Low Molecular Mass Liquid Crystals Research Articles

Gold Nanoparticles Incorporated Nematic Gel Micropillars Capable of Laser Actuation at Room Temperature

Liquid crystal elastomers (LCEs) capable of rapid reversible shape change and ease of actuation at room temperature are highly desired. In this study, a photothermally actuated system based on LCE gel micropillars incorporated with gold nanoparticles is reported for the first time. The photothermal conversion effects of gold nanoparticles (AuNPs) have been studied with a focus on nanoparticle shape (rods and spheres) and loading content. Upon swelling in low molecular mass liquid crystal solvent, the formation of LCE and LCE/AuNP gel enables the reduction of nematic-to-isotropic phase transition temperature (TNI), which allows the gelled micropillar capable of photothermal actuation at room temperature. It has been demonstrated that the LCE gel micropillar with incorporation of 1.0 wt % of gold nanorods can achieve a rapid 30% change in volume within 3 s when under proper laser irradiation at room temperature. The observed discontinuous volumetric transition of the gel micropillars is in good agreement wi...

Experimental Study on the Swelling Behavior of Polymer Networks in a Nematic Solvent

SummarySwelling properties and phase diagrams of binary systems composed of polyacrylate networks and a low molecular mass liquid crystal (LMWLC) have been investigated. The networks were prepared by ultraviolet (UV) irradiation of reactive mixtures including a monomer, a crosslinking agent and a photo‐initiator. Poly(n‐butylacrylate) networks were prepared using two crosslinking agents: 1,6‐hexanedioldiacrylate (HDDA) and a mesogenic acrylic acid 6‐(4'‐(6‐acryloyloxy‐hexyloxy)biphenyl‐4‐yl oxy) hexyl ester (AHBH). The obtained dry polyacrylates were characterized by differential scanning calorimetry, and immersed in an excess of a LMWLC solvent 4‐cyano‐4'‐pentylbiphenyl (5CB), forming polymer gels. A detailed study by polarized optical microscopy allowed to determine the swelling degree of the gels and to follow the phase behavior of the solvent inside the polymer matrix in a wide range of temperature. Swelling data were used to establish the phase diagrams in terms of composition and temperature. It was found that the poly(n‐butylacrylate) network obtained with the nematic crosslinking agent show fluctuations in the region of the nematic‐isotropic transition temperature of 5CB, probably due to the presence of nematic‐nematic coupling effects between the nematic solvent and the nematic moieties of AHBH.

Phase equilibria and photopolymerisation-induced phase transitions of mesogenic diacrylate monomer and low molecular mass liquid crystal mixture

Experimental phase diagrams of binary mesogenic mixtures of reactive mesogenic diacrylate (RM257) monomer and low molar mass liquid crystals (E7) were determined by means of differential scanning calorimetry and optical microscopy. The combined free energy densities of Flory–Huggins for liquid–liquid demixing, Maier–Saupe for nematic ordering, and phase field free energy for crystal solidification was proposed to describe the phase diagrams of the starting E7/RM257 mixtures. The phase diagram thus constructed is an ideal mixing type, exhibiting a narrow loop of isotropic + nematic (I + N) coexistence region followed by the crystal + nematic (Cr1 + N) region in descending order of temperature. Of particular interest is the permanent fixation of the mesophase structures upon photopolymerisation of neat RM257 in the corresponding nematic and crystalline phases. Upon photopolymerisation of a low RM257 content mixture in both isotropic and nematic states, the nematic–isotropic transition of E7 was found to persist. The permanent structural anchoring is seen upon photo-curing of the 90/10 RM257/E7 mixture in the crystalline state.

Stimulus–Responsive Nematic Gels

AbstractNematic elastomers exhibit a rich variety of stimulus–response behaviors owing to their hybrid characters of liquid crystals and elastomers. The nematic elastomers swollen by low molecular mass liquid crystals show anisotropic swelling, and their volume and shape significantly depend on temperature. The swollen nematic elastomers exhibit macroscopic deformation together with a large change in optical birefringence in fast response to electric fields (electro–opto–mechanical effect). The mechanical stretching of the nematic elastomers normal to the initial director drives the director realignment along the elongation axis.

Self-adjusting smart windows based on polymer-dispersed liquid crystals

The control of sunlight can be achieved either by electrochromic or polymer-dispersed liquid crystal (PDLC) smart windows. We have recently shown that it is possible to homeotropically align fluid mixtures of low molecular mass liquid crystal with a negative dielectric anisotropy, and a liquid crystalline monomer, in order to obtain electrically switchable chromogenic devices. They are new materials useful for external glazing. In fact, they are not affected by the classical drawbacks of PDLCs. In this paper we present a new self-switchable glazing technology based on the light-controlled transmittance in a PDLC device. The self-adjusting chromogenic material, which we obtain, is able to self-increase its scattering as a function of the impinging light intensity. The relationship between the electro-optical response and the physical–chemical properties of the components has been also investigated.

Electrically driven director-rotation of swollen nematic elastomers as revealed by polarized Fourier transform infrared spectroscopy

We have investigated the director reorientation behavior of unconstrained nematic gels (nematic elastomer swollen by low molecular mass liquid crystals) under electric fields by means of polarized Fourier transform infrared (FTIR) spectroscopy. The polarized FTIR reveals that the director rotates about the (y) axis normal to the original director ( x axis) and field directions ( z axis), and the nematic order remains unchanged in the plane where the director stays confined during rotation. The rotation angle of director (theta) is estimated as a function of imposed voltage amplitude on the basis of the absorbances of the cyano group which is aligned along the long axis of the mesogen for light linearly polarized in the x and y directions. The director-rotation drives a two-dimensional macroscopic deformation which is characterized by a contraction along the x axis, an extension in the z direction, and nonappreciable length change along the y axis. The strain in the x direction is linearly proportional to sin;{2} theta in agreement with the expectation of soft or semisoft elasticity theory for thin nematic elastomer films where the shear contribution becomes negligibly small.

Electrically switchable chromogenic materials for external glazing

The development of chromogenic materials is important for their potential applications as electrically adjustable or thermally self-adjusting light and heat filters in the external glazing of buildings. Recently, we have reported our investigations on electrolyte–liquid crystal dispersions, which show independent electro-optical and electrochromic properties characterized by fast bleaching times. Nevertheless, for the external glazing of buildings, it is recommended to have a transparent off state, which turns opaque upon application of the external field. In this paper, we present our efforts to homeotropically align by rough surfaces fluid mixtures of a low molecular mass liquid crystal with a negative dielectric anisotropy, a liquid crystalline monomer, and electrolyte molecules in order to obtain transparent films after a polymerization process. Upon application of an ac electric field, the device becomes opaque and if a dc field is added, the cell changes colour too. We have also investigated the relationship between electro-optical/chromogenic response and physical chemical properties of the components.

Dynamics of Electro-Opto-Mechanical Effects in Swollen Nematic Elastomers

We characterize experimentally the dynamics of the electro-opto-mechanical effect (macroscopic deformation with electro-optical effect) in monodomain liquid crystal elastomers swollen by low molecular mass liquid crystals. The optical and mechanical rise times (in response to field-on) decrease in almost proportion to the square of field strength, while the corresponding decay times (in response to field-off) are nearly independent of field strength. The optical rise and decay times are about 1 order of magnitude smaller than the mechanical ones. We propose a simple model to capture the main features of both static and dynamic aspects of the electro-opto-mechanical effect and use it to interpret the experimental data.

The role of defects and boundary and volume inhomogeneities in the field-induced director reorientation in nematic liquid crystals confined between parallel plates

A deep understanding of the director dynamics in nematic liquid crystals confined between parallel plates is important for display technology as well as for basic science. Some aspects of this dynamics are still not clear, in spite of intense research and significant progress in the last thirty years. When a strong magnetic (or electric, with some precautions) field is applied to a well aligned nematic sample, depending on the angle between the director and the field one usually observes either a homogeneous or a spatially periodic (inhomogeneous) director reorientation to the new equilibrium position, parallel to the field, as shown by NMR observations well supported by the Leslie–Ericksen theory [A.F. Martins, P. Esnault and F. Volino, Phys. Rev. Lett. 57, 1745 (1986)]. These are collective modes (observed simultaneously over the whole sample) and the spatially periodic one originates in the long wavelength thermal fluctuations of the director orientation in equilibrium. Recent experiments [G.R. Luckhurst, A. Sugimura, B.A. Timimi and H. Zimmermann, Liquid Crystals 32, 1389 (2005)] have shown that in some practical cases the director reorientation in thin nematic samples confined between two parallel plates does not follows any of those two pictures. We focus on such experiments and present 2D computer simulations of the director dynamics in the presence of topological defects or boundary or volume inhomogeneities in the sample. Deuterium NMR spectra directly computed from the numerical results show good agreement with the experimental data. We propose that in addition to the above mentioned collective modes, a qualitatively different mode of reorientation, that we may call progressive mode, exists and may dominate the director reorientation in thin samples of low molecular mass liquid crystals.

Composite Liquid Crystalline Mixtures for Guided Wave Electro–Optic Applications

A low refractive index multifunctional composite liquid crystalline mixture is proposed to address the issues of in-guide tuning of optical properties. As the standard optical telecommunication line is made of fused silica, signal phase and amplitude modulation using liquid crystal requires that its refractive indices match the refractive index of silica, which is low compared to the majority of commercially available mixtures. In the present work, low molecular mass liquid crystals are mixed with fluorinated additives to adjust refractive index below a critical limit (n < 1.4584 at D-line) without impacting significantly on the nematic temperature range (TCN = − 24°C and TNI = 68°C). A method of reducing the surface anchoring energy of liquid crystals to glass surface is also proposed as a non contact alignment method around the reduced diameter optical fibers. The bulk elastic energy around the fiber acts as an alignment mechanism when LC-fused silica molecular interactions are minimized. The combination of these two properties makes possible the in-guide modulation of the light phase and amplitude with the liquid crystal used as an outer-cladding electro-controllable medium, significantly improving insertion losses.

Effect of polymer structures on electro-optical properties of polymer stabilized liquid crystal films

The polymer stabilized liquid crystal (PSLC) film is a relatively novel electro-optical material, which is generally obtained by dissolving a small amount of a bifunctional photoreactive monomer in a low molecular mass liquid crystal. In this paper, the PSLC films were prepared with photoreactive biphenyl methacrylate monomers by photopolymerization induced phase separation. The effects of liquid crystal concentration, curing time, monomer structures and alignment layer on the electro-optical properties of PSLC films were investigated. The results show that the transmittance in the OFF state (T OFF) increased with the liquid crystal concentration, but the driving voltage decreased. T OFF was also influenced by the curing time. Furthermore, when polyimide was used as alignment layer, the films prepared from the bifunctional monomer shows a higher T OFF, while those from the single functional monomer exhibited a deformed electro-optical curve due to the unsteady polymer networks.

Phase Equilibria of a Mixture of Side-Chain Liquid Crystalline Polymer and Low Molecular Mass Liquid Crystal

A side-chain liquid crystalline polymer based on p-methoxyphenyl-p‘-acryloyloxy benzoate was synthesized for utilization in a polymer dispersed liquid crystal. Phase diagram of the blends of side-chain liquid crystalline polymer and low molecular mass liquid crystal (K21) was established by means of light scattering and differential scanning calorimetry. A theoretical phase diagram was established by self-consistently solving the combined free energy densities of Flory−Huggins, Maier−Saupe, and phase field theory of solidification. The theoretical phase diagram was established to compare with the observed phase diagram of the binary mesogenic mixtures, showing the isotropic, isotropic + nematic (L1 + N2), nematic + nematic (N1 + N2), and crystal + nematic (Cr1 + N2) coexistence regions. The morphologies of these coexistence regions were further confirmed by polarized optical microscopy.

Orientational properties of 1,3,4-oxadiazoles in liquid-crystalline materials determined by electronic absorption and fluorescence measurements

The electronic absorption and fluorescence spectra of two 4-donor/4′-acceptor substituted 2,5-diphenyl-1,3,4-oxadiazole derivatives in isotropic solvents (methanol, toluene), in a commercial mixture of low molecular mass liquid crystals – ZLI 1695 (MERCK, Darmstadt), and in a liquid-crystalline polysiloxane with 4-methoxy-4′-pentoxy-phenylbenzoate side-groups are presented and discussed. The orientational order parameters of the 1,3,4-oxadiazoles, dissolved in the liquid-crystalline films with uniform planar orientation of the mesogenic molecules and in aligned films of the polysiloxane, are determined. The possibility of application in OLEDs of the 1,3,4-oxadiazoles dissolved in homogenously oriented liquid-crystalline polymers is suggested.

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.

Morphology control of liquid crystalline composite gels based on molecular self-assembling kinetics

Liquid crystalline composite gels consisting of a low molecular mass gelator and a low molecular mass liquid crystal were prepared by two types of gelation method (continuous cooling and isothermal gelating), which provide different molecular self-assembling kinetics of the low molecular mass gelator as gelation proceeds. Optical microscopy and atomic force microscopy revealed that numerous fine strands of the one-dimensionally assembled low molecular mass gelators were formed in the composite gels for both the continuous cooling method and the isothermal gelating method. However, the thinner strands were more homogeneously dispersed in the isothermal gelation product at an appropriate temperature, than in the continuous cooling process. This difference in dispersion state of the strands was shown (by polarizing optical microscopy) to have a significant influence on the molecular alignment of the low molecular mass liquid crystal in the liquid crystalline composite gel. The electro-optical response and light scattering–transmitting switching, of the liquid crystalline composite gel in an applied electric field was extremely dependent on the morphology of the gelators. High contrast light switching was achieved for the composite prepared by isothermal gelation. The response time of electro-optical switching was less than 100 µs under 30 Vrms.

Calorimetric studies of poly (2-phenoxyethylacrylate)/5CB mixtures

The thermophysical properties of mixtures of poly (2-phenoxyethylacrylate) and 4-cyano-4′-pentyl-biphenyl, 5CB, are investigated using polarizing optical microscopy (POM) and differential scanning calorimetry (DSC). The polymer has a molar mass M w = 181 000 g mol -1 the low molecular mass liquid crystal exhibits a nematic to isotropic transition at 35.3°C and crystallizes below 23°C. The phase diagram exhibits miscibility gaps in certain regions of temperature and composition where coexisting nematic and isotropic phases are found. From a practical point of view when considering the electro-optical applications of these systems, it proves to be useful to know precisely the amount of small liquid crystal molecules dissolved in the polymer matrix and the concentration of polymer in the nematic phase. The former quantity has a mechanical impact due to a plasticizing effect, an optical impact since it changes the polymer refractive index, while the polymer in the nematic phase shifts the transition temperatures influencing the electro-optical response of the liquid crystal. The present work addresses these important aspects using POM and DSC.

Photoaddressable polymers for liquid crystal alignment

We demonstrate reversible photoinduced in situ reorientation of low molecular mass liquid crystals (LCs) by means of photoaddressable polymers (PAPs). These polymers contain mesogenic azobenzene side chains optimized to reorient cooperatively and effectively upon illumination with polarized light. Various low molecular mass LCs were introduced between two PAP layers and these sandwich devices were tested with respect to stability and reversibility of photoinduced orientation. Dissolution of the PAP layer by the low molecular mass LC was observed for several material combinations and systematically investigated. Different anisotropic dyes were added as fluorescence markers in order to monitor the photoinduced LC orientation. With an optimized material combination, more than 10 reversible reorientation processes could be realized with polarized light of either 514 or 405 nm wavelength, without any reduction in alignment quality. Further, microscopic polarized fluorescence patterns could be produced and erased within short exposure times.

Environmental Stability and Its Improvement in Polymer/Liquid Crystal Hybrid Monolayer

Host-guest hybrid monolayer films composed of an amphiphilic side chain azobenzene (Az) polymer and a low-molecular-mass liquid crystal (LC) were prepared on a solid substrate and the environmental stability was investigated. Hybrid Langmuir-Blodgett (LB) monolayer showed structural changes under conditions of vacuum or elevated temperature due to evaporation of the low mass LC molecule The activation energy for the evaporation of 4'-pentyl-4-cyanobiphenyl (5CB) from the Az side chain array was estimated to be 104 kJ mol -1 . Efforts were made to block the evaporation of LC by capping a transparent LB film barrier. Over-coated polymer LB films in the glassy state having a thickness exceeding 7 nm were capable of blocking evaporation of the LC molecule. This critical thickness was in consistent with the conditions to prepare a defect free ultrathin polymer resist.

Electrooptical characterizations of polymer stabilized cholesteric liquid crystals

Several monomers were added to a cholesteric liquid crystal to obtain after irradiation a polymer stabilized cholesteric texture (PSCT). For that use, two monomers were synthesized. The influence of the nature of the monomer (nematic or cholesteric) and of its structure is investigated as well as parameters such as the UV irradiation. Some of the characteristics of the PSCT can be correlated to the interactions between the polymer network and the low molecular mass liquid crystal molecules. A broadening of the reflection band can also be obtained with such monomers and is also discussed with the aspect of the interactions polymer network/liquid crystal molecules.

Optical activity of a side group cholesteric liquid crystalline silicone

We measured, using a phase modulation technique, the optical rotation in the isotropic and blue phases of a side group cholesteric liquid crystalline silicone polymer. For comparison, similar measurements were performed on cholesteryl nonanoate (CN) and CE6. The polymer has a selective reflection close to that of CE6 and a chiral mesogenic side group chemically similar to CN. The optical activity data for the polymer, unlike those for low molecular mass liquid crystals, does not follow a simple Landau-de Gennes temperature dependence.