Liquid Crystal Networks Research Articles

Morphology and thermal properties of liquid crystalp-PAEB/n-propyl methacrylate copolymers

The copolymers of p-phenylene di{4-[2-(allyloxy) ethoxy]benzoate} (p-PAEB) with n-propyl methacrylate (PMA) were synthesized. The liquid crystalline behavior and thermal properties of copolymers were studied by polarizing optical microscopy (POM), differential scanning calorimetry (DSC), X-ray diffractometer (XRD), and torsional braid analysis (TBA). The results of XRD, POM, and DSC demonstrate that the phase texture of copolymers is affected by the composition of liquid crystal units in copolymers. The POM and XRD reveal that liquid crystal monomer (p-PAEB) and copolymers of p-PAEB with PMA are all smectic phase texture. The dynamic mechanical properties of copolymers are investigated with TBA. The results indicate that the phase transition temperatures and dynamic mechanical loss peak temperature Tp of copolymers are affected by the composition of copolymers and liquid crystal cross networks. The maximal mechanical loss Tp is 114°C and is decreased with added PMA. The behaviors of phase transition are affected by the crosslinking density, and it is revisable for lightly crosslinking LC polymer networks, but it is nonreversible for the densely crosslinking of LC polymer networks. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Distributed Bilayer Photovoltaics Based on Nematic Liquid Crystal Polymer Networks

We discuss a liquid crystal composite approach to provide a distributed interface to vertically separate electron-donating and electron-accepting films in an organic photovoltaic device. Two different methods are used to prepare a nematic liquid crystal polymer network with a porous surface and electron-donating properties. This is infilled with an electron-accepting organic semiconductor to form a bilayer device. The interface is diffuse rather than localized so that more photogenerated excitons can reach it to generate charge before they recombine. Photoinduced absorption of a blend of the donor and acceptor materials confirms that excitons dissociate at the heterointerface. The spatial features of the diffuse interface are examined by Fourier analysis of topographic images. We find a correlation between the in-plane spatial frequencies of the interface and photovoltaic device performance. The device performance is investigated as a function of input irradiance. Any charge combination is monomolecular rather than bimolecular, and the monochromatic power conversion efficiency varies between 0.8% and 0.3% with input irradiance. Equivalent circuit analysis shows that this is limited by a high series resistance, a blocking contact, and nonoptimized spatial features of the porous interface.

Glassy photomechanical liquid-crystal network actuators for microscale devices

In light-driven liquid-crystal network (LCN) actuators, large performance improvements are obtained by varying the orientation of the molecular director through the thickness of the film actuator. Experiments show that sub-millimeter bending radii are achieved using a splayed molecular orientation. Systems with a splayed or twisted nematic (TN) director profile drive greater amplitude and faster bending than uniaxial planar systems with the same chemical composition. The bending radii of these systems are predicted using a simple model including effects of light intensity, material composition and actuator thickness.

Influence of the Molecular Weight of a Polymer on the Gliding of Nematic Liquid Crystals

Gliding of the nematic liquid crystal director on polymethyl-methacrilate layers in a magnetic field is observed. It is found that the kinetics of the gliding process are strongly influenced by the molecular weight of the polymer chains. This fact supports the model of gliding based on the mutual orienting effect of the liquid crystal and polymer network.

A Glassy Bending‐Mode Polymeric Actuator Which Deforms in Response to Solvent Polarity

AbstractSummary: We investigate a series of glassy polymer actuators which are found to bend rapidly and reversibly in response to changes in the solvent environment. The actuators are based on hydrogen‐bonded liquid crystal networks, and bending motion is created using director profiles engineered to take advantage of the network swelling anisotropy. Strongly polar solvents easily swell the network, forcing bending in one direction, while the less polar solvents extract water to force bending in the opposite direction.Shape variation of twisted configuration liquid crystal polymer films in acetone and water.magnified imageShape variation of twisted configuration liquid crystal polymer films in acetone and water.

Thermo-Mechanical Responses of Liquid-Crystal Networks with a Splayed Molecular Organization

Films of liquid-crystal networks with a splayed molecular alignment over their cross-section display a well-controlled deformation as a function of temperature. The deformation can be explained in terms of differences in thermal expansion depending on the average molecular orientation of the mesogenic centers of the monomeric units. The thermal expansion of the anisotropic polymers has been characterized as a function of their molecular structure and the polymerization conditions. As a reference, films with an in-plane 90° twist have also been studied and compared with the splayed, out-of-plane molecular rotation. The twisted films show a complex macroscopic deformation owing to the formation of saddle-like geometries, whereas the deformation of the splayed structured is smooth and well controlled. The deformation behavior is anticipated to be of relevance for polymer-based microelectromechanical system (MEMS) technology.

Role of network nematicity in swelling and phase equilibria of polymer networks in nematic solvents

Swelling and phase equilibria of polymer networks in a low molecular mass liquid crystal (LC) have been investigated as a function of network nematicity. LC networks with varying nematicity were prepared by copolymerization of mixtures of mesogenic monomers and non-mesomorphic (styrene) monomers with various compositions. Molar fraction of mesogenic monomer ( x) in copolymer network strongly influences the swelling behavior as well as the nematic–isotropic (N–I) transitions in both dry and swollen states. The swollen networks of sufficiently high x with strong nematicity exhibit a sharp N–I transition and simultaneously undergo a discontinuous change in gel volume, i.e. volume phase transition. Meanwhile, the swollen networks of x≤0.8 with less nematicity show a broad N–I transition, and the resulting volume change proceeds continuously over a finite temperature range. When x decreases further down to less than 0.5, the nematicity of the dry networks vanishes. The nematic ordering in the swollen copolymer networks of x<0.5 occurs at the same temperature as the N–I transition temperature of the pure nematic solvent ( T NI S), which yields the inflection in the swelling-temperature curve. A mean field theory considering network nematicity as a variable describes the effects of x on volume phase transition such as a shift of T NI G and a change in the magnitude of volume transition, apart from the emergence of continuous volume transition due to the broad N–I transition. The purely isotropic network of x=0 appreciably swells in the nematic solvent. The solvent inside the gel forms the nematic phase at the temperatures below T NI S *(=T NI S −2 ° C), which conflicts with the classical theoretical prediction that nematic ordering of LC solvent in fully swollen isotropic network never occurs.

Photomechanics: directed bending of a polymer film by light.

Polymer solutions and solids that contain light-sensitive molecules can undergo photo-contraction, whereby light energy is converted into mechanical energy. Here we show that a single film of a liquid-crystal network containing an azobenzene chromophore can be repeatedly and precisely bent along any chosen direction by using linearly polarized light. This striking photomechanical effect results from a photoselective volume contraction and may be useful in the development of high-speed actuators for microscale or nanoscale applications, for example in microrobots in medicine or optical microtweezers.

Liquid Crystal H-Bonded Polymer Networks under Mechanical Stress

The orientation behavior of liquid crystal networks having the Sm CA structure stabilized by H-bonds has been studied. The mesogenic groups formed by hydrogen bonds generate a layered structure of the polymer. The hydrogen bonds are responsible for the elastic properties of the liquid crystal polymer network and the mechanism of its plastic flow above the threshold strain. The mechanical response is interpreted in terms of the mechanism of breaking and reconnecting H-bonds under stress. The local sliding of layers is possible if H-bonds are temporarily disconnected and then reconnected again to restore the LC structure. The anisotropy of the mechanical properties of the H-bonded polymer materials is observed in the monodomain LC networks.

2C09 コレステリック液晶ネットワークの光スイッチング : らせん構造の光制御に及ぼすキラルアゾベンゼンの影響(フォトニクス・光デバイス)

2C09 コレステリック液晶ネットワークの光スイッチング : らせん構造の光制御に及ぼすキラルアゾベンゼンの影響(フォトニクス・光デバイス)

Liquid crystal polysiloxane networks as materials for molecular imprinting technology: memory of the mesomorphic organization

A novel approach to the synthesis of molecularly imprinted polymers via non-covalent linkages has been studied. It relies on the use of thermotropic side group liquid crystal polymer networks. The polysiloxane networks obtained after extraction of the template preserved the mesomorphic organization set up in the presence of the guest molecule. A first batch rebinding analysis was performed: this study revealed that the imprinted polymer has a much greater affinity for the template molecule than has the non-imprinted polymer, and a significant selectivity.

Probing matrix isolation effects in lyotropic liquid crystal nanocomposites using water-soluble PPv

The incorporation of water-soluble poly(sodium phenylenevinylenedicarboxylate) in the ordered nanochannels of a hexagonal lyotropic liquid crystal network is described. The absorption and emission behavior of the pristine, pre-formed PPV derivative in solution, in the bulk solid state, and in the nanocomposite are compared. The effect of matrix isolation in the nanostructured assembly on the conjugation length, conformation, stability, and alignment of the encapsulated PPV chains will be discussed.

Pretransitional phenomena in acrylate-based liquid crystal networks

The static Kerr effect is used to study the pretransitional phenomena in polymer networks even in the presence of inherent optical anisotropy of crosslinked materials. The pretransitional phenomena in a liquid crystal polymer and its networks are sensitive to the structure of the crosslinks and their distribution within the network. The bulky network junctions distributed more or less homogeneously within the network serve as defects which decrease the hypothetical second order phase transition temperature T* and suppress the nematic fluctuations in the polymer network above the clearing point.

Catalytic Pd Nanoparticles Synthesized Using a Lyotropic Liquid Crystal Polymer Template

Ion exchange of Pd(II) ions into a nanostructured lyotropic liquid crystal network, followed by reduction with hydrogen gas, generates a polymer composite containing finely dispersed, catalytically active Pd nanoparticles 4−7 nm in diameter. Loss of order upon nanoparticle growth appears to be due to residual mobility in the polymer matrix near the ionic headgroups. The Pd nanoparticle composite exhibits excellent product selectivity in benzaldehyde hydrogenation and high reactivity in catalyzing Heck olefinations.

Preparation of liquid-crystal thermosets:In situ photopolymerization of oriented liquid-crystal diacrylates

The photoinitiated polymerization of 2-chloro-1,4-phenylene bis(4-(6-(acry- loyloxy)hexyloxy)benzoate) (1M) was studied. The monomer 1M exhibited a broad nematic phase between 24.9 and 113.7 °C on a DSC cooling scan. It was oriented in its nematic phase at a substrate coated with polyimide and unidirectionally rubbed with a nylon cloth. During polymerization, the ordering of the liquid- crystal molecules was fixed, yielding a uniaxially crosslinked network. The clear liquid- crystal networks (LCNs) exhibited a birefringence between 0.14 and 0.19, depending on the polymeriza- tion temperature. Finally, a nonmesogenic diluent, tetra(ethylene glycol)diacrylate, was mixed with 1M, subsequently decreasing the birefringence of the obtained LCNs. The LCNs containing nonmesogenic diluent exhibited not only a smaller birefringence but also a weaker birefringence dispersion in the visible region. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 3929 -3935, 1999

Polarized Photoluminescence and Measurement of Degree of Order in Mixed Oriented Polymer Liquid Crystal Films

Optical anisotropy in a new form of cross-linked liquid crystal network incorporating a fluorescent chromophore has been demonstrated. Linearly polarized absorption and fluorescence is observed in oriented thin films. From measurement of absorption and fluorescence anisotropy, the order parameter was found to be 0.65 and 0.80 respectively. Temperature dependent birefringence measurements reveal that there is no appreciable loss of alignment up to 200°C. Raman spectroscopy was used to investigate the discrepancy between the absorption and fluorescence measurements. It is found that the fluorescent chromophore is less well ordered than the liquid crystal. The reasons for this are discussed.

Liquid crystal acrylate-based networks: polymer backbone–LC order interaction

New synthetic pathways to poly- and monodomain acrylate-based networks via gamma-irradiation and chemical crosslinking are described. Strain-induced orientation transitions and phase behavior are studied by X-ray scattering, differential scanning calorimetry and optical microscopy. Analysis of the observed phenomena is given in terms of crosslinking conditions, network topology, structure and spatial distribution of crosslinks. The pretransitional behavior and the observed shifts in clearing point Tn–i are in good agreement with theoretical predictions. The reorientation process in monodomain networks is regulated via strain gradients generated by external mechanical field. The variation in geometrical shape of the samples gives an opportunity to control and realize two different reorientation mechanisms: uniform director rotation and reorientation via stripe domains. Gamma-irradiation is found to be a powerful tool for fixation of any current distribution of poly- and monodomain regions within the LC film. ‘Macroscopic memory’ of the information recorded on the films is observed and discussed.

Electro-Optical Effect Without Hysteresis for (Polymer/Liquid Crystal) Composite Films

Phase-separation behavior of the composite films composed of polymer of a three dimensional network and low molecular weight liquid crystals (LCs) embedded in the network was studied on the basis of the evolution of optical heterogeneity in a casting solution during a film formation process and the morphology of the polymer networks. The electro-optical properties of the composite films based on electric field controlled light scattering were strongly dependent on the size of LC domains which was controlled by the solvent evaporation rate. When poly(diisopropyl fumarate), which is incompatible with the LC, was used as the matrix polymer, no electro-optical hysteresis was detected, while considerable electro-optical hysteresis was done for the composite film consisting of poly(methyl methacrylate), which is fairly compatible with the LC. However, when poly(methyl methacrylate) with high molecular weight (996k) was used as the matrix polymer, hysteresis decreased due to a decrease of compatibility.

Phase-Separation Behavior and Electro-Optical Properties for (Polymer/Liquid Crystal) Composite Films

AbstractDominant factors that control the aggregation structure and electro-optical properties were presented for (polymer/liquid crystal) composite films prepared by a solvent cast method from homogeneous solutions. Phase-separation behavior of the composite films consisting of a three dimensional polymer network and low molecular weight liquid crystals (LCs) embedded in the network was studied by evaluating the development of optical heterogeneity in the casting solution during film formation and by observing the morphology of the polymer networks. It was confirmed from light scattering studies of the casting solution during solvent evaporation that the phase-separated structure of polymer and LC was formed through spinodal decomposition. The resultant size of the LC domain in the composite film was controlled by regulating the solvent evaporation rate. The morphology of the polymer network in the composite film was also strongly influenced by the solubility between the polymer and the casting solvent. The electro-optical properties based on field controlled light scattering of the composite film were strongly dependent on the size of the LC domain. Though the rise response time decreased, the decay response time increased with an increase in the size of LC domains in the composite film. When poly(diisopropyl fumarate), which is not compatible with the LC, was used as the matrix polymer, no electro-optical hysteresis was observed, while considerable electro-optical hysteresis was observed for the composite film consisting of poly(methyl methacrylate), which shows a certain compatibility with the LC.

Molecular architectures in thin plastic films by in‐situ photopolymerization of reactive liquid crystals

Abstract— The process of in‐situ photopolymerization of reactive mesogens leads to densely crosslinked liquid crystal (LC) networks. The molecular organization within the thin films can be accurately controlled by combining the various alignment techniques known for low molar mass LCs with the structuring techniques known for photopolymerization. The films thus produced exhibit optical properties useful for polarization control and/or colour control in the various types of LCDs.