A cold-responsive liquid crystal elastomer provides visual signals for monitoring a critical temperature decrease

Critical temperature indicators have been extensively utilized in various fields, ranging from healthcare to food safety. However, the majority of the temperature indicators are designed for upper critical temperature monitoring, indicating when the temperature rises and exceeds a predefined limit, whereas stringently demanded low critical temperature indicators are scarcely developed. Herein, we develop a new material and system that monitor temperature decrease, e.g., from ambient temperature to the freezing point, or even to an ultra-low temperature of −20 °C. For this purpose, we create a dynamic membrane which can open and close during temperature cycles from high temperature to low temperature. This membrane consists of a gold-liquid crystal elastomer (Au-LCE) bilayer structure. Unlike the commonly used thermo-responsive LCEs which actuate upon temperature rise, our LCE is cold-responsive. This means that geometric deformations occur when the environmental temperature decreases. Specifically, upon temperature decrease the LCE creates stresses at the gold interface by uniaxial deformation due to expansion along the molecular director and shrinkage perpendicular to it. At a critical stress, optimized to occur at the desired temperature, the brittle Au top layer fractures, which allows contact between the LCE and material on top of the gold layer. Material transport via cracks enables the onset of the visible signal for instance caused by a pH indicator substance. We apply the dynamic Au-LCE membrane for cold-chain applications, indicating the loss of the effectiveness of perishable goods. We anticipate that our newly developed low critical temperature/time indicator will be shortly implemented in supply chains to minimize food and medical product waste.

We confirmed the polydomain structure of the LCE by performing XRD analysis. Figure S1 indicates that regardless of PEGDA concentration the LCE exhibits isotropic alignment. Figure S1. 2D-XRD patterns of the elastomers consisting of different contents of PEGDA ranging from 20 mol% to 28 mol% after first-step thiol-acrylate addition reaction.
The nematic-isotropic transition temperature T ni was determined by DSC measurement. We can see from Figure S2 that regardless of PEGDA concentration T ni is below room temperature. With increasing the PEDTA concentration, T ni slightly decreases. We measured the stress-strain curve of the polydomain elastomers after thiol-acrylate addition reaction. Figure S3 shows that the failure strain appears after 100% applied strain, irrespective of PEGDA concentration. Figure S3. Stress-strain curves of the polydomain elastomers with different PEGDA concentrations after primary thiol-acrylate addition reaction. The experiment was conducted at room temperature.
The compositions of the LCEs are shown in Table S1. The PEGDA concentration out of the total acrylate derivatives is varied from 20 mol% to 28 mol%. The ratio of the rest of composition remains a constant. Table S1. Compositions for the fabrication of the LCEs.
We characterized the molecular structure of the LCE by conducting XRD analysis. The 2D patterns demonstrate the typical nematic liquid crystal structure, regardless of PEGDA concentration and measuring temperature ( Figure S4). Figure S4. 2D patterns of XRD analysis showing the molecular structure of nematic LCE containing various PEGDA concentrations at room temperature and 2 C.
We chose methyl yellow as the reactive dye for chemical reaction. The protonation of the dye is shown in Figure S5a. The chemical reaction of methyl yellow dispersed in the LCE was conducted with an acid mixture of propionic acid/sulfuric acid. The color of the LCE changes from yellow to bright red ( Figure S5b). The two-dimensional LCTTI is constructed with an Au-LCE, an acid reservoir, and an acid-inert polyethylene sealant film ( Figure S6a). As seen from Figure S6b, when jumping from room temperature to freezing point, the color changes from yellow to bright red after 15 min. When returning to room temperature, the color remains red. Figure S6. (a) Prototype of two-dimensional (2D) LCTTI. (b) Photographs showing that the 2D LCTTI demonstrating color change when placed on an ice surface and staying red when returning to room temperature. The volume of the applied acid is 30 µL.
We investigated the LCTTI's color exhibition at different temperature decrease rate. When cooling to 2 C at 20 C/min, the color appears red in 4 min and stabilizes in 15 min ( Figure  S7a). In comparison, when jumping from room temperature to 2 C, the generated red color appears slightly less brighter after 4 min, while the stabilized color after 15 min is comparable ( Figure S7b). Based on this, we can conclude that the color exhibition of the LCTTI in 15 min after activation is barely affected by the temperature reduction rate. Figure S7. Photographs showing the color change of the LCTTI (a) after cooling down from room temperature to 2 C at temperature decrease rate of 20 C/min, and (b) jumping from room temperature to 2 C.
We characterized the absorbance of methyl yellow by using UV-Vis spectroscopy. The methyl yellow absorbs the light of wavelength ranging from 350 nm to 500 nm and peaks at 410 nm. When treating with an acid, the absorption band shifts to the range of 400 nm to 600 nm with maximum absorption at 510 nm. Captions for Video S1 to S6 Video S1. Cross polarized optical microscopy video showing the uniaxial expansion of the LCE upon temperature decrease.

Video S2.
The uniaxial expansion of the LCE induced by temperature decrease recorded by camera.

Video S3.
Optical microscopy video showing the Au film cracking upon temperature decrease.

Video S4.
The LCTTI demonstrating color change when cooled from room temperature to -10 C.