Flexible thermal responsive infrared reflector based on cholesteric liquid crystals and polymer stabilized cholesteric liquid crystals

Polymer-Stabilized Liquid Crystal Films Containing Dithienyldicyanoethene-Based Chiral Photoswitch: Multi-Modulation for Environment-Adaptative Smart Windows

A polymer-stabilized liquid crystal (PSLC)-based environment-adaptative smart window with multi-modulations is demonstrated. This PSLC system contains a right-handed dithienyldicyanoethene-based chiral photoswitch and a chiral dopant, S811, with opposite handedness, of which the reversible cis-trans photoisomerization of the switch can drive self-shading of the smart window under UV light stimulus because of the transition from nematic phase to cholesteric one. With the assistance of solar heat, the opacity of the smart window can be deepened because the heat promotes the isomerization conversion rate of the switch. This switch has no thermal relaxation at room temperature, therefore, the smart window exhibits dual stabilization: transparent state (cis-isomer) and opaque state (trans-isomer). Moreover, the incident intensity of sunlight can be regulated by an electric field, which allows the smart window to adapt to some specific situations. Such an energy-saving device can be used in buildings and vehicles to control indoor temperature and adapt to the required ambiance.

Preparation of transmittance-switchable composites from a coexistent system of polymer-dispersed and polymer-stabilized cholesteric liquid crystals

Polymer-dispersed and polymer-stabilized liquid crystals (PD&PSLCs) are the coexistent systems of PDLCs and PSLCs, in which a mesogenic polymer network forms in the LC droplets to construct the PSLC system and the combination of the LC domains and the isotropic polymer matrix makes up the PDLC system. In this work, a PD&PS cholesteric LC (PD&PSCLC) system, where the isotropic polymer walls make up the PDCLC microstructure and the liquid crystalline polymer network in the CLC droplets constructs the PSCLC system, is fabricated successfully. The CLCs are composed of a negative dielectric anisotropy nematic LC and a chiral dopant. Owing to the stabilizing effect of the liquid crystalline polymer network, the planar texture of the CLCs can be retained after the preparation process, and the PD&PSCLC sample is in a large-transmittance state. The action of a low-frequency electric field induces the disordered texture of the CLCs and the light-scattering (low-transmittance) state because of the electro-hydrodynamic effect and dynamic scattering effects. After the removal of the low-frequency electric field, the stabilizing effect of the liquid crystalline polymer network induces the CLC molecules to return to the homogeneous orientation state, and the sample also re-obtains the large transmittance. This work may provide a facile approach to fabricating polymer/CLC composites with switchable transmittance for reverse-mode smart windows.

Reduction of solar infrared heating by using highly transparent thin films based on organic chiral nematic liquid crystal polymer

This paper demonstrates a thin and transparent reflector film for the near infrared, based on chiral nematic liquid crystal (CLC) polymers. Two films reflect almost 50% of unpolarized incident light from 730 to 820 nm and from 880 to 1030 nm, while remaining completely transparent in the visible region with transmittance >90%. An efficient window uses the combination of two reflectors. After exposing two window-cubes for 2 h to direct sunlight, the temperature inside the cube with reflector windows was 4°C lower than in cube with plain windows. This reveals that the infrared (IR) reflectors can effectively control the indoor temperature. These films, which are 8 µm in thickness, can be detached from the glass substrates and used as a free-standing film, or be attached to a flexible optical foil or a solid window. The foils can be applied in buildings, offices, and automobiles to statically reduce the energy consumption required for air conditioning or lighting. The free-standing foils show acceptable resistance to polar protic solvents and are thermally stable up to 100°C.

Controlling the Phase Behavior and Reflection of Main-Chain Cholesteric Oligomers Using a Smectic Monomer

Cholesteric liquid crystals (CLCs) are a significant class of temperature-responsive photonic materials that have the ability to selectively reflect light of a specific wavelength. However, the fabrication of main-chain CLC oligomers with dramatic reflection band variation upon varying the temperatures remains a challenge. Here, a feasible method for improving and controlling the responsiveness of main-chain cholesteric liquid crystal oligomers by the incorporation of a smectic monomer is reported. The smectic monomer strengthens the smectic character of the oligomers and enhances the magnitude of the change of the pitch as a function of temperature upon approaching the cholesteric–smectic phase transition temperature. The central wavelength of the reflection band can be easily modified by mixing in an additional chiral dopant. This promising method will open the door to the preparation of temperature-responsive photonic devices with excellent responsiveness.

Thermochromic Cholesteric Liquid Crystal Microcapsules with Cellulose Nanocrystals and a Melamine Resin Hybrid Shell

Thermochromic coatings that can change their color in response to variations in ambient temperature have various potential applications. Cholesteric liquid crystals (CLCs) are promising thermochromic materials due to their selective light reflection and wide regulation range. However, it remains a challenge to fabricate thermochromic coatings that combine good responsivity, mechanical strength, fabrication feasibility, and flexibility. In this study, CLC microcapsules containing cellulose nanocrystals (CNCs) and a melamine-formaldehyde (MF) resin hybrid shell were fabricated via in situ polymerization using CNC-stabilized Pickering emulsions as templates. The CNCs were employed as both Pickering emulsifiers and alignment agents of CLCs to prepare CLC Pickering emulsions. The CLC microcapsules were mixed with curable binders to obtain coating slurries, and thermochromic coatings were prepared by painting the slurries on substrates and drying. The thermochromic coatings could adjust their color in the visible wavelength range in a temperature range of 12 to 42 °C. Moreover, the obtained thermochromic coatings displayed a relatively high reflectance of up to 30-40% and can even be applied to flexible substrates. The CLC microcapsules with CNCs and an MF hybrid shell are promising in the field of smart decorative paints, anti-counterfeit labels, and artificial skins.

Reversible Thermochromic Photonic Coatings with a Protective Topcoat

The fabrication of reversible and robust thermochromic coatings remains challenging. In this work, a temperature-responsive photonic coating with a protective topcoat is fabricated. A cholesteric oligosiloxane liquid crystal possessing a smectic-to-cholesteric phase-transition temperature response is synthesized. A planar alignment of its cholesteric phase is possible with blade coating. By stabilizing with 3 wt % of a crosslinked liquid crystal network, the photonic coating shows a color change ranging from red to blue upon heating. High transparency is retained, and the structural color changes are fully reversible. A transparent polysiloxane layer can be directly applied on top of the cholesteric layer to protect it against damage without affecting its optical properties. This approach satisfies the basic requirements of thermochromic polymer coatings, as it combines easy processability, coating robustness, and a reversible temperature response.

Temperature-dependent chirality of cholesteric liquid crystal for terahertz waves

Recently, the terahertz (THz) chiral field control opens a new window to THz devices and their applications. In this Letter, the active manipulation for THz chiral states based on the cholesteric liquid crystal (CLC) has been demonstrated by THz time domain cross-polarization spectroscopy. The results show that the CLC has strong THz optical activity and circular dichroism (CD) effect, and the strongest THz CD of 22 dB and a polarization rotation angle of 88.4° occur around the phase transition temperature T_S-N=250K. Rising to a room temperature of 300 K, the CLC turns from a chiral state to an isotropic state for THz waves with the phase transition processes of CLC molecules. Therefore, this CLC device can be performed as a thermally active THz circular polarizer, which brings potential applications in THz polarization imaging, broadband communication, and spectroscopy.

Strain-enhanced sensitivity of polymeric sensors templated from cholesteric liquid crystals

Detection of volatile organic compounds (VOCs) is an important issue due to their harmful impact on human health. In this study, we aimed at enhancing the sensitivity of the anisotropic polymeric films templated from cholesteric liquid crystals (CLCs) in the identification of VOCs at concentrations on the order of 100 ppm. To increase sensitivity, we introduced negative strain to the films in the direction parallel to the helical axis and evaluated its effect on the sensitivity. Specifically, we used LC mixtures of reactive [4-(3-acryloyoxypropyloxy)benzoic acid 2-methyl-1,4-phenylene ester (RM257)], nonreactive E7 mesogen and chiral dopant [4-((1-methylheptyloxycarbonyl)phenyl-4-hexyloxybenzoate) (S-811)] to synthesize CLC-templated polymeric films with programmed strain profiles using a curved wedge cell, and measured their response against a range of toluene vapor concentrations. Based on the obtained results, we demonstrated a relationship between the negative strain in the cholesteric pitch and the sensitivity of the sensor based on spacial responses evaluated from the change in coloring of the film. Our results showed that negative strain helps to increase the sensitivity of the sensors up to 15 times compared to their unstrained counterparts. Moreover, 90% of the equilibrium response is achieved in less than one minute of exposure which offers rapid diagnosis of VOCs. Our tests for the reversibility of the sensors showed that the CLC-templated polymeric films can be used multiple times without a significant loss of sensitivity.

Polymer Stabilized Cholesteric Liquid Crystal Siloxane for Temperature-Responsive Photonic Coatings

Temperature-responsive photonic coatings are appealing for a variety of applications, including smart windows. However, the fabrication of such reflective polymer coatings remains a challenge. In this work, we report the development of a temperature-responsive, infrared-reflective coating consisting of a polymer-stabilized cholesteric liquid crystal siloxane, applied by a simple bar coating method. First, a side-chain liquid crystal oligosiloxane containing acrylate, chiral and mesogenic moieties was successfully synthesized via multiple steps, including preparing precursors, hydrosilylation, deprotection, and esterification reactions. Products of all the steps were fully characterized revealing a chain extension during the deprotection step. Subsequently, the photonic coating was fabricated by bar-coating the cholesteric liquid crystal oligomer on glass, using a mediator liquid crystalline molecule. After the UV-curing and removal of the mediator, a transparent IR reflective polymer-stabilized cholesteric liquid crystal coating was obtained. Notably, this fully cured, partially crosslinked transparent polymer coating retained temperature responsiveness due to the presence of non-reactive liquid-crystal oligosiloxanes. Upon increasing the temperature from room temperature, the polymer-stabilized cholesteric liquid crystal coating showed a continuous blue-shift of the reflection band from 1400 nm to 800 nm, and the shift was fully reversible.