Polymer Network Liquid Crystal Research Articles

Protruding organic surfaces triggered by in-plane electric fields

Coatings with a dynamic surface topography are of interest for applications in haptics, soft robotics, cell growth in biology, hydro- and air dynamics and tribology. Here we propose a design for creating oscillating surface topographies in thin liquid crystal polymer network coatings under an electric field. By applying an alternating electric field, the coating surface deforms, and pre-designed local corrugations appear. The continuous AC electric field further initiates oscillations superimposed on the formed topographies. This effect is based on microscopic free volume creation. By exciting the liquid crystal network at its resonance frequency, maximum free volume is generated and large surface topographies are formed. Molecular simulation is used to examine this behaviour in microscopic detail as a function of oscillation frequency. Surface topography formation is fast and reversible. Excess free volume is energetically unfavourable, thus the surface topographies disappear within seconds once the electric field is removed.

Non‐Uniform Optical Inscription of Actuation Domains in a Liquid Crystal Polymer of Uniaxial Orientation: An Approach to Complex and Programmable Shape Changes

AbstractAchieving complex shape change of liquid‐crystal polymer networks (LCNs) under stimulation generally requires spatial configuration of the orientation direction, that is, patterned directors, of liquid crystal monomers prior to polymerization by means of treated surfaces. A strategy is demonstrated that needs only the simple uniaxial orientation of mesogens (monodomain) induced by mechanical stretching of LCNs. Using a rationally designed liquid crystal polymer, photocrosslinking is utilized to pattern or spatially organize the actuating monodomains in order to generate a differential contractile and/or extensional force field required for targeted shape change. Moreover, the approach enables versatile actuation modes and allows multiple shape changes to be programmed on a single piece of the polymer. This important feature is demonstrated with a specimen cut to have eight strips that, upon thermal stimulation, simultaneously display eight types of shape morphing.

Non-Uniform Optical Inscription of Actuation Domains in a Liquid Crystal Polymer of Uniaxial Orientation: An Approach to Complex and Programmable Shape Changes.

Achieving complex shape change of liquid-crystal polymer networks (LCNs) under stimulation generally requires spatial configuration of the orientation direction, that is, patterned directors, of liquid crystal monomers prior to polymerization by means of treated surfaces. A strategy is demonstrated that needs only the simple uniaxial orientation of mesogens (monodomain) induced by mechanical stretching of LCNs. Using a rationally designed liquid crystal polymer, photocrosslinking is utilized to pattern or spatially organize the actuating monodomains in order to generate a differential contractile and/or extensional force field required for targeted shape change. Moreover, the approach enables versatile actuation modes and allows multiple shape changes to be programmed on a single piece of the polymer. This important feature is demonstrated with a specimen cut to have eight strips that, upon thermal stimulation, simultaneously display eight types of shape morphing.

Theory of liquid crystal elastomers and polymer networks : Connection between neoclassical theory and differential geometry.

In liquid crystal elastomers and polymer networks, the orientational order of liquid crystals is coupled with elastic distortions of crosslinked polymers. Previous theoretical research has described these materials through two different approaches: a neoclassical theory based on the liquid crystal director and the deformation gradient tensor, and a geometric elasticity theory based on the difference between the actual metric tensor and a reference metric. Here, we connect those two approaches using a formalism based on differential geometry. Through this connection, we determine how both the director and the geometry respond to a change of temperature.

Doping Liquid Crystal Cells with Photocurable Monomer via Holographic Exposure to Realize Optical-Scattering-Free Infrared Phase Modulators with Fast Response Time

Photocurable monomer-doped liquid crystal (LC) cells were processed via holographic exposure using a low-power He–Ne laser to generate holographic polymer networks. The polymer network LC (PNLC) cells are used to fabricate infrared phase modulators at 1550 nm wavelength possessing favorable electro-optical performance. Compared with our previous work, the percentages of ingredients in the LC mixture filled in PNLC cells underwent a slight change. The 2 wt% concentration of anisotropic monomer RM257 were in place of isotropic monomer N–vinyl–2–pyrrolidinone (NVP). As a result, the fabricated phase modulators also maintained well homogeneous LC alignments and optical-scattering-free characteristics. Furthermore, NVP dopant successfully reduced the operating voltages from 95 Vrms to 79 Vrms to prevent polymer network deformation when electrically operating with higher voltages. The fabricated infrared phase modulators had a good average response time (i.e., rising time of 0.88 ms and falling time of 0.40 ms).

Light-Responsive Hierarchically Structured Liquid Crystal Polymer Networks for Harnessing Cell Adhesion and Migration.

Extracellular microenvironment is highly dynamic where spatiotemporal regulation of cell-instructive cues such as matrix topography tightly regulates cellular behavior. Recapitulating dynamic changes in stimuli-responsive materials has become an important strategy in regenerative medicine to generate biomaterials which closely mimic the natural microenvironment. Here, light responsive liquid crystal polymer networks are used for their adaptive and programmable nature to form hybrid surfaces presenting micrometer scale topographical cues and changes in nanoscale roughness at the same time to direct cell migration. This study shows that the cell speed and migration patterns are strongly dependent on the height of the (light-responsive) micrometer scale topographies and differences in surface nanoroughness. Furthermore, switching cell migration patterns upon in situ temporal changes in surface nanoroughness, points out the ability to dynamically control cell behavior on these surfaces. Finally, the possibility is shown to form photoswitchable topographies, appealing for future studies where topographies can be rendered reversible on demand.

Electro-optical properties of vertically aligned polymer network liquid crystals for normally transparent light shutters

ABSTRACTThis study examined the electro-optical properties of black-background, normally transparent polymer network liquid crystal cells suitable for light shutters applicable to transparent flexible displays. The cells were fabricated using a polymer induced phase separation method from the mixture of negative liquid crystal, reactive-mesogen and black dye. A parametric study for key material and process parameters was carried out to determine the parameter dependence of the E-O properties and to achieve high E-O performance. The E-O properties were strongly dependent on RM concentration and UV exposure intensity, not on UV exposure energy. The optimal conditions for high E-O performance are described.

Modulated Visible Light Upconversion for Luminescence Patterns in Liquid Crystal Polymer Networks Loaded with Upconverting Nanoparticles

Modulated Visible Light Upconversion for Luminescence Patterns in Liquid Crystal Polymer Networks Loaded with Upconverting Nanoparticles

Voxel resolution in the directed self-assembly of liquid crystal polymer networks and elastomers.

Monomeric mixtures formulated to prepare a liquid crystal polymer network (LCN) or elastomer (LCE) can be "programmed" by surface alignment to retain complex and arbitrary spatial distributions of the director orientation upon polymerization. The localized control of orientation in a given volume (voxel) within these materials is the subject of intense research, currently motivated by the prospect of distinctive mechanical responses (both active and passive). Here, we report on a rapid and scalable photopatterning method to prepare alignment surfaces with a throughput of 10 mm2 s-1, using a commercial spatial light modulator and projection optics. Enabled by this method, we detail that the resolution limit of the inscribed director profile is not dictated by the optical system but is determined by the elastic-mediated orientational relaxation of the liquid crystalline materials. A simple model is experimentally validated and the implications for device design are discussed.

Polymer network liquid crystal grating/Fresnel lens fabricated by holography

ABSTRACTIn this article, polymer network liquid crystal (PNLC) grating/Fresnel lens is fabricated by holography. The exposure light pattern for the grating is obtained by interfering two planar wave fronts, while the Fresnel pattern is achieved by interfering a planar wave front and a spherical wave front. Owing to the alignment effect and anchoring power of polymer network, the holographic PNLC grating achieves improved diffraction efficiency, and remarkably reduced operation voltage (reduced by 80%) compared with holographic polymer-dispersed-liquid-crystal and holographic polymer-stabilised blue-phase liquid-crystal gratings, while maintaining submillisecond response. Moreover, it achieves high spatial frequency with a 2-μm grating period, thanks to the holographic fabrication. The holographic PNLC Fresnel lens also exhibits attractive electro-optical properties.

Preparation of biomimetic photoresponsive polymer springs.

Polymer springs that twist under irradiation with light, in a manner that mimics how plant tendrils twist and turn under the effect of differential expansion in different sections of the plant, show potential for soft robotics and the development of artificial muscles. The soft springs prepared using this protocol are typically 1 mm wide, 50 μm thick and up to 10 cm long. They are made from liquid crystal polymer networks in which an azobenzene derivative is introduced covalently as a molecular photo-switch. The polymer network is prepared by irradiation of a twist cell filled with a mixture of shape-persistent liquid crystals, liquid crystals having reactive end groups, molecular photo-switches, some chiral dopant and a small amount of photoinitiator. After postcuring, the soft polymer film is removed and cut into springs, the geometry of which is determined by the angle of cut. The material composing the springs is characterized by optical microscopy, scanning electron microscopy and tensile strength measurements. The springs operate at ambient temperature, by mimicking the orthogonal contraction mechanism that is at the origin of plant coiling. They shape-shift under irradiation with UV light and can be pre-programmed to either wind or unwind, as encoded in their geometry. Once illumination is stopped, the springs return to their initial shape. Irradiation with visible light accelerates the shape reversion.

Polymer Network Liquid Crystal (PNLC) Lenticular Microlens Array With No Surface Treatment

A polymer network liquid crystal (PNLC) lenticular microlens array with no surface treatment is prepared. Before polymerization, the LC and diacrylate monomers are reoriented with gradient refractive index distribution by a fringing field. After polymerization, the oriented LC is stabilized by the polymer network. The prepared PNLC lenticular microlens array is optically anisotropic. Without surface treatment, the device fabrication is simplified, and it can present almost the same optical performance as the PNLC lenticular microlens with surface treatment. Integrating with a 90 $^\circ $ twisted-nematic LC cell, the PNLC lenticular microlens array can be switched between a focusing state and non-focusing state. Such a PNLC lenticular microlens array has potential application in switchable 2D/3D displays.

Fluorinated Azobenzenes for Shape-Persistent Liquid Crystal Polymer Networks.

Liquid crystal polymer networks respond with an anisotropic deformation to a range of external stimuli. When doped with molecular photoswitches, these materials undergo complex shape modifications under illumination. As the deformations are reversed when irradiation stops, applications where the activated shape is required to have thermal stability have been precluded. Previous attempts to incorporate molecular switches into thermally stable photoisomers were unsuccessful at photogenerating macroscopic shapes that are retained over time. Herein, we show that to preserve photoactivated molecular deformation on the macroscopic scale, it is important not only to engineer the thermal stability of the photoswitch but also to adjust the cross-linking density in the polymer network and to optimize the molecular orientations in the material. Our strategy resulted in materials containing fluorinated azobenzenes that retain their photochemical shape for more than eight days, which constitutes the first demonstration of long-lived photomechanical deformation in liquid-crystal polymer networks.

Hysteresis-free and submillisecond-response polymer network liquid crystal.

We demonstrate a polymer network liquid crystal (PNLC) with negligible hysteresis while keeping submillisecond response time. By doping about 1% dodecyl acrylate (C12A) into the liquid crystal/monomer precursor, both hysteresis and residual birefringence are almost completely eliminated. The operating voltage and scattering properties remain nearly intact, but the tradeoff is enhanced double relaxation. This hysteresis-free PNLC should find applications in spatial light modulators, laser beam control, and optical communications in infrared region.

Volume generation towards dynamic surface morphing in liquid crystal polymer networks

ABSTRACTThin coatings based on liquid crystal networks (LCNs) modified with azobenzene moieties are able to create dynamic surface topographies in the micrometre range by exposure with UV light. The surface corrugations can be erased and restored by switching ‘off’ and ‘on’ the UV illumination. Various configurations were presented. The formation of the protrusions was proven to be induced mainly by excessive volume formation when the order in the LCNs is reduced. It is suggested that this extra volume formation can be further enhanced by stimulating the oscillatory dynamics of trans-cis and cis-trans isomerisation. Therefore, dual-wavelength exposure not only exciting the trans state of azobenzene by 365 nm UV light but simultaneously also the cis state by 455 nm blue light was shown to enhance the effect.

Localised polymer networks in chiral nematic liquid crystals for high speed photonic switching

Self-assembled periodic structures based upon chiral liquid crystalline materials have significant potential in the field of photonics ranging from fast-switching optoelectronic devices to low-threshold lasers. The flexoelectro-optic effect, which is observed in chiral nematic liquid crystals (LCs) when an electric field is applied perpendicular to the helical axis, has significant potential as it exhibits analogue switching in 10–100 μs. However, the major technological barrier that prohibits the commercial realisation of this electro-optic effect is the requirement of a uniform, in-plane alignment of the helix axis between glass substrates. Here, it is shown that periodic polymer structures engineered in the nematic phase of a chiral nematic LC device using direct laser writing can result in the spontaneous formation of the necessary uniform lying helix (ULH) state. Specifically, two-photon polymerization is used in conjunction with a spatial light modulator so as to correct for aberrations introduced by the bounding glass substrates enabling the polymer structures to be fabricated directly into the device. The ULH state appears to be stable in the absence of an externally applied electric field, and the optimum contrast between the bright and dark states is obtained using polymer structures that have periodicities of the order of the device thickness.

P-139: Polymer Network Liquid Crystal Grating Cured with Interfered Visible Light

In this paper, we report a polymer network liquid crystal grating fabricated using interfered visible light. Exposed to the periodic light pattern, polymer concentration gradient is achieved as the precursor is cured. The grating is simple to fabricate and has low voltage, sub-millisecond response and high spatial resolution.

Thermo-responsive shape and optical memories of photonic composite films enabled by glassy liquid crystalline polymer networks.

We propose a novel shape and optical memories of a photonic composite film based on a silica opal photonic crystal (PC) template and a liquid crystal polymer network (LCN). Here, the photonic composite film was fabricated by introducing a LCN precursor into a silica opal PC template, followed by UV photo-polymerization and then by the removal of the template. The obtained bilayer-structure photonic film was found to spontaneously form a three-dimensional (3D) temporary bending shape in response to heating, and thus the corresponding reflection color of the photonic composite film shows a blue shift during bending deformation. The inherent mechanisms of these two observations could be attributed to the variations of the LC molecule orientation and the light reflection in the photonic composite film during the thermal process. More intriguingly, the resulting temporary bending shape was fixed by applying mechanical force during slowly cooling down to the room temperature or autonomously fixed by a rapid cooling in liquid nitrogen. Additionally, this temporary state could restore back to the permanent flat shape when the film is cooled from the heat source without an external force. Finally, more complex 3D shape-memory samples could also be achieved by simply controlling the LC alignment or designing the sample geometry. This work opens up a new way to develop a novel shape-memory polymer photonic film.

Regulating the modulus of a chiral liquid crystal polymer network by light.

We report a novel way to modulate the elastic modulus of azobenzene containing liquid crystal networks (LCNs) by exposure to light. The elastic modulus can cycle between different levels by controlling the illumination conditions. Exposing the polymer network to UV light near the trans absorption band of azobenzene gives a small reduction of the glass transition temperature thereby lowering the modulus. The addition of blue light addressing the cis absorption band surprisingly amplifies this effect. The continuous oscillatory effects of the trans-to-cis isomerization of the azobenzene overrule the overall net cis conversion. The influence on the chain dynamics of the network is demonstrated by dynamic mechanical thermal analysis which shows a large shift of the glass transition temperature and a modulus decrease by more than two orders of magnitude. The initial high modulus and the glassy state are recovered within a minute in the dark by switching off the light sources, despite the observation that azobenzene is still predominantly in its cis state. Based on these new findings, we are able to create a shape memory polymer LCN film at room temperature using light.

Programmable and adaptive mechanics with liquid crystal polymer networks and elastomers.

Liquid crystals are the basis of a pervasive technology of the modern era. Yet, as the display market becomes commoditized, researchers in industry, government and academia are increasingly examining liquid crystalline materials in a variety of polymeric forms and discovering their fascinating and useful properties. In this Review, we detail the historical development of liquid crystalline polymeric materials, with emphasis on the thermally and photogenerated macroscale mechanical responses--such as bending, twisting and buckling--and on local-feature development (primarily related to topographical control). Within this framework, we elucidate the benefits of liquid crystallinity and contrast them with other stimuli-induced mechanical responses reported for other materials. We end with an outlook of existing challenges and near-term application opportunities.