Photoactuation and thermal isomerisation mechanism of cyanoazobenzene-based liquid crystal elastomers.

4-Alkoxy-4'-cyanoazobenzenes are organic chromophores with great applicability within present nanotechnology. However, the use of such azo dyes for obtaining light-triggered artificial muscle-like actuators remains still unexplored. Achieving further knowledge about the thermal back reaction and isomerisation mechanism for these types of azoderivatives is essential to get photo-actuators with the desired abilities.

Orientability of the minor director of homeotropically aligned smectic-a elastomers in external mechanical fields.

We present studies on bulk smectic-A copolymer networks with end-on attached homeotropically oriented mesogens that show spontaneous optical biaxiality at room temperature. Orthoscopic and conoscopic investigations under uniaxial extension in the layer planes give first evidence of the orientability of the minor director in mechanical fields yielding biaxial monodomains with 3-d orientational long-range order of all three principle axes. This is an important step towards the synthesis of permanently oriented biaxial monodomain elastomers for which highly interesting mechanical and optical properties are expected.

Anisotropic ionic mobility of lithium salts in lamellar liquid crystalline polymer networks.

New mesogens presenting smectic A (SmA) phases and capable of hosting lithium salts are designed. The mesogens comprise a vinyl-functionalized spacer to allow further reaction to the polymer backbone, an aromatic core and ethylene oxide chains, able to coordinate lithium ions. Copolymerizing these monomers with a suitable crosslinker yields the first lithium containing liquid crystalline elastomers (LCEs).

Mechanics of Homeotropically Ordered Smectic-A Elastomers with Global Oblate Chain Conformation.

Smectic-A elastomers combine one-dimensional translational order of rod-like segments with the rubber elasticity of a polymer network. In recent years, detailed investigations were carried out on elastomers showing a global prolate chain conformation. In this communication, the first experiments on fluorinated SA elastomers exhibiting a global oblate chain conformation are presented, where the polymer chains are on average compressed along the layer normal of the lamellar phase structure.

Random disorder and the smectic-nematic transition in liquid-crystalline elastomers.

We report effects of disorder due to random cross-linking on the nematic to smectic-A phase transition in smectic elastomers. Thermoelastic data, stress-strain relations and high-resolution x-ray scattering profiles have been analyzed for two related compounds with a small and a larger nematic range, respectively, each for 5% as well as 10% cross-links. At 5% cross-link density the algebraic decay of the positional correlations of the smectic layers survives in finite-size domains, providing a sharp smectic-nematic transition.

Role of the local order on the thermal cis-to-trans isomerisation kinetics of azo-dyes in nematic liquid crystals.

Azoderivatives show faster thermal cis-to-trans isomerisation kinetics when they are under the influence of the nematic mean field than dissolved in isotropic solution. The local order parameter induced in the azo-dye by the host mesogen determines the rate of its thermal cis-to-trans isomerisation process. In this way, AZO6-LC mixtures with the same order parameter exhibit identical isomerisation rates independently of the mesogen chemical nature.

Increasing the isomerisation kinetics of azo dyes by chemical bonding to liquid-crystalline polymers.

It is well known that the proper substitution of the azobenzene core allows tuning the thermal cis-to-trans isomerisation kinetics of azo dyes. The thermal isomerisation process of nitro-substituted azobenzenes is accelerated up to 13 times with respect to that in the common isotropic solvents when they are doped in nematic low molar mass liquid crystals. This kinetic acceleration is even stronger when these azo dyes are covalently linked to a nematic siloxane polymer.

Opto-mechanical effect in photoactive nematic side-chain liquid-crystalline elastomers.

The mechanical behaviour of monodomain nematic side-chain liquid-crystalline elastomers containing azoderivatives as pendant groups or crosslinkers has been studied under UV irradiation and in the darkness at different temperatures. From the evaluation of the opto-mechanical experiments, the mechanical efficiency, kinetic rates, activation energies and the isomerization mechanism of the azocompounds in the liquid-crystalline matrix could be determined, as well as the effect of the chemical constitution of the azobenzene derivatives and their role in the elastomeric network.

Azophenol-based liquid-crystalline elastomers for light-driven actuators.

Para-substituted azophenols exhibit a fast thermal cis-to-trans isomerization rate in ethanol, which can be transferred to the solid state by obtaining liquid-crystalline elastomeric systems. The absence of protic solvent is compensated by the establishment of hydrogen bonding between azophenol monomers that are close to each other. Opto-mechanical experiments reveal that azophenol-containing liquid single-crystal elastomers are valuable materials for light-controlled actuators exhibiting relaxation times of 1 s at room temperature.

Polydomain-monodomain orientational process in smectic-C main-chain liquid-crystalline elastomers.

The polydomain-monodomain (PM) transformation takes place when a polydomain of a smectic-C main-chain liquid-crystalline elastomer (SmC MCLCE) is uniaxially stretched. We present results based on a combination of mechanical and X-ray experiments which show how the domains initially rearrange to finally form a perfect conical layer distribution (monodomain) when the sample is fully stretched. The rearrangement and orientational process of the domains is quantified and compared to the parallel and perpendicular uniaxial stress-strain deformations of a monodomain sample.

Calorimetric study of the Paranematic-to-Nematic transition of polydomain side-chain liquid-crystalline elastomers with different mesogen composition.

The phase transition behaviour of various nematic side-chain liquid-crystalline elastomers with different mesogen composition has been explored by means of high-resolution ac calorimetry. Polydomain samples of the same crosslinking density and different type of mesogens have been investigated. The results show a strong dependence of the phase transition features upon the composition of the mesogen.

Kinetico-mechanistic study of the thermal cis-to-trans isomerization of 4,4'-dialkoxyazoderivatives in nematic liquid crystals.

4,4'-Dialkoxy-substituted azobenzenes are usually required for technical applications. Here, we study the mechanism through which the azo-dye thermally isomerizes from the unstable cis isomer to the more stable trans isomer when it is incorporated in the nematic liquid-crystalline state. We have determined the kinetic and thermal activation parameters for this process in different nematic environments.

Gel-like elasticity in glass-forming side-chain liquid-crystal polymers.

We study the complex shear modulus G of two side-chain liquid-crystal polymers (SCLCPs), a methoxy-phenylbenzoate substituted polyacrylate (thereafter called PAOCH3 ), and a cyanobiphenyl substituted polyacrylate supplied by Merck (thereafter called LCP105) using a piezoelectric rheometer. Two methods of filling the cell are used: (a) a capillary method, which can be used only at high temperature because of the low value of the viscosity, and (b) the classical one, thereafter called compression method, which consists in placing the sample between the two slides of the cell and to bring them closer. By filling the cell at high temperature either with the compression or the capillary method, we show that the response of both compounds is liquidlike ( G' approximately f2 and G'' approximately f , where f is the frequency) for temperatures higher than a certain temperature T0 and gel-like (G' approximately const, G'' approximately f) below T0.

Phase biaxility in smectic-a side-chain liquid crystalline elastomers.

(2) H NMR investigations on the biaxial phase behavior of smectic-A liquid crystalline side-chain elastomers are presented. Biaxiality parameters were determined by measuring the quadrupolar splitting of two spin probes, namely benzene-d(6) and hexamethylbenzene-d(18) , at various angles between the principal director and the external magnetic field: while for a uniaxial sample the angular dependence can be described by the second Legendre polynomial, an additional asymmetric term needs to be included to fit the data of the two investigated biaxial systems. Two elastomers synthesized from mesogens that differ in the molecular geometry in order to study the molecular origin of biaxiality were compared.

Shear-induced tilt in smectic-A elastomers.

Smectic-A elastomers combine the positional long-range order of mesogenic molecules in one dimension with the rubber elasticity of a polymer network. While the influence of uniaxial mechanical fields on the phase structure has been investigated intensively during recent years, the impact of shear forces on the orientation of mesogens remains unclear. We present x-ray experiments under shear strain, showing an induced macroscopic tilt depending on the applied shear angle and geometry of the setup.

Piezoresistivity and electro-thermomechanical degradation of a conducting layer of nanoparticles integrated at the liquid crystal elastomer surface.

When a liquid crystal elastomer (LCE) is reprocessed with conducting nanosized particles a conducting layer can be formed at the LCE surfaces. Here, two different LCE materials and two different conducting carbon particles were used. These four reprocessed LCEs were investigated when subject to a thermal phase transition and mechanical extension.

Criticality controlled by cross-linking density in liquid single-crystal elastomers.

A high-resolution calorimetry and deuteron-nuclear magnetic resonance study of a paranematic-nematic phase transition was performed on liquid single-crystal elastomers. We show that density variations of both rodlike and pointlike cross-links strongly affect the mean value and the dispersion of local mechanical fields. The system exhibits an inherent weakly disordered orientational state composed of regions with the temperature profile of the nematic order parameter ranging from first order to supercritical.

Order and strain in main-chain smectic liquid-crystalline polymers and elastomers.

The layer correlations in main-chain smectic liquid-crystal polymer and elastomer systems have been studied using high-resolution X-ray scattering. In contrast to side-chain smectic polymers, in main-chain systems the polymer chains are oriented parallel to the layer normal. As a result they couple directly to the lamellar structure and any polymer defect is translated into layer distortions.

Enantioselective absorption of chirally doped liquid crystalline networks studied by the use of an electronic microbalance.

A polydomain cholesteric elastomer was obtained by cross-linking a nematic side-chain polysiloxane in the presence of a chiral dopant. After extraction of the chiral dopant, sorption experiments were performed, by the use of an electronic microbalance, in the presence of each enantiomer of a chiral amine molecule. The sorption kinetics corresponds to a Fickian diffusion behavior.

Phase biaxiality in nematic liquid crystalline side-chain polymers of various chemical constitutions.

In a previous deuterium NMR study conducted on a liquid crystalline (LC) polymer with laterally attached book-shaped molecules as the mesogenic moiety, we have revealed a biaxial nematic phase below the conventional uniaxial nematic phase (Phys. Rev. Lett.

Shear mechanical anisotropy of side chain liquid-crystal elastomers: influence of sample preparation.

We study the mechanical anisotropy of a series of uniaxial side chain nematic elastomers prepared with the same chemical composition but with different preparation protocols. For all the compounds, the experiments performed as a function of temperature show no discontinuity in both G' (//) and G' ( perpendicular) (the labels // and perpendicular stand for the director parallel, respectively perpendicular to the shear displacement) around the nematic-isotropic (N-I) phase transition temperature determined by DSC. They also all show a small decrease in G' (//) starting at temperatures well above this temperature (from approximately 4( degrees ) C to approximately 20( degrees ) C depending on the compound studied) and leading to a small hydrodynamic value of the G' ( perpendicular)/G' (//) ratio.

Main-chain smectic liquid-crystalline polymers as randomly disordered systems.

We report a high-resolution X-ray lineshape study of main-chain smectic polymers. The results indicate that the layer ordering differs fundamentally from the algebraic decay typical for other smectic liquid-crystalline systems. The lineshapes are best described by broad squared Lorentzians indicating some form of short-range correlations.

Evidence of supercritical behavior in liquid single crystal elastomers.

Temperature profiles of the first and the second moment of the nematic order parameter distribution function, as determined from the deuteron nuclear magnetic resonance line shapes, as well as heat capacity response, provide support for the supercritical scenario of the nematic-paranematic phase transition in liquid single crystal elastomers. The relative strength of the locked-in internal mechanical field with respect to the critical field can be decreased by swelling the elastomer samples with low molecular mass nematogen. By increasing the concentration of the dopant, critical and below-critical behavior is promoted.

Low-voltage-driven electromechanical effects of swollen liquid-crystal elastomers.

We experimentally investigate, in detail, electromechanical effects in liquid-crystal elastomers (LCEs) previously swollen with low-molecular-weight liquid crystals (LMWLCs). Both polydomain (POLY) and monodomain (MONO) LCEs were studied. We used a well known LMWLC, 4-n-pentyl-4-cyanobiphenyl (5CB) as a solvent.