Polymer Network Liquid Crystal Research Articles

Dynamics of the photo-thermo-mechanical actuations in NIR-dye doped liquid crystal polymer networks.

We describe photo-thermo-mechanical actuation and its dynamics in thin films of a liquid crystal networks (LCN) under near infrared (NIR) illumination through experiments and simulations. Splay aligned films of different thicknesses (25 μm to 100 μm) were obtained by crosslinking a mixture of mono-functional and bi-functional liquid crystal monomers. The NIR-driven thermo-mechanical actuation was achieved by adding an NIR dye to the monomer mixture. The absorption of incoming radiation by the dye molecules raises the local temperature of the film causing an order-disorder (nematic-isotropic) transition, thereby resulting in a macroscopic shape change. We have investigated the effect of film thickness, NIR laser power and dye concentration on the tip displacement of the films in a cantilever configuration. The experimental findings and finite element simulation results are in reasonably good quantitative agreement. Despite using lower NIR powers than typically employed, the films show high actuation and large displacements. After achieving saturation in actuation, the films exhibit a flutter behavior which is discussed in light of the observed overshoot in the tip displacement for certain intensities and thicknesses. Finally, using a solar simulator, we also show the visible light response of the film.

Biomimetic jagged micropatterns templated from photoswitchable liquid crystal topography for energy harvesting and sensing applications

Biomimetic jagged micropatterns templated from photoswitchable liquid crystal polymer network topographies have been demonstrated for triboelectric energy harvesting and wearable sensing applications.

Unconventional Approaches to Light-Promoted Dynamic Surface Morphing on Polymer Films

Abstract Recent progress in research of light-promoted surface morphing on polymer materials is reviewed. Photoinduced mass transport motions in azobenzene polymers leading to surface relief gratings (widely called SRGs) formation are well-known and extensively discussed. Among many efforts, this article focuses on unconventional approaches, most of which are seemingly less highlighted. The first part introduces investigations using azobenzene polymers; introducing supramolecular approaches with tunable and removable azobenzene units, self-structuring of surface morphology upon single laser beam irradiation, photo-triggered migration via Marangoni flow, SRG formation and other functions of polymer brushes etc. The second part overviews the mass transport motions driven by other photoreaction classes such as photopolymerization, photocrosslinking, and photoisomerization of non-azobenzene components. The final part describes new dynamic surface morphing processes observed in fingerprint texture of liquid crystals and liquid crystal polymer networks, which can be applied to reversible friction control and self-cleaning. These uncommon attempts at photo-assisted dynamic morphing extend the possibilities of microfabrication on polymer films and are expected to find new opportunities to create new surface functions in soft materials.

A fully self‐powered, natural‐light‐enabled fiber‐optic vibration sensing solution

AbstractFiber‐optic sensors have been developed to monitor the structural vibration with advantages of high sensitivity, immunity to electromagnetic interference (EMI), flexibility, and capability to achieve multiplexed or distributed sensing. However, the current fiber‐optic sensors require precisely polarized coherent lasers as the lighting sources, which are expensive in cost and suffer from the power supply issues while operating at outdoor environments. This work aims at solving these issues, through developing a fully self‐powered, natural‐light‐enabled approach. To achieve that, a spring oscillator‐based triboelectric nanogenerator (TENG), a polymer network liquid crystal (PNLC), and an optical fiber were integrated. The external vibration drove the PNLC to switch its transparency, allowing the variation of the incident natural light in the optical fiber. Compared with the majority of conventional TENG‐based active vibration sensors, the developed paradigm does not suffer from the EMI, without requirements of the signal preamplification which consumes additional energy. The vibration displacement monitoring was performed to validate the sensing effectiveness of the developed paradigm.

Photopatterning Crystal Orientation in Shape-Morphing Polymers.

Shape-morphing polymers have gained particular attention due to their unique capability of shape transformation under numerous external stimuli such as light, pH, and temperature. Their shape-morphing properties can be used in various applications such as robotics, artificial muscles, and biomedical devices. To take advantage of the stimuli-responsive properties of the smart polymers in such applications, programming shape change precisely through a facile synthetic procedure is essential. Programmable shape-morphing is readily obtained in hydrogels and liquid crystal polymer networks, but shape programming of semicrystalline polymers usually relies on low-resolution mechanical deformation. In this paper, a semicrystalline shape-morphing polymer with a controlled shape programmability was developed via photopatterning crystal orientation using a spatially controlled photopolymerization technique. The semicrystalline polymer network forms aligned crystallites at the boundaries between dark and bright regions during photopolymerization using a projector, which introduces an anisotropic stimulus response in the films. The semicrystalline polymer films with photoaligned crystallites expand 9-15% in the direction perpendicular to the patterned lines when heated above the melting temperature. Furthermore, spatially patterning the crystal orientation enables the formation of various complex 3D structures including a helical coil, a coil with a handedness inversion, a cone, a saddle, and a twisting flower. Finally, the magnitude of the shape transformation was controlled by varying the polymerization temperatures, and the actuation temperature was tuned by changing the amount of crystallinity in the polymer films. The simplicity and ease of control of our approach to program complex 3D structures from 2D semicrystalline polymer films make it a promising system for the aforementioned applications.

Chemically Triggered Changes in Mechanical Properties of Responsive Liquid Crystal Polymer Networks with Immobilized Urease.

Liquid crystal polymer networks (LCNs) are stimuli-responsive materials that can be programmed to realize spatial variation in mechanical response and undergo shape transformation. Herein, we report a process to introduce chemical specificity to the stimuli response of LCNs by integrating enzymes as molecular triggers. Specifically, the enzyme urease was immobilized in LCN films via acyl fluoride conjugation chemistry. Activity assays and confocal fluorescence imaging confirmed retention of urease activity after immobilization as well as widespread distribution of enzyme on the film. The addition of urea triggered a response in the LCN whereby newly generated ammonia reacted with free acyl fluorides to form benzamide moieties. These moieties were capable of dimerizing through the formation of supramolecular hydrogen bonds, which was reflected in a 4-fold increase in Young's modulus. Through dynamic mechanical analysis and calorimetry, we further confirmed that the degree of hydrogen bonding in the LCNs could be judiciously designed to fine-tune the mechanical properties and glass transition temperature. These findings demonstrate the untapped potential of biochemical mechanisms as molecular triggers in LCNs and open the door to the use of nucleophilic chemistries in modulating the mechanical properties of LCNs.

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 τNI but reflective scattering states at τ values above τNI. 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.

Flexible polymer network liquid crystals using imprinted spacers bonded by UV-curable reactive mesogen for smart window applications

We propose junction-type, vertically aligned polymer network liquid crystals using plastic substrates for flexible smart windows, in which the spacers are formed with the imprinting method and are bonded to the substrate surface by UV-curable reactive mesogen (RM). We clarified that the optical property can be improved by suppressing the aggregation of RMs around the spacers through thinning of the coating of the RM and the simultaneous improvement of the wettability of the parallel alignment film. We achieved excellent haze properties of 3.0% with voltage off and 92.7% with voltage on, and high curvature performance with a small curvature radius of 9.0 mm.

Programmable Liquid Crystal Defect Arrays via Electric Field Modulation for Mechanically Functional Liquid Crystal Networks.

The arrangement of mesogenic units determines mechanical response of the liquid crystal polymer network (LCN) film to heat. Here, we show an interesting approach to programming three-dimensional patterns of the LCN films with periodic topological defects generated by applying an electric field. The mechanical properties of three representative patterned LCN films were investigated in terms of the arrangement of mesogenic units through tensile testing. Remarkably, it was determined that LCN films showed enhanced toughness and ductility as defects increased in a given area, which is related to the elastic modulus mismatch that mitigates crack propagation. Our platform can also be used to modulate the frictional force of the patterned LCN films by varying the temperature, which can provide insight into the multiplex mechanical properties of LCN films.

Crack Programming: From Chaos to Control: Programmable Crack Patterning with Molecular Order in Polymer Substrates (Adv. Mater. 22/2021)

Cracking is typically an uncontrolled material failure process. In article number 2008434, Hyun Kim, Taylor H. Ware, and co-workers describe a process that enables unprecedented control of crack formation. By depositing thin-film chromium on photoaligned liquid-crystal polymer networks, patterns of cracks on the surface of a polymer can be freely programmed in a high-resolution and high-throughput manner. This crack programming process provides new opportunities for unconventional microfabrication. The image depicts light transmitted through a polymer film coated with chromium with azimuthally patterned cracks.

Liquid crystal polymer networks directed by scanning wave photopolymerization of oxetane monomer and crosslinker

In this study, we fabricated liquid crystal polymer networks (LCNs) containing a large amount of crosslinker by scanning wave photopolymerization, and examined the effect of crosslinker concentration on molecular alignment behavior. Samples with different crosslinker concentrations were photopolymerized by unidirectionally scanning ultraviolet light, resulting in LCN films with uniaxial molecular alignment. The LCN films obtained from the compositions of only monomer or monomer and crosslinker had mesogens aligned in parallel to the light scanning direction. Moreover, the LCN films with high crosslinker concentrations were able to maintain their initial molecular alignment after repeated heating cycles.

39‐2: Student Paper: Fast‐Switchable Scattering Device Based on Liquid‐Crystal Fibonacci Grating

In this work, we proposed a fast‐switchable quasi‐continuous grating (Fibonacci grating) as the scattering device for viewing angle control. The viewing angle varies from nearly 0 to 30 degrees. The proposed device requires a relatively small driving voltage (<10 V), and shows a fast response time (<7 ms), comparing with traditional scattering devices such as high retardation film, polymer dispersed liquid crystal and polymer network liquid crystal, etc. The proposed device shows promising potential in controllable viewing angle and wavefront coding.

P‐52: Realization of Transparent LCD with Field Sequence

A 2.6‐inch Transparent LCD prototype has been developed by using network polymer liquid crystal PNLC (Polymer Network Liquid Crystal) combined with field‐sequential color display technology. Moreover, the display principle of transparent LCD with field sequence was expounded and a contrast ratio enhancement algorithm has been developed to improve the image CR in this paper. As there is an intrinsic problem for the transparent LCD with field sequence that uniformity of brightness is not excellent enough caused by the panel polymer and the side‐in backlight, a novel light‐guiding structure has been developed to provide the uniform surface light source, the picture shows better effect than the traditional processing method which achieved high efficiency light emission (83%) and uniformity (64%). The simulation and experimental results verify the effectiveness of the contrast ratio enhancement algorithm and the novel light‐guiding structure.

Tribo-Induced Smart Reflector for Ultrasensitive Self-Powered Wireless Sensing of Air Flow.

Air-flow sensing is essential in broad applications of weather forecasting, ocean monitoring, gas leakage alarming, and health monitoring. However, in severe environments where electrical power supply and cable connection are not available, the sensing of air flow in a self-powered way is a challenging issue. In this work, we reported a tribo-induced smart reflector to achieve the self-powered wireless sensing of the air flow by combining an aerodynamics-driven triboelectric nanogenerator (TENG) and a silver-coated polymer network liquid crystal. Upon being driven by the air flow, the developed reflector performed specular and diffused reflectance without and with charging by the TENG, respectively, enabling wireless sensing through mechanical-electrical-optical signal conversion. In the developed sensing paradigm, the sensing module can be fully self-powered without the need of signal pre-amplification, which is electrically separated from the light source and detection modules without cable connections. The applications of self-powered wireless wind speed sensing and breath monitoring were performed to demonstrate the effectiveness of the developed paradigm toward self-powered wireless sensing nodes in the internet of things.

From Chaos to Control: Programmable Crack Patterning with Molecular Order in Polymer Substrates.

Cracks are typically associated with the failure of materials. However, cracks can also be used to create periodic patterns on the surfaces of materials, as observed in the skin of crocodiles and elephants. In synthetic materials, surface patterns are critical to micro- and nanoscale fabrication processes. Here, a strategy is presented that enables freely programmable patterns of cracks on the surface of a polymer and then uses these cracks to pattern other materials. Cracks form during deposition of a thin film metal on a liquid crystal polymer network (LCN) and follow the spatially patterned molecular order of the polymer. These patterned sub-micrometer scale cracks have an order parameter of 0.98± 0.02 and form readily over centimeter-scale areas on the flexible substrates. The patterning of the LCN enables cracks that turn corners, spiral azimuthally, or radiate from a point. Conductive inks can be filled into these oriented cracks, resulting in flexible, anisotropic, and transparent conductors. This materials-based processing approach to patterning cracks enables unprecedented control of the orientation, length, width, and depth of the cracks without costly lithography methods. This approach promises new architectures of electronics, sensors, fluidics, optics, and other devices with micro- and nanoscale features.

Modeling the combined photo-chemo/thermo-mechanical actuation in azobenzene-doped liquid crystal thin films

Azo-doped liquid crystal polymer network (LCN) thin films are promising candidates for the development of soft-robots. These azo-doped LCNs exhibit either photo-chemo-mechanical or photo-thermo-mechanical or both deformations based on the type of azo-molecule used for doping, the type of external stimulus, and the surrounding environment. This paper presents a computational model capable of simulating this combined photo-chemo/thermo-mechanical deformation phenomena. Moreover, the influence of attenuation of light through the thickness and the influence of variation in the through-thickness alignment of mesogens are considered in the model. A detailed investigation of the influence of isomerization parameters, light intensity, and thermal parameters on light-induced temperature rise and the resultant deformations is also presented. The developed model shows good agreement with the experimental measurements available in the literature for the temperatures and deformations attained by azo-doped LCN films under illumination while placed in air/water.

Security use of bilayer dichroic films made of liquid crystal polymer networks

ABSTRACT We report on bilayer dichroic dye-doped liquid crystal (BDLC) films for patternable color codes, which can be used in anti-counterfeiting applications such as security codes and, unlike the conventional single-layer LC color films, show unique color changes depending on the direction of the polarization of the incident light that passes through them. This is a facile way to enhance the security level of security codes. BDLC films are fabricated by laminating two single-layer LC polymer network films with crossed optical axes. We believe that BDLC films can be used as a new platform for a security-enhanced anti-counterfeiting code that can replace complex chiral materials or expensive plasmonic particles.

Improved saturation and wide-viewing angle color filters based on multi-twist retarders.

Birefringent color filters serve a critical role in next-generation display systems, including augmented-/virtual-/mixed-reality headsets, and many types of optical remote sensing. Most prior polarization interference filters (PIFs) employ many individually aligned plates that enable only relatively thick color filters (≥100s of µm), are usually limited to small clear apertures (few cm), and offer poor off-axis performance. Here, we report on a family of monolithic, thin-film, birefringent PIFs formed using liquid crystal polymer (LCP) network materials, also known as reactive mesogens. These multi-twist retarders (MTRs) are only a few µm thick and have a single alignment surface. They offer high color saturation with a notch-type pass/stopband, analogous to Solc PIFs and stable off-axis performance. Here, we apply simplifying assumptions inspired by Solc PIFs, and develop a design method resulting in MTRs with an alternating achiral/chiral architecture. We theoretically and experimentally presented three types of MTR color filters (blue-yellow, green-magenta, and cyan-red), which manifest strong color filtering behavior and improved angular performance (up to ±20°) with larger color space coverage and high total light efficiency compared to their Solc filters counterparts. Such high-saturated and wide-viewing MTR color filters can be promising elements to maintain the system field of view (FOV) in the next-generation displays or spectral imaging applications.

Submillisecond-Response Polymer Network Liquid Crystal Phase Modulators.

A submillisecond-response and light scattering-free polymer-network liquid crystal (PNLC) for infrared spatial light modulators is demonstrated. Our new liquid crystal host exhibits a higher birefringence, comparable dielectric anisotropy, and slightly lower visco-elastic constant than a commonly employed commercial material, HTG-135200. Moreover, the electro-optical performance of our PNLCs with different monomer concentrations, cell gaps, and liquid crystal (LC) hosts is compared and discussed from four aspects: operating voltage, hysteresis, relaxation time, and light scattering loss. The temperature effect on hysteresis is also analyzed. Potential applications of PNLCs for laser beam steering and spatial light modulators especially in the infrared region are foreseeable.

Effect of 1-ethynyl-4-(trifluoromethyl) benzene on polymer network liquid crystal

ABSTRACT The polymer network liquid crystals (PNLCs) device has the advantages of large manufacturing area, high transmittance, but still has the disadvantages of high driving voltage and large hysteresis. In this paper, 1-ethynyl-4-(trifluoromethyl) benzene (ETB) was used in polymer network liquid crystal system to reduce the threshold and saturation voltage and hysteresis. The functional group of ETB is an alkynyl group, which can be regarded as polymer monomer or diluent because of its polymerisation less than the alkenyl group. When the whole weight content of polymer is 5 wt% and the 1 wt% ETB as a monomer, the operating voltage of PNLCs is reduced by 63.1%, and the hysteresis is reduced to 0.18 V. As a diluent, when the weight content of 1,4-Bis-[4-(3-acryloyloxypropyloxy)benzoyloxy]-2-methylbenzene (RM257) and ETB is 5% and 1%, the operating voltage of the polymer network liquid crystal is reduced by 51.5%, and the hysteresis is reduced to 0.38 V. From these two experiments, a small amount of ETB material doping can significantly reduce the operating voltage and hysteresis effect of PNLCs devices. Moreover, the equivalent rotational viscosity of the polymer network liquid crystals with suitable content of ETB as diluent can be reduced to be similar to that of the pure liquid crystal.