Cholesteric liquid crystals (CLCs) are a major class of photonic materials that display selective reflection properties arising from their helical ordering. The temperature response of CLCs, comprising dynamic reflection color changes upon variation of temperature, is exploited using material systems consisting of small mesogenic molecules, polymer‐dispersed liquid crystals (PDLCs), polymer‐stabilized liquid crystals (PSLCs), or liquid‐crystalline polymers. Taking advantage of the easy processability and flexibility of the molecular design, these temperature‐responsive CLCs are fabricated into different forms of photonic devices, including cells, coatings, free‐standing films, and 3D objects. Temperature‐responsive devices developed from CLCs are integrated for application in temperature sensors, energy‐saving smart windows, smart labels, actuators, and adding aesthetically pleasing features to common objects. Herein, the device capabilities of the different material systems of temperature‐responsive CLCs are summarized: small mesogenic molecules, PDLCs, PSLCs, and CLC polymers. For each system, examples of different device forms are presented, with their temperature responsiveness and the underlying mechanisms discussed. In addition, the potential of each material system for future device applications and product developments is envisioned.
Producing lightweight polymeric actuators able to generate high stresses typical of hard metals and/or ceramics remains challenging. The photo-mechanical responses of ultra-drawn ultrahigh molecular weight polyethylene (UHMWPE) actuators containing azobenzene photo-switches with symmetrically attached polyethylene (PE) side chains are reported. Long PE side chains promote dispersion within the apolar UHMWPE matrix, and the ultra-drawn films are highly aligned. The ultra-drawn azobenzenedoped UHMWPE films have high Young's moduli ($100 GPa) and are viscoelastic at room temperature at strains below 1%. The photo-mechanical response of the films is fast (<1 s), showing a high specific actuation stress response (>6 3 10 4 Pa (kg m À3 ) À1 ) to UV or visible light at a low strain ($0.06%). The actuator responds to rotating linearly polarized light, causing a photoinduced stress wave response. Such rapid, high-stress, low-strain, photo-mechanical responses are unique in soft polymer systems with physical values approaching hard metals/ceramics.
A transparent color‐tunable device is presented based on an electrothermal response by using interdigitated electrode patterns. The response is generated by applying an in‐plane AC electric field that heats up a thermosensitive cholesteric liquid crystal mixture. The induced temperature elevations cause band gap shifting (Δλ > 350 nm) up until a colorless state is reached, corresponding to the isotropic phase. Color shifting can be tuned manually by varying the electric field or autonomously by the surrounding temperature. Broadband dielectric spectroscopy reveals that the electrothermal response originates from resistive heating of the transparent electrode pattern in conjunction with the cell capacitance and is therefore largely dependent on the electrode configuration. Hence, the electrothermal response can be easily modified by changing the electrode pattern, frequency and/or voltage, dependent on the user's requirements. Therefore, the ability of this technique to manipulate the autonomous thermal response by an electric field, using only one conductive substrate, shows promise in the field of optoelectronics, sensors, and smart windows.
Low molecular weight cholesteric liquid crystals (CLCs) can be trapped as droplets or particles inside a polymer binder, forming polymer dispersed cholesteric liquid crystal (PDCLC) systems, which are typically used for smart windows, displays, and optical sensors. While dispersing a single CLC mixture via emulsification, rendering reflective PDCLC films upon drying with a stable optical response, is well studied, the incorporation of distinct microdroplets inside a PDCLC coating is barely explored. Here, structural colored PDCLC coatings are prepared by mixing distinctive thermosensitive photonic microdroplets, featuring an optical time–temperature response that is visualized as a reflective color loss or shift dependent on the composition of the CLC mixtures. The presented optical time–temperature response is induced by diffusion of CLCs between adjacent liquid crystal domains via interconnecting passageways, causing an irreversible optical response until homogeneous mixing is attained. Hence, blending distinct CLC microdroplets in a polymer matrix provides a facile and scalable method for fabricating optical time–temperature integrators; moreover, alternative optical time–temperature responses can be realized when adjusting the composition of the photonic mixtures, demonstrating the potential of this method to acquire novel photonic applications based on low molecular weight CLCs.
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