Tunability of the elastocaloric response in main-chain liquid crystalline elastomers

Abstract

Materials exhibiting a large caloric effect could lead to the development of new generation of heat-management technologies that will have better energy efficiency and be potentially more environmentally friendly. The focus of caloric materials investigations has shifted recently from solid-state materials toward soft materials, such as liquid crystals and liquid crystalline elastomers. It has been shown recently that a large electrocaloric effect exceeding 7 K can be observed in smectic liquid crystals. Here, we report on a significant elastocaloric response observed by direct elastocaloric measurements in main-chain liquid crystal elastomers. It is demonstrated that the character of the nematic to paranematic/isotropic transition can be tuned from the supercritical regime towards the first-order regime, by decreasing the density of crosslinkers. In the latter case, the latent heat additionally enhances the elastocaloric response. Our results indicate that a significant elastocaloric response is present in main-chain liquid crystalline elastomers, driven by stress fields much smaller than in solid elastocaloric materials. Therefore, elastocaloric soft materials can potentially play a significant role as active cooling/heating elements in the development of new heat-management devices.

Elastocaloric effect in amorphous polymer networks undergoing mechanotropic phase transitions

“…The liquid crystal-containing elastomers studied in this work undergo a mechanotropic phase transition, from disordered to nematic, which is asssociated with an elastocaloric temperature change of up to ∆T + = 2.9 K. This represents a large improvement over existing reports on the elastocaloric response in other liquid crystalline polymer networks, which typically demonstrate ∆T + ∼ 1 K, 14,22 . Further, these initial results approach the magnitude of the temperature changes of 5.2 and 6.5 K reported for electrocaloric LC systems.…”

“…Recent literature reports detail the elastocaloric response of LCEs. Experimental efforts have so far demonstrated elastocaloric temperature changes of only ∆T ∼ 1 K. 14,22 Lavrič et al observed ∆T = 1 K at a strain of ε = 90% and detailed that this ∆T was observed across a range of crosslink density. 22 Skačej has reported molecular simulations that suggest the elastocaloric effect in mainchain LCEs could yield a ∆T > 10 K. 23 However, the simulations underestimate the specific heat of liquid crystal-containing systems, which likely leads to an overestimate of the elastocaloric temperature change.…”

“…Experimental efforts have so far demonstrated elastocaloric temperature changes of only ∆T ∼ 1 K. 14,22 Lavrič et al observed ∆T = 1 K at a strain of ε = 90% and detailed that this ∆T was observed across a range of crosslink density. 22 Skačej has reported molecular simulations that suggest the elastocaloric effect in mainchain LCEs could yield a ∆T > 10 K. 23 However, the simulations underestimate the specific heat of liquid crystal-containing systems, which likely leads to an overestimate of the elastocaloric temperature change. LC systems also have been explored as electrocaloric materials, notably harnessing a smectic-to-isotropic transition to achieve a ∆T + of up to 5.2 to 6.5 K. 14,24 Our group has recently quantified the transition from disorder (order parameter Q ≈ 0) to appreciable order (Q ≈ 0.4, i.e., the nematic phase) in a liquid crystal-containing elastomeric network upon application of uniaxial strain.…”

Reprogrammable Humidity-Driven Liquid Crystalline Polymer Actuator Enabled by Dynamic Ionic Bonds

“…In recent years, stimulus responsive polymers have become a hotspot in research of advanced functional materials. , One of the most competitive candidates refers to liquid crystalline polymer (LCP), in which the ordered liquid crystal mesogens are frozen by covalently cross-linked polymer network. − Because of the combination of the programmable orientation of liquid crystal and the elasticity of polymer matrix, LCP has been proved as an outstanding matrix to prepare smart actuators and soft robots, presenting great potentials in multiple fields such as biomimetic actuators, artificial muscles, , functional sensors, , and so on. − In order to broaden the practical applications of the LCP materials, various LCP devices with different stimulus responses were prepared, including light, − electricity, , heat, , magnetism, force, and humidity. , …”

“…3−7 Because of the combination of the programmable orientation of liquid crystal and the elasticity of polymer matrix, LCP has been proved as an outstanding matrix to prepare smart actuators and soft robots, presenting great potentials in multiple fields such as biomimetic actuators, 8 artificial muscles, 9,10 functional sensors, 11,12 and so on. 13−15 In order to broaden the practical applications of the LCP materials, various LCP devices with different stimulus responses were prepared, including light, 16−19 electricity, 20,21 heat, 22,23 magnetism, 24 force, 25 and humidity. 26,27 Compared with other stimuli, humidity is a nontoxic, harmless trigger that can be easily obtained.…”