Photoinduced Topographical Feature Development in Blueprinted Azobenzene‐Functionalized Liquid Crystalline Elastomers

All‐optical deformation and recovery of complex topographical features is demonstrated within elastic sheets composed of main‐chain type azobenzene‐functionalized liquid crystalline elastomers (azo‐LCEs). The azo‐LCEs are synthesized via an orthogonal, two‐step reaction between commercially available LC monomers and n‐butylamine. By employing surface alignment, the local orientation of the nematic director is spatially complex (“blueprinted”). Exposing the blueprinted LCE films to light as an actinic stimulus generates a photomechanical response which yields reversible shape changes between 2D and 3D shapes. The deformation of azo‐LCEs strongly depends on the azobenzene concentration as well as the network structure (i.e., crosslink density). Blueprinting complex director profiles within azo‐LCEs yield reconfigurable elastic sheets that can be addressed both remotely and selectively which may have benefit in a variety of applications in aerospace, medicine, and optics.     

Stimuli‐Responsive Materials Based on Interpenetrating Polymer Liquid Crystal Hydrogels

Stimuli‐responsive materials based on interpenetrating liquid crystal‐hydrogel polymer networks are fabricated. These materials consist of a cholesteric liquid crystalline network that reflects color and an interwoven poly(acrylic acid) network that provides a humidity and pH response. The volume change in the cross‐linked hydrogel polymer results in a dimensional alteration in the cholesteric network as well, which, in turn, leads to a color change yielding a dual‐responsive photonic material. Furthermore a patterned coating having responsive and static interpenetrating polymer network areas is produced that changes both its surface topography and color.     

Well‐Defined Pillararene‐Based Azobenzene Liquid Crystalline Photoresponsive Materials and Their Thin Films with Photomodulated Surfaces

Photoresponsive materials (PRMs) have long been a hot topic and photo‐modulated smart surface is very appealing. Particularly, liquid crystalline PRMs are able to amplify and stabilize photoinduced orientation thanks to their self‐assembling and ordering characteristics. Herein, the first pillararene‐based azobenzene liquid crystalline PRM with well‐defined structure is presented, which can avoid the usually ill‐defined composition drawback of polymer PRMs and prevent the severe H‐aggregation from suppressing or even completely blocking photoresponse in simple azobenzene derivatives. The pillar[5]arene‐based macrocyclic azobenzenes with variant length spacers show wide temperature range smectic liquid crystalline mesophases and excellent film‐formation property. The tubular pillar[5]arene macrocyclic framework provides sufficient free volume for azobenzene moieties to achieve reversible photoisomerization and photoalignment; thus, their thin films demonstrate excellent light‐triggered modulation of surface free energy, wettability, and even photoalignment‐mediated orientation of an upper layer discotic liquid crystal columnar mesophase. Such pillararene‐based azobenzene liquid crystals represent novel and promising PRMs with extensive fascinating applications.     

Photodriven,Flexural–Torsional Oscillation of Glassy Azobenzene Liquid Crystal Polymer Networks

Cantilevers composed of glassy, photoresponsive liquid crystalline polymer networks (LCNs) are shown to oscillate at high frequency (～50 Hz) and large amplitude when exposed to light from a 442 nm coherent wave (CW) laser. Added dimensionality to previously reported in‐plane oscillations is enabled by adjusting the orientation of the nematic director to the long axis of the cantilever yielding in‐plane bending accompanied by out‐of‐plane twisting (flexural–torsional oscillation). The fundamental photoresponse of this class of glassy azobenzene liquid crystal polymer networks (azo‐LCN) is further probed by examining the influence of cantilever aspect ratio, laser intensity, and temperature. The frequency of photodirected oscillations is strongly correlated to the length of the cantilever while the amplitude and threshold laser intensity for oscillation is strongly correlated to temperature.     

Control of the Properties of Micrometer‐Sized Actuators from Liquid Crystalline Elastomers Prepared in a Microfluidic Setup

In this article new results on the preparation of monodisperse particles from a liquid crystalline elastomer in a microfluidic setup are presnted. For this, droplets from a liquid crystalline monomer are prepared in a microfluidic device and polymerized while they are flowing inside a microtube. The parti­cles obtained by this method possess an internal orientation, which gives them actuating properties. When they are heated into the isotropic phase of the liquid crystalline material they show a reversible change in shape whereby they change their length in one direction by almost 100%. It is shown how the variation of experimental parameters during their synthesis impacts the properties of these micro‐actuators. Influence over their primal shape, the strength of their shape changing properties, their size, and their mechanical properties is demontrated. From the systematic variation of experimental parameters a deep understanding of the complex processes taking place in a flowing droplet of a liquid crystalline material is obtainted. Additionally NMR analysis and swelling experiments on these actuating materials are provided.     

Controlling the Self‐Assembly of Periodic Defect Patterns in Smectic Liquid Crystal Films with Electric Fields

Large‐area periodic defect patterns are produced in smectic A liquid crystals confined between rigid plate electrodes that impose conflicting parallel and normal anchoring conditions, inducing the formation of topological defects. Highly oriented stripe patterns are created in samples thinner than 2 μm due to self‐assembly of linear defect domains with period smaller than 4 μm, whereas hexagonal lattices of focal conic domains appear for thicker samples. The pattern type (1d/2d) and period can be controlled at the nematic–smectic phase transition by applying an electric field, which confines the defect domains to a thin surface layer with thickness comparable to the nematic coherence length. The pattern morphology persists in the smectic phase even after varying the field or switching it off. Bistable, non‐equilibrium patterns are stabilized by topological constraints of the smectic phase that hinder the rearrangement of defects in response to field variations.     

Tunable Photonic Materials via Monitoring Step‐Growth Polymerization Kinetics by Structural Colors

The functional and responsive properties of elastomeric materials highly depend on crosslink density and molecular weight between crosslinks. However, tedious analytical steps are needed to obtain polymer network structure–property relationships. In this article, an in situ structure–property characterization method is reported by monitoring the structural color change in a photonic elastomeric material. The photonic materials are prepared in a two‐step polymerization process. First, linear chain extension occurs via Michael addition. Second, photopolymerization ensures crosslinking, resulting in the formation of an elastomeric photonic network. During the first step, the step‐growth polymer process can be monitored by following the photonic reflection band redshift, allowing to program the molecular weight between the crosslinks. During network formation, the crosslink density, chain length between crosslinks, and the colors are “frozen in.” These processes can be locally controlled creating both single‐layered multicolor patterned and broadband reflective coatings at room temperature. The scalability of the coating process is further demonstrated by using a gravure printing technique. Additionally, the final coatings are made responsive toward specific solvents and temperature. Here the modulus, response, and color of the coating are controlled by tuning the crosslink density and molecular weight between crosslinks of the elastomeric material.     

Double Stimuli‐Responsive Isoporous Membranes via Post‐Modification of pH‐Sensitive Self‐Assembled Diblock Copolymer Membranes

Double stimuli‐responsive membranes are prepared by modification of pH‐sensitive integral asymmetric polystyrene‐b‐poly(4‐vinylpyridine) (PS‐b‐P4VP) diblock copolymer membranes with temperature‐responsive poly(N‐isopropylacrylamide) (pNIPAM) by a surface linking reaction. PS‐b‐P4VP membranes are first functionalized with a mild mussel‐inspired polydopamine coating and then reacted via Michael addition with an amine‐terminated pNIPAM‐NH₂ under slightly basic conditions. The membranes are thoroughly characterized by nuclear magnetic resonance (¹H‐NMR), Fourier transform infrared spectroscopy and X‐ray‐induced photoelectron spectroscopy. Additionally dynamic contact angle measurements are performed comparing the sinking rate of water droplets at different temperatures. The pH‐ and thermo‐double sensitivities of the modified membranes are proven by determining the water flux under different temperature and pH conditions.     

Temperature‐ and Light‐Regulated Gas Transport in a Liquid Crystal Polymer Network

Azobenzene‐containing liquid crystal polymer networks (LCNs) are developed for temperature‐ and light‐regulated gas permeation. The order in a chiral‐nematic LCN (LCN*) is found to be essential to couple the unique structure of the membrane and its gas permeation responses to external stimuli such as temperature and varying irradiation conditions. An LCN membrane polymerized in the isotropic phase exhibits enhanced N₂ permeation with increasing temperature, like most traditional polymers, but barely responds to exposure with 455 and 365 nm light. In sharp contrast, a reversible decrease of N₂ transport is observed for the LCN* membrane of exactly the same chemical composition, but molecularly ordered, when submitted to an elevated temperature. More importantly, alternating in situ illumination with 455 and 365 nm light modulates reversibly N₂ permeation performance of the LCN* membrane, through the trans–cis isomerization of azo moieties. The authors postulate that, besides the anisotropic deformation of LCN*, the decreased order in LCN* membrane caused by external stimuli (i.e., increasing temperature or UV light illumination) is responsible for an inhibition of gas permeation. These results show potential applications of liquid crystal polymers in the gas transport and separation, and also contribute to the development of “smart” membranes.     

Stimuli‐Responsive Bioinspired Materials for Controllable Liquid Manipulation: Principles,Fabrication, and Applications

Many emerging interfacial technologies, such as self‐cleaning surfaces, oil/water separation, water collection, and microfluidics, are essentially liquid manipulation processes. In this regard, micro‐nanostructures of the living organisms are highly preferable, by virtue of the evolutionary pressure and the adaptation to the specific environments, to inspire the optimization of man‐made interfaces. With the increasing demands of modern life, research, and industry, intelligent materials with stimuli‐responsive liquid manipulation functions have gained substantial attention from interfacial scientists. This review introduces the recent progress in the development of stimuli‐responsive liquid‐manipulating materials with bioinspired structures and surface chemistry according to two classified manipulation modes: (i) smart manipulation of liquid wetting behaviors, including lyophobic/lyophilic and superlyophobic/superlyophilic, and (ii) smart manipulation of liquid motion behaviors, including coalescence, transportation, rolling/adhesion, and sliding/pinning. At the beginning of the presentation of each classification, the theoretical basis and the sources of inspiration are introduced comprehensively to ensure a better understanding. This review mainly focuses on the mechanisms, fabrication, and applications of the state‐of‐the‐art works related to smart and biomimetic liquid‐manipulating materials. Finally, conclusions and future prospects are provided, and the remaining problems and promising breakthroughs in fabricating large‐scale, cost‐effective, and efficient smart liquid‐manipulating materials are outlined.     

Microactuators: Control of the Properties of Micrometer Sized Actuators from Liquid Crystalline Elastomers Prepared in a Microfluidic Setup (Adv. Funct. Mater. 24/2010)

In this article new results on the preparation of monodisperse particles from a liquid crystalline elastomer in a microfluidic setup are presnted. For this, droplets from a liquid crystalline monomer are prepared in a microfluidic device and polymerized while they are flowing inside a microtube. The parti­cles obtained by this method possess an internal orientation, which gives them actuating properties. When they are heated into the isotropic phase of the liquid crystalline material they show a reversible change in shape whereby they change their length in one direction by almost 100%. It is shown how the variation of experimental parameters during their synthesis impacts the properties of these micro‐actuators. Influence over their primal shape, the strength of their shape changing properties, their size, and their mechanical properties is demontrated. From the systematic variation of experimental parameters a deep understanding of the complex processes taking place in a flowing droplet of a liquid crystalline material is obtainted. Additionally NMR analysis and swelling experiments on these actuating materials are provided.     

Solid‐State NMR Investigations of the Unusual Effects Resulting from the Nanoconfinement of Water within Amphiphilic Crosslinked Polymer Networks

Two types of solid‐state ¹⁹F NMR spectroscopy experiments are used to characterize phase‐separated hyperbranched fluoropolymer–poly(ethylene glycol) (HBFP–PEG) crosslinked networks. Mobile (soft) domains are detected in the HBFP phase by a rotor‐synchronized Hahn echo under magic‐angle spinning conditions, and rigid (hard) domains by a solid echo with no magic‐angle spinning. The mobility of chains is detected in the PEG phase by ¹H → ¹³C cross‐polarization transfers with ¹H spin‐lock filters with and without magic‐angle spinning. The interface between HBFP and PEG phases is detected by a third experiment, which utilized a ¹⁹F → ¹H–(spin diffusion)–¹H → ¹³C double transfer with ¹³C solid‐echo detection. The results of these experiments show that composition‐dependent PEG inclusions in the HBFP glass rigidify on hydration, consistent with an increase in macroscopic tensile strength.     

Reversibly and Irreversibly Humidity‐Responsive Motion of Liquid Crystalline Network Gated by SO2 Gas

Manipulating functional liquid crystalline networks (LCNs) by multiple stimuli has been a hot topic in the research of smart actuators and fabrication of biomimetic robots. Here, a single‐layer LCN film showing dual responsiveness to humidity and SO₂ gas is prepared. Interestingly, the humidity sensitivity of the LCN film can be gated by exposing the film to SO₂. At the same time, the SO₂ sensitivity is strongly affected by the ambient relative humidity (RH) and the response time is able to be tuned by changing the RH over a wide range. The mechanism of this dual response of the LCN film is ascribed to the neutralization of carboxylic acid and in situ acidification of the carboxylic salt. The reversibly and irreversibly humidity‐induced motion gated by SO₂ renders the LCN film valuable in the design and preparation of multifunctional devices.     

Biosensor Array of Interpenetrating Polymer Network with Photonic Film Templated from Reactive Cholesteric Liquid Crystal and Enzyme‐Immobilized Hydrogel Polymer

A biosensor array is fabricated using an interpenetrating polymer network consisting of photonic film templated from reactive cholesteric liquid crystal (CLC) and enzyme‐immobilized polyacrylic acid (PAA). The solid‐state photonic film on the glass substrate is successfully templated by ultraviolet (UV) curing of the reactive CLC mixture of a reactive mesogen mixture of RMM 727 (from Merck) and a nonreactive chiral dopant of (S)‐4‐cyano‐4′‐(2‐methylbutyl)biphenyl following the extraction of the chiral dopant. The acrylic acid monomer mixed with a cross‐linker of tri(propylene glycol) diacrylate is infiltrated into the extracted space of the photonic film, and UV‐cured with a photomask to obtain a patterned array‐dot film. The interpenetrated cholesteric liquid crystal/hydrogel polymer network (CLC‐hydrogel‐IPN) array is further functionalized in the individual dots with urease, for a model study of biosensor array applications. The dots of the CLC‐hydrogel‐IPN array respond independently to the urea by a color change with high sensitivity and stability. Thus, the patterned CLC‐hydrogel‐IPN can be used as a new biosensor array for cost‐effective and easy visual detection without any sophisticated instruments.     

Exploiting Microphase‐Separated Morphologies of Side‐Chain Liquid Crystalline Polymer Networks for Triple Shape Memory Properties

We report a new strategy to achieve triple shape memory properties by using side‐chain liquid crystalline (SCLC) type random terpolymer networks (XL‐ TP‐n), where n is the length of flexible methylene spacer (n = 5, 10, and 15) to link backbone and mesogen. A lower glass transition temperature (T_g = T_low) and a higher liquid crystalline clearing temperature (T_cl = T_high) of XL‐TP‐n serve as molecular switches to trigger a shape memory effect (SME). Two different triple shape creation procedures (TSCPs), thermomechanical treatments to obtain temporary shapes prior to the proceeding recovery step, are used to investigate the triple shape memory behavior of XL‐TP‐n. The discrete T_g and T_cl as well as unique microphase‐separated morphologies (backbone‐rich and mesogen‐rich domains) within smectic layers of XL‐TP‐n enables triple shape memory properties. Motional decoupling between backbone‐rich and mesogen‐rich domains is also critical to determine the resulting macroscopic shape memory properties. Our strategy for obtaining triple shape memory properties will pave the way for exploiting a broad range of SCLC polymers to develop a new class of actively moving polymers.     

From Membrane to Skin: Aqueous Permeation Control Through Light‐Responsive Amphiphilic Polymer Co‐Networks

The functionalization of amphiphilic polymer co‐networks with light‐responsive spiropyran and spirooxazine derivatives leads to a new type of light‐responsive materials. The material consisting of hydrophilic nanochannels shows desirable properties such as light‐responsive permeability changes of aqueous caffeine solutions, an exceptional repeatability of the photochromism, and tunable basic permeability rates. The versatility of the system is demonstrated by using different functionalization routes such as copolymerization of light‐responsive monomers or crosslinker as well as postmodification of the preformed amphiphilic network. Moreover, light‐responsive spirobenzopyran and novel spirooxazine derivatives are synthesized, which changes the properties of the light‐responsive membranes after inclusion into the amphiphilic co‐networks. Finally, the permeability of the delivery membrane can be tailored to match the properties of porcine skin, an in vitro model of human neonatal skin. One possible application might be the use of the light‐responsive membranes as key‐unit of a transdermal caffeine‐delivery system for preterm neonates.     

In‐Film Bioprocessing and Immunoanalysis with Electroaddressable Stimuli‐Responsive Polysaccharides

Advances in thin‐film fabrication are integral to enhancing the power of microelectronics while fabrication methods that allow the integration of biological molecules are enabling advances in bioelectronics. A thin‐film‐fabrication method that further extends the integration of biology with microelectronics by allowing living biological systems to be assembled, cultured, and analyzed on‐chip with the aid of localized electrical signals is described. Specifically, the blending of two stimuli‐responsive film‐forming polysaccharides for electroaddressing is reported. The first, alginate, can electrodeposit by undergoing a localized sol–gel transition in response to electrode‐imposed anodic signals. The second, agarose, can be co‐deposited with alginate and forms a gel upon a temperature reduction. Electrodeposition of this dual polysaccharide network is observed to be a simple, rapid, and spatially selective means for assembly. The bioprocessing capabilities are examined by co‐depositing a yeast clone engineered to display a variable lymphocyte receptor protein on the cell surface. Results demonstrate the in‐film expansion and induction of this cell population. Analysis of the cells' surface proteins is achieved by the electrophoretic delivery of immunoreagents into the film. These results demonstrate a simple and benign means to electroaddress hydrogel films for in‐film bioprocessing and immunoanalysis.