an isothermal decrease in the entropy and emission of heat. Mechanical deformation of conventional LCEs either can result in a mechanotropic phase transition 41,42 or cooperative organization of domains (e.g., polydomain to monodomain transition). Both mechanisms have been reported to result in temperature increases to deformation followed by cooling on release. 42 Several studies have demonstrated elastocaloric temperature changes in the range of 1-2 °C in polydomain LCEs under relatively low mechanical stress. 39 Computational predictions suggest even larger temperature changes can be realized with optimized composition and LC content of the material. 43 Recent reports have introduced the use of mechanotropic phase transitions in liquid-crystalline containing amorphous materials to produce moderate elastocaloric exects. 41,42 Thus, far, the attainable temperature change has reached ± 3.5 °C under minimal stress and strain with near-zero hysteresis which may position LCEs as promising candidates for practical elastocaloric applications. 44 In addition to the conventional elastocaloric exect, where materials heat under strain and cool upon release, the inverse elastocaloric exect, typified by cooling under applied stress and heating upon relaxation, has been observed in shape memory alloys (SMAs) 43,45 with the associated thermodynamics of the inverse system attributable to stress induced increase in entropy that results in a concurrent decrease in temperature. 33 The Maxwell relation describing this exect is given by

where S is entropy, σ is applied stress, ε is strain, and T is temperature. When ( ) T > 0, the material cools under stress due to an increase in entropy. 46 This behavior is observed during phase transitions that involve changes in symmetry but do not result in significant volume changes such as the martensitic transitions of some metal alloys. In these materials, the interplay between stress and strain governs the transformation, and significant entropy changes near the transition point can lead to substantial cooling exects. 47 In SMAs, the inverse exect arises due to the interplay between structural transformations and secondary phenomena such as internal stress fields or magnetism. For example, alloys such as Co-Cr-Al-Si have demonstrated inverse elastocaloric output associated with a reentrant martensitic phase transformation driven by magnetic interactions. 31,48 While ultimately the design of an elastocaloric cooling system can use either a conventional or inverse elastocaloric material, we speculate that the realization of both exects in polymeric materials could be used in parallel or series in advanced designs. While prior examinations detail inverse elastocaloric exects in SMAs, we are unaware of any reports of inverse elastocaloric output in soft, polymeric systems. Accordingly, we report what we believe is a distinctive inverse elastocaloric output in supramolecular LCE. We demonstrate that stretching this new subset of LCEs disrupts the hydrogen bonds, which is an endothermic process involving a positive enthalpy change. The energy absorbed during bond disruption contributes to the observed cooling exect, as the system draws in heat to compensate for bond breaking. This cooling exect is accompanied by an increase in entropy resulting from the disruption of hydrogen bonds that compose the dimeric liquid crystal units in the LCE. Accordingly, deformation eventually reduces the order. The combined exect of enthalpy (heat absorption) and entropy (increased disorder) during deformation results in a temperature decrease with minimum applied stress. Furthermore, these elastomers rapidly recover to strain cycling due to their low nematic-to-isotropic transition temperatures (T NI ) and accordingly, exhibit minimal mechanical hysteresis.

The supramolecular LCE composition examined here were prepared via thiol-acrylate photopolymerization of liquid crystalline diacrylate monomers C6BAPE, the supramolecular monomer M 6 OBA, and thiol-based cross-linker and chain extender (Figure 1a). M 6 OBA dimerizes to form a liquid crystalline phase. Mixtures were filled at room temperature into glass substrates spaced by 100 μm spacers. Upon photopolymerization (Figure 1b), the polymer network forms and is composed of both covalent bonds and noncovalent bonds (via hydrogen bonding). Second heating curve of dixerential scanning calorimetry (DSC) analysis is presented in Figure 1c indicating that the glass transition temperature (T g ) is well below room temperature. The T NI of this supramolecular LCE composition is around room temperature. These transitions were also observed during dynamic mechanical analysis (DMA) with a temperature ramp under dynamic oscillation at a frequency of 1 Hz (Figure S2). The maximum peak of the tan(δ) curve, representing T g , appeared at ≈ -3 °C, while the other peak at ≈27 °C corresponded to T NI .

The mechanical properties of the supramolecular LCE are evident in tensile testing (Figure 2a). With a near-ambient T NI , the material deforms similar to the "isotropic" LCE subject to our recent examination. 44 The material exhibited elastic deformation with a modulus of 0.87 MPA. The supramolecular LCE was also examined for hysteresis and creep recovery. The material is fully reversible at room temperature (Figure S3). Infrared spectroscopy was used to assess the potential mechanochemical response of the supramolecular LCE. The LCE was deformed and placed on an ATR crystal. IR spectra were taken immediately. Although the material is soft and recovers rapidly due to minimal hysteresis, the samples were initially cooled to their T g to immobilize the polymer network, and slow the time scale of recovery to enable spectral acquisition. Notably, the stretching is only up to 75%, the peak at 1680 cm -1 corresponding to the carbonyl stretch of dimerized M 6 OBA decreases in height upon stretching is attributed to the disruption of hydrogen bonds (Figure 2b) within the supramolecular LCE network, which dilutes the liquid crystalline phases and leads to a uniform optical state. [49][50][51][52] Previous studies of LCEs have established elastocaloric phenomena. When LCEs prepared of entirely covalent bonds are subjected to stress, the polymer network aligns, increasing its order, which causes an initial rise in temperature. As the material alignment saturates, the temperature increases causes heat (Q) to be lost to the environment, and the sample returns to the ambient temperature. Upon stress release, the material reverts back to the disordered state accompanied by a decrease in temperature. 42,44 This phenomenon is attributed to entropy changes. The transition from a disordered to an ordered state decreases entropy, leading to a temperature increase (exothermic), while the return to disorder upon unloading increases entropy, resulting in cooling (endothermic). The elastocaloric curve for a conventional LCE (e.g., entirely covalent) is shown in Figure S4.

The elastocaloric response of the supramolecular LCE was observed during deformation by thermal imaging (Figure 3a, Movie S1). Here, the material was subject to deformation at the strain rate of 16.67%/s to a value of 75% strain. Upon deformation, the material exhibits an initial -0.2 °C decrease in temperature. This decrease in temperature was repeatedly observed in replicate samples as well as in other related compositions. However, the magnitude of the temperature change was highly irregular axecting statistical analysis.

Accordingly, we choose to present a representative sample that illustrates the observation of the response. At maximum strain, the sample is held and the temperature further drops an additional -0.5 °C. The tensile deformation is released, and the material consistently exhibits a -0.1 °C decrease in temperature on release. The total temperature decrease on deformation in this sample was -0.7 °C. The evolution of strain and temperature as a function of time for this experiment is presented in Figure 3b. To ensure the reproducibility, we Figure 1. continued supramolecular mesogens M 6 OBA reversibly cleaves at moderate strain values. (c) Second heating curve of diUerential scanning calorimetry (DSC) for supramolecular LCEs showing glass transition temperature (T g ) and nematic-to-isotropic transition temperature (T NI ) of the material. conducted five repeated deformation-recovery cycles as shown in Figure S5.

Informed by IR analysis, we believe that a majority of the cooling is associated with mechanochemical disruption of the hydrogen bonds in the material. We were not able to deconflict the contribution of enthalpic disruption of the bonds versus the entropic contributions. However, for this particular material system, we note the rate dependence of the process. We do not observe the inverse elastocaloric exect to instantaneous (rapid) deformation to 75% shown in Figure S6. We speculate that the slow application of strain at 16.67%/s as well as the extended holding period allows for the force to be sustained in the material, which could enhance the amount of overcoming the energy barrier for dimerization. The exects of strain rate and stretching percentage are shown in Figure S7 and S8. Careful experimentation is necessary to elucidate the underlying mechanism in detail. However, the decrease in temperature aligns with our hypothesis that deformation of supramolecular LCE can disrupt hydrogen bonds within the LCE structure and ultimately result in an inverse elastocaloric exect. The contrast between the response of conventional and supramolecular LCEs is shown in Figure S9.

As an initial confirmation of the association of deformation of supramolecular LCE and order decrease, we utilized birefringent imaging between crossed polarizers (Figure 4a, Movie S2). With the proximity of T NI to room temperature, the material is generally isotropic (initially lacks birefringence). Some birefringence is observed proximate to the grips. As strain is applied, the material becomes strongly birefringent. This indicates the material is exhibiting a classical mechanotropic phase transition (entropy decrease). However, to continued deformation the birefringence largely vanishes and the film exhibits a milky-white texture between crossed polarizers. This is indicative of some residual retention of order at a considerably reduced magnitude. This experiment supports the conclusion that deformation of the supramolecular LCE does axect organization (e.g., entropy increase). Upon unloading, the material once again becomes strongly birefringent before returning to a generally isotropic state. For comparison, photographs of a conventional LCE subject to a similar deformation cycle are shown in Figure S10.

From the data presented in Figure 3a and 3b, the deformation of these materials is consistent with prior examinations that refer to the inverse elastocaloric exect. However, the evolution of entropy in these materials is nuanced. The material begins in an isotropic state. Upon deformation, we propose the material transitions into a nematic phase. This is a decrease in entropy which should manifest as an increase in temperature. However, although this birefringent image is evident in the process, it is limited in time such that the emanation of heat in this state may be limited and not detectable with our methodology. Sustaining and eventually holding deformation results in hydrogen bond disruption. The exact amount of disruption is unclear. The mechanochemical disruption of hydrogen bonds is itself an endothermic process (cooling). However, birefringence measurements confirm changes in organization that suggest that the mechanically induced cooling is a combination of enthalpic and entropic components. Notably, upon release, the elastocaloric process reported here does not exhibit a positive temperature change. Although initially counterintuitive, this may be indirect evidence of the contribution of the mechanotropic phase transition to the process in this particular material system. At the deformed state, the material clearly has a limited amount of order. When strain is released, the material is further cooled before the temperature returns to ambient. We speculate that the temperature drop on strain release, which is repeatedly seen in samples, is associated with the reverse mechanotropic process, where some amount of order is initially regained. The elastic recovery of the material is rapid (Figure S3b). The duration of the ordered state is limited in duration. The recovery in temperature may be a counterbalance of the exothermic nature of hydrogen bond reformation balanced with the entropic recovery of order and subsequent return to the isotropic state. A summary of the deformation and recovery processes is presented in Figure 4b.

This work reports the observation of inverse elastocaloric exect in supramolecular liquid crystalline elastomers (LCEs) composed of intramesogenic hydrogen bonds. The inverse elastocaloric response of the LCE is associated with mechanochemical dissociation of the hydrogen bonds. Thermodynamically, the mechanically induced order-disorder transition increases entropy and manifests as cooling. In this initial report, the elastocaloric output is < -1 °C. These findings establish a foundation for tuning LCE properties through supramolecular design, enabling highly responsive, lowhysteresis, and cyclable caloric materials capable of inverse elastocaloric response.