Liquid Crystal Networks Research Articles

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.

Light-deformable dynamic surface fabricated by ink-jet printing.

Dynamic surfaces which can change their topography with external stimuli have wide application prospects. Liquid crystal network (LCN) is an ideal material for making dynamic surfaces, but traditional methods for LCN dynamic surface manufacturing are difficult to scale up, which limits its applications. This research proposes a new method to fabricate a responsive surface using ink-jet printing technology. Using a liquid crystal monomer mixture as the ink, we printed arrays of droplets onto a glass substrate with a homeotropic alignment layer and polymerized the droplets into deformable LCN hemispheres. An azobenzene diacrylate was copolymerized into the hemispheres to make them photo-responsive to UV light. Because the ink-jet printing method can potentially be used to print countless hemispheres on a large area substrate, large area dynamic surfaces consisting of a multitude of separate dynamic structures can be manufactured. Since the deformation of the entire surface is a periodic repetition of the deformation of a single hemisphere, we characterized the deformation of individual hemispheres, and found that the optical image of hemispheres between crossed polarizers shows a "maltese cross" texture, and 3D surface profiling shows the top surface depresses into a valley after UV-irradiation. This is caused by an order parameter decrease of the homeotropically aligned LC molecules, which leads to a contraction in the alignment direction. The deformation amplitude can be modulated by UV intensity and temperature. This kind of dynamic surface fabricated by ink-jet printing technology can easily be scaled up and is promising for applications such as adjustable micro-lenses or surface wettability.

Ultralong cycling and wide temperature range of lithium metal batteries enabled by solid polymer electrolytes interpenetrated with a poly(liquid crystal) network

With the in situ polymerization of liquid crystal (LC), more free lithium ions are released, which enhances the performance of P-PLC-IL.

Effects of polarized light on the optical and self-oscillation behaviors of liquid crystal network polymers

The different polarizations of light show different oscillating behaviors in terms of amplitude and frequency of the LCN oscillation.

Stimulus-driven liquid metal and liquid crystal network actuators for programmable soft robotics.

Sophisticated soft matter engineering has been endorsed as an emerging paradigm for developing untethered soft robots with built-in electronic functions and biomimetic adaptation capacities. However, the integration of flexible electronic components into soft robotic actuators is challenging due to strain mismatch and material incompatibilities. Herein, we report a general strategy to integrate electrically conductive liquid metals (LMs) and shape-morphing liquid crystal networks (LCNs) towards multifunctional and programmable soft robotics. A unique colloidal LM ink with superior adhesion and photothermal conversion efficiency was judiciously designed and fabricated by ultrasonicating LMs and miniature carboxylated gold nanorods (MiniGNR-COOH) in an aqueous suspension of biological bacterial cellulose. The designed nanocellulose-based colloidal LM ink is used for shape-deformable and electrically conductive LM-LCN soft robots that can be electro- and photo-thermally actuated. As proof-of-concept demonstrations, we present a light-fueled soft oscillator, an inchworm-inspired soft crawler and programmable robotic Shadow Play exhibiting multifunctional controllability. The strategy disclosed here could open up a new technological arena for advanced multifunctional soft materials with potential utility in bioinspired soft machines, integrated soft electronics, human-computer interaction and beyond.

Numerical analysis and design of a light-driven liquid crystal polymer-based motorless miniature cart.

Liquid crystal polymers are a special class of soft materials that can change their shape in response to numerous stimuli such as light, heat, electric field, and chemicals. The ability to tailor the deformed shape by tuning the alignment of mesogens across the film has enabled the researchers to generate unique motions from these liquid crystal polymer thin films. Simulating such motions might allow us to understand the underlying mechanisms better and could lead to novel designs. In this paper, we analyze the kinematics of the light-driven rolling motion of wheels fabricated with azobenzene-doped glassy liquid crystal networks through a one-way coupled transient photo-mechanical model. The influence of the isomerization parameters and the alignment of mesogens through the thickness on the kinematics of the wheel is presented. The developed model is further used to assess the feasibility of a light-actuated four-wheeled cart with wheels made of azobenzene-doped liquid crystal network thin films.

Dynamical Landau-de Gennes theory for electrically-responsive liquid crystal networks.

Liquid crystal networks combine the orientational order of liquid crystals with the elastic properties of polymer networks, leading to a vast application potential in the field of responsive coatings, e.g., for haptic feedback, self-cleaning surfaces, and static and dynamic pattern formation. Recent experimental work has further paved the way toward such applications by realizing the fast and reversible surface modulation of a liquid crystal network coating upon in-plane actuation with an AC electric field [Liu, Tito, and Broer, Nat. Commun. 8, 1526 (2017)10.1038/s41467-017-01448-w]. Here, we construct a Landau-type theory for electrically-responsive liquid crystal networks and perform molecular dynamics simulations to explain the findings of these experiments and inform on rational design strategies. Qualitatively, the theory agrees with our simulations and reproduces the salient experimental features. We also provide a set of testable predictions: the aspect ratio of the nematogens, their initial orientational order when cross-linked into the polymer network, and the cross-linking fraction of the network all increase the plasticization time required for the film to macroscopically deform. We demonstrate that the dynamic response to oscillating electric fields is characterized by two resonances, which can likewise be influenced by varying these parameters, providing an experimental handle to fine-tune device design.

A Flexible Bilayer Actuator Based on Liquid Crystal Network and PVDF–TrFE for Low‐Grade Waste Heat Harvesting

Flexible low‐grade waste heat generators are a promising device that can both recycle thermal energy and exhibit elasticity, are light weight, and have easily customized shapes. However, most thermal energy recovery technologies only work at large temperature gradients. Herein, a dynamic generator based on liquid crystal network (LCN) and poly(vinylidenedifluoride–trifluoroethylene) (PVDF–TrFE) is reported. The generator can operate in a temperature of ≈30 °C with a temperature difference of 6 K. LCN expands and shrinks in the temperature fluctuation, which is beneficial for PVDF–TrFE exposure to greater temperature changes. The device is found to exhibit a maximum output voltage of ≈4.5 mV based on pyroelectric and piezoelectric effect. This simple strategy may broaden applications of liquid crystalline actuators for harvesting low‐grade waste heat.

Heat-Resistant Polymer Materials Based on Liquid Crystal Compounds

Data on heat-resistant linear and crosslinked polymer materials containing mesogenic groups in their structure have been systematized. Principles of the macromolecular design of thermoplastic liquid-crystal (LC) polymers providing high performance characteristics and their relationship with the supermolecular organization and processing conditions have been discussed. Methods for synthesizing LC precursors with reactive terminal groups cured to form three-dimensional LC networks have been discussed; the effect of the chemical nature of the terminal groups on the thermal stability of the resulting material has been shown.

3D Microstructures of Liquid Crystal Networks with Programmed Voxelated Director Fields.

The shape-shifting behavior of liquid crystal networks (LCNs) and elastomers (LCEs) is a result of an interplay between their initial geometrical shape and their molecular alignment. For years, reliance on either one-step in situ or two-step film processing techniques has limited the shape-change transformations from 2D to 3D geometries. The combination of various fabrication techniques, alignment methods, and chemical formulations developed in recent years has introduced new opportunities to achieve 3D-to-3D shape-transformations in large scales, albeit the precise control of local molecular alignment in microscale 3D constructs remains a challenge. Here, the voxel-by-voxel encoding of nematic alignment in 3D microstructures of LCNs produced by two-photon polymerization using high-resolution topographical features is demonstrated. 3D LCN microstructures (suspended films, coils, and rings) with designable 2D and 3D director fields with a resolution of 5 μm are achieved. Different shape transformations of LCN microstructures with the same geometry but dissimilar molecular alignments upon actuation are elicited. This strategy offers higher freedom in the shape-change programming of 3D LCN microstructures and expands their applicability in emerging technologies, such as small-scale soft robots and devices and responsive surfaces.

Unique two-way free-standing thermo- and photo-responsive shape memory azobenzene-containing polyurethane liquid crystal network

In this work, azobenzene-containing polyurethane liquid crystal networks (PULCN(AZO)s) were synthesized using a one-pot strategy to demonstrate excellent two-way free-standing thermo-/photo-responsive shape memory effects. Based on the step-growth nature of hydroxyls and isocyanates, the architectures of the networks were adjusted by controlling the stoichiometries of chemical materials. A uniform monodomain sample was prepared by external stress relaxation via a reversible addition reaction of a dynamic carbamate bond. Two independent transition temperatures assigned to glass transition temperature/melting temperature and liquid crystal phase transition temperature were employed to thermally trigger triple shape memory effects and two-way autonomous actuation. In addition, taking advantage of the trans-cis photoisomerization of azobenzene, the programmed network showed a reversible bending and unbending shape change upon irradiation by visible light at 450 and 550 nm, respectively. Coupling the autonomously thermo-induced contraction/extension actuation and reversible photo-induced bending/unbending behaviors of PULCN(AZO)s in one system will expand their potential applications in emerging multifunctional devices.

Synthesis of well-defined PS-based Azo-liquid crystals with control of phase transitions and photo-behaviors for liquid crystal networks with photomechanical deformation

Benefiting from their good mechanical processability and high tunability, Si–H and vinyl-functionalized polystyrenes (PSs), i.e., poly(4-vinylphenyl)-dimethyl-silane (PVPDMS/PS-SiH) and poly(4-vinylphenyl)-1-butene (PVSt/PS-Vinyl), produced by living anionic polymerization (LAP) were treated with four vinyl- and Si–H-terminated azobenzene mesogens (Azo-Vinyl, Azo-SiH, AzoB-Vinyl, and AzoB-SiH, referred to as M1, M2, M3, and M4, respectively) via hydrosilylation for the fabrication of PS-based side-chain liquid crystal polymers (SCLCPs) containing different spacers, i.e., PS-SiC/SiOSi-Azo/AzoB. As a strategy for basic molecular control in PS-based SCLCPs, the cooperative influences of the spacers and the degree of polymerization (DP) on the mesomorphic transitions were explored. The phase transitions could be tuned slightly based on the DP but dramatically by adjusting the spacers. For example, the temperature ranges of mesomorphic formation (ΔT) were in the order PS-[SiOSi]-Azo (ΔT = 34–49 °C) < PS-[SiC]-Azo (ΔT = 49–60 °C) < PS-[SiOSi]-AzoB (ΔT = 154–164 °C) < PS-[SiC]-AzoB (ΔT = 162–193 °C). Taking advantage of the quantitative structure-property correlation of PS-based SCLCPs, PS-based liquid crystal networks (LCNs) were further designed for the photomechanical deformation. PS-based LCNs show good mechanical properties with a storage modulus of 120 MPa, controlled phase transitions, and glass transition temperatures (Tg) around room temperature. Crosslinked PS-based LCNs of rigid PS homopolymers with selective crosslinking can exhibit directional bending upon irradiation with UV light at 50 °C after stretching, and this was shown to be a photomechanical deformation, which expands the applicability of these materials.

Artificial Organic Skin Wets Its Surface by Field-Induced Liquid Secretion

SummaryLiving organisms enhance their survival rate by excreting fluids at their surface, but man-made materials can also benefit from liquid secretion from a solid surface. Known approaches to secrete a liquid from solids are limited to passive release driven by diffusion, surface tension, or pressure. Remotely triggered release would give active control over surface properties but is still exceptional. Here, we report on an artificial skin that secretes functional fluids by means of radiofrequency electrical signals driven by dielectric liquid transport in a (sub-)microporous smectic liquid crystal network. The smectic order of the polymer network and its director determine the flow direction and enhance fluid transport toward the surface at pre-set positions. The released fluid can be reabsorbed by the skin using capillary filling. The fluid-active skins open avenues for robotic handling of chemicals and medicines, controlling tribology and fluid-supported surface cleaning.

Light Tracking and Light Guiding Fiber Arrays by Adjusting the Location of Photoresponsive Azobenzene in Liquid Crystal Networks

AbstractThe integration of light tracking and light guiding within fiber arrays is an intriguing challenge. In this study, an advanced drop casting/drawing method is applied to fabricate a fiber array capable of not only performing sunflower‐inspired light tracking, but also light guiding. The fiber arrays are constructed with various liquid crystal networks by adjusting the location of photoresponsive azobenzene moieties in the polymer network to understand the correlation between the extent of photoresponse and azobenzene location. Incorporating the azobenzene in the main chain oligomer renders these fibers with advanced photoresponse in both air and water. The fibers are able to track a light source and to guide the collected light, making these responsive actuator arrays potentially attractive for advanced photovoltaic and optical elements.

Spontaneous photo-deformation of a liquid crystal network membrane

Photo-sensitive characteristic of the liquid crystal networks (LCNs) provides a distinct way to control the deformation of the LCN membrane. In this paper, the spontaneous photo-deformation of a simply supported square liquid crystal network membrane is investigated. Firstly, the governing equation for the LCN membrane with an arbitrary pre-designed uniaxial director (common orientation vector) pattern is established. By using the finite difference method, the deflection configurations are obtained, and four basic flexure modes for the LCN membrane are observed. The evolution of the flexure modes with respect to the director orientation is discussed in detail, and the critical division condition is given. The results given in this paper provide a theoretical guidance on the design of LCN based intelligent light driven membrane elements.

Reactive mesogens for ultraviolet-transparent liquid crystal polymer networks

ABSTRACT Transparency and stability to UV light are important and desirable properties for modern tunable optical elements and active soft robots. A library of novel reactive mesogens for liquid crystal polymer networks resilient and transparent to UV light has been synthetised and characterised. Phase behaviours of the reactive mesogens have been determined by polarised optical microscopy and differential scanning calorimetry. Liquid crystal polymer networks based on the combination of these novel reactive mesogens have been evaluated and compared to those based on common commercially available compounds. The results showed a twofold increase in transparency in a broad UV spectral region (200–400 nm) and importantly showed no degradation upon prolonged UV exposure contrary to the networks composed from commercial counterparts.

Electroplasticization of Liquid Crystal Polymer Networks.

Shape-shifting liquid crystal networks (LCNs) can transform their morphology and properties in response to external stimuli. These active and adaptive polymer materials can have impact in a diversity of fields, including haptic displays, energy harvesting, biomedicine, and soft robotics. Electrically driven transformations in LCN coatings are particularly promising for application in electronic devices, in which electrodes are easily integrated and allow for patterning of the functional response. The morphing of these coatings, which are glassy in the absence of an electric field, relies on a complex interplay between polymer viscoelasticity, liquid crystal order, and electric field properties. Morphological transformations require the material to undergo a glass transition that plasticizes the polymer sufficiently to enable volumetric and shape changes. Understanding how an alternating current can plasticize very stiff, densely cross-linked networks remains an unresolved challenge. Here, we use a nanoscale strain detection method to elucidate this electric-field-induced devitrification of LCNs. We find how a high-frequency alternating field gives rise to pronounced nanomechanical changes at a critical frequency, which signals the electrical glass transition. Across this transition, collective motion of the liquid crystal molecules causes the network to yield from within, leading to network weakening and subsequent nonlinear expansion. These results unambiguously prove the existence of electroplasticization. Fine-tuning the induced emergence of plasticity will not only enhance the surface functionality but also enable more efficient conversion of electrical energy into mechanical work.

Flexible Nanoporous Liquid Crystal Networks as Matrixes for Förster Resonance Energy Transfer (FRET)

Förster resonance energy transfer (FRET) is important, not only in the fields of biology and biophysics but also in optoelectronics and light guiding systems. Different matrixes are being investigated that facilitate FRET, including zeolites and metal–organic frameworks. In this work, a matrix for FRET generation is proposed: nanoporous liquid crystal networks. These liquid crystal networks can be easily processed and can align dichroic fluorescent dyes. A base treatment can create nanopores in the network, which are then able to absorb a second fluorescent dye in an aqueous phase while still retaining good alignment. Using lifetime measurements, we provide proof that even in this nonoptimized system, around 70% of the energy was transferred via the FRET mechanism from one dye to the other. Liquid crystal networks have many advantages over current matrixes as they are easy to fabricate as well as flexible and could be modified to selectively and reversely absorb dyes, allowing many applications.

Liquid Crystal Networks on Thermoplastics: Reprogrammable Photo-Responsive Actuators.

Arbitrary shape (re)programming is appealing for fabricating untethered shape‐morphing photo‐actuators with intricate configurations and features. We present re‐programmable light‐responsive thermoplastic actuators with arbitrary initial shapes through spray‐coating of polyethylene terephthalate (PET) with an azobenzene‐doped light‐responsive liquid crystal network (LCN). The initial geometry of the actuator is controlled by thermally shaping and fixing the thermoplastic PET, allowing arbitrary shapes, including origami‐like folds and left‐ and right‐handed helicity within a single sample. The thermally fixed geometries can be reversibly actuated through light exposure, with fast, reversible area‐specific actuation such as winding, unwinding and unfolding. By shape re‐programming, the same sample can be re‐designed and light‐actuated again. The strategy presented here demonstrates easy fabrication of mechanically robust, recyclable, photo‐responsive actuators with highly tuneable geometries and actuation modes.

Liquid Crystal Networks on Thermoplastics: Reprogrammable Photo‐Responsive Actuators

AbstractArbitrary shape (re)programming is appealing for fabricating untethered shape‐morphing photo‐actuators with intricate configurations and features. We present re‐programmable light‐responsive thermoplastic actuators with arbitrary initial shapes through spray‐coating of polyethylene terephthalate (PET) with an azobenzene‐doped light‐responsive liquid crystal network (LCN). The initial geometry of the actuator is controlled by thermally shaping and fixing the thermoplastic PET, allowing arbitrary shapes, including origami‐like folds and left‐ and right‐handed helicity within a single sample. The thermally fixed geometries can be reversibly actuated through light exposure, with fast, reversible area‐specific actuation such as winding, unwinding and unfolding. By shape re‐programming, the same sample can be re‐designed and light‐actuated again. The strategy presented here demonstrates easy fabrication of mechanically robust, recyclable, photo‐responsive actuators with highly tuneable geometries and actuation modes.