Normal- and Reverse-Mode Thermoresponsive Controllability in Optical Attenuation of Polymer Network Liquid Crystals

Using pretrained machine learning models to predict luminous and solar transmittance controllability of liquid crystal/polymer composites from microstructural images

Polarized optical microscopy (POM) images of polymer network liquid crystals (PNLCs) were first analyzed using a pretrained machine learning model for feature extraction and hierarchical clustering. The analyses worked well in predicting and improving the thermoresponsive changes individually in direct luminous and hemispheric solar transmittance, both of which are crucial properties of energy-saving smart windows. The features of a 1280 × 1920-pixel color POM image were extracted by the latest pretrained algorithm, EfficientNet-B7, as a 2560-dimensional vector and then reduced into a two-dimensional space for clustering and visualization using the uniform manifold approximation and projection (UMAP) algorithm while efficiently preserving the global structures of the distance relationship in a high-dimensional space. The feature vectors in the UMAP space were correlated with the thermoresponsive transmittance and classified using hierarchical clustering analysis. The extracted features belonging to some clusters were also correlated with the fabrication parameters. The PNLCs here were produced from various raw materials under different fabrication conditions. These analyses and predictability are extensively applied to different PNLCs for stimuli-responsive optical devices, such as solar- and privacy-control windows.

Effect of Electrospinning Network Instead of Polymer Network on the Properties of PDLCs

In this study, polymer-dispersed liquid crystal (PDLC) membranes were prepared by combining prepolymer, liquid crystal, and nanofiber mesh membranes under UV irradiation. EM, POM, and electro-optic curves were then used to examine the modified polymer network structure and the electro-optical properties of these samples. As a result, the PDLCs with a specific amount of reticular nanofiber films had considerably improved electro-optical characteristics and antiaging capabilities. The advancement of PDLC incorporated with reticulated nanofiber films, which exhibited a faster response time and superior electro-optical properties, would greatly enhance the technological application prospects of PDLC-based smart windows, displays, power storage, and flexible gadgets.

Electrospun Core-Sheath Fibers with a Uniformly Aligned Polymer Network Liquid Crystal (PNLC)

Electrospun polymer-liquid crystal (PLC) fibers have potential applications such as wearable sensors and adaptive textiles because of their rapid response and high flexibility. However, existing PLC fibers only have a narrow responsive range and poor resistance to heat and chemicals. Herein, a new type of PLC fiber is prepared by using a coaxial electrospinning process. The core solution is 4'-pentyl-4-biphenylcarbonitrile (5CB), and the sheath solution is a mixture containing 13 wt % PVP and 10 wt % reactive mesogen (RM). After UV exposure of the fibers, 5CB in the core and RM diffusing from the core are cross-linked into an LC polymer. The fibers have a highly uniform morphology with an average diameter of 3.2 ± 0.5 μm, and mesogens inside the fibers align unidirectionally with the long axis of the fibers. The fibers show a broad phase-transition temperature range between 13.5 and 155.5 °C and have a response time of less than 10 s. The temperature range can also be controlled by adjusting components in the electrospun fibers and UV exposure time. The core-sheath fibers prepared in such a manner exhibit excellent heat and chemical resistance with reversible optical responses. Moreover, when the fibers are exposed to volatile organic compounds (VOCs) such as toluene, the fibers show a rapid optical response to toluene vapor within 25 s. This study demonstrates that the fibers are potentially useful for preparing flexible temperature and chemical sensors.

Advanced liquid crystal-based switchable optical devices for light protection applications: principles and strategies

With the development of optical technologies, transparent materials that provide protection from light have received considerable attention from scholars. As important channels for external light, windows play a vital role in the regulation of light in buildings, vehicles, and aircrafts. There is a need for windows with switchable optical properties to prevent or attenuate damage or interference to the human eye and light-sensitive instruments by inappropriate optical radiation. In this context, liquid crystals (LCs), owing to their rich responsiveness and unique optical properties, have been considered among the best candidates for advanced light protection materials. In this review, we provide an overview of advances in research on LC-based methods for protection against light. First, we introduce the characteristics of different light sources and their protection requirements. Second, we introduce several classes of light modulation principles based on liquid crystal materials and demonstrate the feasibility of using them for light protection. In addition, we discuss current light protection strategies based on liquid crystal materials for different applications. Finally, we discuss the problems and shortcomings of current strategies. We propose several suggestions for the development of liquid crystal materials in the field of light protection.

Thermoresponsive mobility of liquid crystals and reactive mesogens during photopolymerization-induced phase separation

Molecular interactions between liquid crystals (LCs) and reactive mesogens (RMs) at temperatures across the phase transition regions were comprehensively studied during photopolymerization-induced phase separation (PPIPS) beginning with raw mixtures until the formation of polymer network liquid crystals (PNLCs). Then, the molecules were found to be nonuniformly more and less mobile in response to temperature as PPIPS progressed. Optical birefringence and infrared absorption were carefully measured throughout PPIPS, using 4-cyano-4'-hexylbiphenyl (6CB) and 1,4bis-[4-(3-acryloyloxypropyloxy) benzoyloxy]-2-methylbenzene (RM257) as typical LCs and RMs. Microscopic views of thermoresponsive changes in the molecular orientation order of both LCs and RMs were obtained: LCs and RMs in raw mixtures interacted with one another but uniformly transformed their molecular orientation. Such interactions continuously change to become nonuniform with progress in PPIPS. At the incipient stages of PPIPS, RMs, which are polymerized but not completely networked, inhibit LCs from changing their molecular orientation and vice versa. As PPIPS progresses, some LCs become more mobile and some less mobile owing to RM constraints. The domain configuration of the submicrometer phase separation affects the thermoresponsive mobility of LCs and RMs, that is, LCs become more mobile in LC-richer areas. The quantitative knowledge here provides comprehensive insight that LCs and RMs are mutually constrained and that such interactive behavior varies nonuniformly as PPIPS progresses.

Thermally responsive polymer-dispersed liquid crystal diffusers fabricated using laser speckle pattern irradiation

This study examined the thermal response of polymer-dispersed liquid crystal (PDLC) diffusers, patterned using a two-lens imaging system. Optical modulation was achieved by modifying the PDLC transmittance using temperature-induced changes to liquid crystal (LC) orientation. PDLCs with controllable scattering properties were obtained by irradiating LC-polymer composites with laser speckle patterns. The variation of the scattering characteristics of the PDLCs with temperature, average speckle size, and LC orientation order was analyzed to determine the most suitable parameters for a diffuser for smart window solar-ray control applications. The findings of these experiments demonstrate that using speckle patterns, a one-time laser exposure process, can provide a simple fabrication method of novel optical devices.

Thermoresponsive Reflective Scattering of Meso-Scale Phase Separation Structures of Uniaxially Orientation-Ordered Liquid Crystals and Reactive Mesogens

Polymer network liquid crystals (PNLCs) capable of thermoresponsive change in reflective scattering were fabricated using a self-organization technique called photopolymerization-induced phase separation. These PNLCs exhibit nonscattering states at temperatures τ below the nematic-to-isotropic (NI) phase transition temperature τ_NI but reflective scattering states at τ values above τ_NI. The magnitude of change of optical clarity is 80% and of solar transmittance is 20% in PNLCs with a thickness of 50 μm. The microscopic structures consist of wavelength- or meso-scale phase separation domains of liquid crystals (LCs) and polymerized reactive mesogens (RMs) in which cyanobiphenyl (CB) groups are thermoresponsively transformed between uniaxially orientation-co-ordered and disordered states. Such thermoresponsive structures were fabricated by employing the CB groups as mesogenic bodies, which were expected to mutually associate due to their physicochemical structures. Cross-linkers stabilized the meso-scale domains and made the PNLCs durable through repeated temperature changes. Polarizing optical microscopy (POM) and scanning electron microscopy showed meso-scale composites that reflectively scatter visible and near-infrared light. POM and Fourier-transform infrared spectroscopy at different temperatures suggest that the orientation order of the CB groups changes in the LC phase in response to temperature but remains ordered in the RM phase. Such a thermoresponsive change in the orientation order produces the switchability in meso-scale nonuniformity and consequently in reflective light scattering. The thermoresponsive PNLCs are not only effective as energy-saving smart windows but also advantageous at stages of manufacture, installation, and operation.