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. The distance from the critical point, reflected in the sharpness of the heat capacity anomalies, increases when adding a shorter-length mesogen. The results provide new insight for the impact of mesogens on the thermodynamic behaviour and, thus, on the thermomechanical response of nematic liquid-crystalline elastomers.

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.

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. They allowed us to determine the diffusion coefficients and to show that the doped polymer has a more pronounced affinity toward one of the enantiomers.

The first asymmetrically beta-polysubstituted porphyrin-based hexagonal columnar liquid crystal.

The preparation of a new beta-tetrasubstituted porphyrin through a short synthesis with liquid crystalline properties in a wide range of temperatures including room temperature is described.

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. This change in behavior from the conventional flow response to a gel-like response, when approaching the glass transition, is observed for nonsliding conditions and for very weak-imposed shear strains. It can be explained by a percolation-type mechanism of preglassy elastic clusters, which correspond to long-range and long-lived density fluctuations that are frozen at the time scale of the experiment. The sample response is therefore the sum of two contributions: one is due to the flow response of the polymer melt and the other to the elastic response of the network formed by the preglassy elastic clusters. By filling the cell below T0 with the compression method, both compounds exhibit a gel-type behavior by gently bringing closer the slides of the cell and an anomalous low-frequency behavior characterized by G'=const and G''=const by increasing the pressure used to bring closer the slides of the cell. A compression-assisted aggregation of the preglassy elastic clusters can explain both the increase in the low-frequency elastic plateau when the sample thickness is decreased and the anomalous low-frequency behavior. Further evidence for the existence of these elastic clusters is provided by the following results: (a) the nonlinear response of the samples as a function of the strain amplitude, which can be explained by the Payne effect, and (b) the aggregation effects, which can be mimicked by a polydimethylsiloxane melt filled with silica particles, the silica particles playing the role of the preglassy elastic clusters. All these observations show that PAOCH3 is not a macroscopically solidlike material with an unconventional type of elasticity, as claimed by Mendil [Phys. Rev. Lett. 96, 077801 (2006)]. The gel-type behavior observed here on two SCLCPs ( PAOCH3 and LCP105) and previously on some conventional flexible polymers (atactic polystyrene, poly-n-butylacrylate) seems to be a generic effect of the glass transition. The presence of the preglassy elastic clusters questions the widely accepted hypothesis of ergodicity in the supercooled state.

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. For the homopolymers the resulting X-ray lineshapes are well described by Lorentzians. This is interpreted as an average of algebraically decaying order in domains with dimensions of hundreds of nm and a wide dispersion of sizes. The elastomers show much broader peaks than the correponding polymers. This is attributed to strong non-uniform strain within the finite-size domains due to defects of the layer structure.

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. The measurements taken as a function of frequency show that the second plateau in G' (//) and the associated dip in G (//)" expected from dynamic semi-soft elasticity are not observed. These results can be described by the de Gennes model, which predicts small elastic anisotropy in the hydrodynamic and linear regimes. They correspond to the behavior expected for compounds beyond the mechanical critical point, which is consistent with the NMR and specific heat measurements taken on similar compounds. We also show that a reduction in the cross-linking density does not change the non-soft character of the mechanical response. From the measurements taken as a function of frequency at several temperatures we deduce that the time-temperature superposition method does not apply. From these measurements, we also determine the temperature dependence of the longest relaxation time tau(E) of the network for the situations where the director is either parallel or perpendicular to the shear velocity. Finally, we discuss the influence on the measurements of the mechanical constraint associated with the fact that the samples cannot change their shape around the pseudo phase transition, because of their strong adherence on the sample-bearing glass slides.

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. However, several higher harmonics are observed, which excludes simple liquid-like short-range order. This behaviour is tentatively attributed to a random field of defects associated with entangled hairpins in the main-chain polymer structure.

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.

Strain-induced reorientation and mobility in nematic liquid-crystalline elastomers as studied by time-resolved FTIR spectroscopy.

Polarized Fourier transform infrared (FTIR) spectroscopy is employed to study the segmental orientation and mobility of liquid-crystalline elastomers (LCEs) with a monodomain structure in response to external mechanical fields parallel and perpendicular to the initial nematic director. The mean orientation and the molecular order parameter of the different molecular moieties referring to the mesogen, the spacer and the network are analyzed in detail. Parallel stretch leaves the mean orientation of the different molecular moieties and its molecular order parameter nearly uninfluenced. Perpendicular stretch results in a threshold-like dependence: for elongation ratios lambda < or = lambda(c) = 1.3 (10 mol% crosslinker density), respectively lambda < or = lambda(c) = 1.6 (5 mol% crosslinker density) no change of the mean orientation and the molecular order parameters is observed, while for lambda > or = lambda(c) all molecular units reorient and their molecular order parameters are strongly decreased. The present studies give no indications that the reorientational dynamics of the network and the mesogens differ as long as the elongation ratio is smaller than lambda(c).

A new opto-mechanical effect in solids.

We propose that large, reversible shape changes in solids, of between 10%-400%, can be induced optically by photoisomerizing monodomain nematic elastomers. Empirical and molecular analysis of shape change and its relation to thermal effects is given along with a simple model of the dynamics of response. Our experiments demonstrate these effects for the first time and theory is compared qualitatively with our results.

Mechanical properties of mono-domain side chain nematic elastomers.

We investigate the behavior of the shear rigidity modulus G = G' + iG" of three mono-domain side chain liquid-crystalline elastomers composed of side chain polysiloxanes cross-linked by either flexible or rigid cross-linkers. The measurements were taken in a frequency domain ranging from approximately 0.02 Hz to approximately 10(4) Hz applying the shear in a plane perpendicular to or containing the director. The measurements as a function of temperature show an anisotropy of G' which appears around T(NI), when decreasing the temperature, and which is due to the expected lowering of G'(//) coming from the coupling between the shear and the director. The measurements as a function of frequency show that G has two components for both geometries, in both the isotropic phase and in the nematic phase around the phase transition. One reflects the network behavior in its hydrodynamic regime ( G' is constant and G'' is approximately f, where f is the frequency), the other which appears at higher frequencies is characterized by a scaling law behavior (G' is approximately G" is approximately f(0.5)) of the Rouse type. We discuss the results in the framework of available theories and show that the three elastomers present a non-soft behavior, even for the elastomer for which the contrary was claimed, and that there is no separation of time scales between the director and the network. We also present data on a poly-domain sample and a non-mesomorphic one which complement these results.

Deoxylysolecithin and a new biphenyl detergent as solubilizing agents for bovine rhodopsin. Functional test by formation of metarhodopsin II and binding of G-protein.

The protein-detergent interaction in rhodopsin-detergent micelles has been investigated by using formation of metarhodopsin II (MII) as a monitor. Two detergents of different structural rigidity have been applied. One of them is [3-(lauroyloxy)propyl]phosphorylcholine, which has a high conformational flexibility in its hydrophobic moiety like most of the known detergents for rhodopsin. This deoxylysolecithin was originally designed as a detergent for membrane proteins by Weltzien [Weltzien, H. U. (1979) Biochim. Biophys. Acta 559, 259-287]. The other detergent, which is highly rigid in its hydrophobic part, has been developed for this study. It consists of a biphenyl derivative and a hydrophilic octaethylene oxide group. Both the formation kinetics of MII and the position of its equilibrium with its tautomeric form, metarhodopsin I (MI), strongly differed in the deoxylysolecithin and biphenyl detergent. Deoxylysolecithin caused very fast MII formation and shifted the equilibrium strongly to MII, like other detergents with alkyl chains as the hydrophobic part. In the biphenyl detergent, however, formation of MII was slow and the MI/MII equilibrium similar to that in the native system. For rhodopsin reconstituted in lipid bilayers, normal MII formation requires a well-adjusted fluidity of the hydrocarbon environment of the protein [Baldwin, P. A., & Hubbell, W. L. (1984) Biochemistry 24, 2633-2639], which was explained by an appropriate interfacial pressure at the protein-lipid interface. Extension of this concept would indicate that in the micellar core a degree of fluidity comparable to that of the disk membrane is just achieved with the highly rigid biphenyl structure.(ABSTRACT TRUNCATED AT 250 WORDS)

Texture Evolution during the Polydomain-Monodomain Transition in Nematic Elastomers.

We have studied the behavior of a liquid crystalline elastomer undergoing a polydomain-monodomain transition. The textures emerging under increasing extensional load have been examined with a combination of optical microscopy, X-ray, and small-angle polarized light scattering. The experimental data are interpreted in terms of reorientation of the local director with increasing extension. The results of this combination of techniques at low extensions are consistent with a two-dimensionally periodic director texture. The amplitude of this modulation decreases continuously as the mesogens are pulled into the extensional direction at high loads. In this manner the samples which are polydomain under no load become essentially monodomain with increasing extension.