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. Despite the push-pull nature of the 4-alkoxy-4'-cyanoazobenzene chromophore, it has been experimentally demonstrated that it isomerises via an inversion mechanism. The opto-mechanics of the prepared elastomeric material has also been investigated. For this system, a maximum opto-mechanical response of ca. 2.5 kPa has been registered, which is independent of the working temperature of the photoactuator.

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). The SmA structure where the ethylene oxide chains are microphase separated in layers is fixed by the crosslinking and permanent macroscopic orientation is obtained. Diffusion and conductivity measurements of the monomer sample show a large anisotropy of the ion mobility (100 for the cation and 400 for the anion). In the elastomer the anisotropy of the lithium mobility is comparable to that in the monomers.

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. The mechanical anisotropy is studied by means of thermoelastic experiments and stress-strain measurements. For the first time, the layer compression modulus B of smectic elastomers is directly measured. B is significantly larger as compared to conventional low molar mass liquid crystals and decreases significantly with increasing local disorder introduced by the isotropic crosslinker.

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. At an increased cross-link concentration of 10% the smectic order disappears and gives way to extended short-range layer correlations. In this situation neither a smectic-nematic nor a nematic-isotropic transition is observed anymore. The occurrence of disorder at a relatively large cross-link concentration only, indicates that smectic elastomers are rather resistant to a random field. The temperature dependence of the correlation lengths and thermoelastic behavior suggest a shift to a "parasmectic" regime of a first-order smectic-isotropic 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. The temperature dependence of the local order parameter of the chromophore is related with the thermal activation parameters of the isomerisation process and they are characteristic for each AZO6-LC nematic mixture.

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. In this environment, the isomerisation process is accelerated more than 10(3) times. This effect is presented herein for the first time. The possible application of the networks obtained as possible photo-actuators has been also considered.

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. The stress-strain behaviour of the polydomain lays between the uniaxial deformations, parallel and perpendicular to the director, of the monodomain sample.

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. Their comparison with those values obtained in isotropic media demonstrates that the mechanism through which the thermal cis-to-trans isomerization takes place is the inversion pathway in all the physical states studied. The nematic order increases the rate of the thermal cis-to-trans isomerization process. This fact is related to a cooperative interaction established between the mesogen molecules and the azo-dye. This effect is not present in the isotropic state.

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. Biaxiality is observed for both elastomers when approaching the glass transition, suggesting that the network dynamics dominate the formation of the biaxial phase.

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. Here it is shown that the resistance change with strain ('piezoresistivity') for these reprocessed LCEs can be described through lattice percolation and geometrical changes in the LCE shape. The mechanisms and rate of degradation are also described for the conducting layer as a function of the number of electro-thermomechanical strain cycles performed.

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. On increasing the cross-linking density the predominantly first order thermodynamic response transforms into a predominantly supercritical one.

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. 2004, 92, 125501). To elucidate details of its formation, we here report on deuterium NMR experiments that have been conducted on different types of LC side-chain polymers as well as on mixtures with low-molar-mass mesogens. Different parameters that affect the formation of a biaxial nematic phase, such as the geometry of the attachment, the spacer length between the polymer backbone and the mesogenic unit, as well as the polymer dynamics, were investigated. Surprisingly, also polymers with terminally attached mesogens (end-on polymers) are capable of forming biaxial nematic phases if the flexible spacer is short and thus retains a coupling between the polymer backbone and the LC phase. Furthermore, the most important parameter for the formation of a biaxial nematic phase is the dynamics of the polymer backbone, as the addition of a small percentage of low molar mass LC to the biaxial nematic polymer from the original study served to shift both the glass transition and the appearance of detectable biaxiality in a very similar fashion. Plotting different parameters for the investigated systems as a function of T/Tg also reveals the crucial role of the dynamics of the polymer backbone and hence the glass transition.

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. After swelling POLY and MONO LCEs (LSCE) with 5CB, shape changes were measured by recording the displacement of the edge of the swollen LCE at different voltages, V, and temperature. With 100 microm distance between electrodes, measurable shape changes (approximately 1-20 microm) are observed with small voltages (V approximately 0.5-10 V). In particular, we note that, compared to unswollen L(S)CEs, a dramatic approximately 200 times decrease of the threshold field was found for electromechanical effects in swollen L(S)CEs. While swollen MONO LCEs showed electromechanical effects in the planar geometry, homeotropic MONO swollen with homeotropically oriented 5CB did not. This is easy to understand because, in the homeotropic case, the liquid-crystal preferred axis is already aligned with the field so the field has no reorienting effect. The inverse of the response time when the field was switched on in both POLY and MONO was proportional to E2, which is the same field dependence as the response time of LMWLCs. When the field was switched off, the relaxation time showed a field dependence different from that of LMWLCs that we attribute to relaxation of the LCE network.

Disorder by random crosslinking in smectic elastomers.

We present a high-resolution x-ray study of the effects of disorder due to random crosslinking on the one-dimensional translational ordering in smectic elastomers. At a small crosslink density of about 5%, the elastomer network stabilizes the smectic structure against layer-displacement fluctuations, and the algebraically decaying layer ordering extends up to several micrometers. With increasing concentration of crosslinks, the finite size of these domains is strongly reduced, indicating that disordering takes over. Finally, at a crosslink concentration of 20%, the structure factor can be described by a Lorentzian, which signals extended short-range correlations. The findings are discussed in terms of recent theories of randomly quenched disorder.

Fast liquid-crystal elastomer swims into the dark.

Liquid-crystal elastomers (LCEs) are rubbers whose constituent molecules are orientationally ordered. Their salient feature is strong coupling between the orientational order and mechanical strain. For example, changing the orientational order gives rise to internal stresses, which lead to strains and change the shape of a sample. Orientational order can be affected by changes in externally applied stimuli such as light. We demonstrate here that by dissolving-rather than covalently bonding-azo dyes into an LCE sample, its mechanical deformation in response to non-uniform illumination by visible light becomes very large (more than 60 degrees bending) and is more than two orders of magnitude faster than previously reported. Rapid light-induced deformations allow LCEs to interact with their environment in new and unexpected ways. When light from above is shone on a dye-doped LCE sample floating on water, the LCE 'swims' away from the light, with an action resembling that of flatfish such as skates or rays. We analyse the propulsion mechanism in terms of momentum transfer.

Swelling dynamics of liquid crystal elastomers swollen with low molecular weight liquid crystals.

We experimentally investigated the swelling behavior of thin films (approximately 150 microm) of liquid crystalline elastomers (LCEs) by low molecular weight liquid crystals (LMWLCs). The two LMWLCs used are the well-known nematic liquid crystals, 4-n-pentyl-4-cyanobiphenyl, and 4-methoxy-benzilidene-4-butyl-aniline. Both polydomain (POLY) and monodomain (MONO) LCE swelling are studied. In MONO LCEs (LSCEs), the director n empty set is uniformly oriented throughout the film. POLY films are made of many domains with different orientations. Its swelling behavior was similar to isotropic gels. In contrast, LSCEs revealed interesting results not anticipated by any theory. First, the LMWLC enters the LSCE by front propagation about three-times faster axially n empty set than radially n empty set. Second, only the LSCE dimensions radially n empty set expanded, while that axially n empty set did not change at all. Third, when the LMWLC director and the LSCE director are aligned (MONO2 samples), swelling takes place about twice as fast as when they are not aligned. Volume change dynamics of swollen L(S)CEs investigated as a function of temperature revealed several phase transitions by optical and calorimetry techniques.