arXiv subjects

E. M. Terentjev

Publications and source records attributed to E. M. Terentjev.

At least 37 records · Page 2Linked to original sources

arXiv2006

Chirality transfer and stereo-selectivity of imprinted cholesteric networks

Imprinting of cholesteric textures in a polymer network is a method of preserving a macroscopically chiral phase in a system with no molecular chirality. By modifying the elastics properties of the network, the resulting stored helical twist can be manipulated within a wide range since the imprinting efficiency depends on the balance between the elastics constants and twisting power at network formation. One spectacular property of phase chirality imprinting is the created ability of the network to adsorb preferentially one stereo-component from a racemic mixture. In this paper we explore this property of chirality transfer from a macroscopic to the molecular scale. In particular, we focus on the competition between the phase chirality and the local nematic order. We demonstrate that it is possible to control the subsequent release of chiral solvent component from the imprinting network and the reversibility of the stereo-selective swelling by racemic solvents.

cond-mat.soft↗

arXiv2006

Concentration-Temperature Superposition of Helix Folding Rates in Gelatin

We study the kinetics of helix-coil transition in water solutions of gelatin (collagen protein) by optical rotation techniques combined with thermal characterization. By examining the rates of secondary helix folding, and covering a very wide range of solution concentrations, we are able to identify a universal exponential dependence of folding rate on concentration and quench temperature. We demonstrate a new concentration-temperature superposition of data at all temperatures and concentrations, and build the corresponding master curve. The results support the concept of a diffuse helix-coil transition. We find no concentration dependance of the normalized rate constant, suggesting first order (single) kinetics of secondary helix folding dominate in the early stages of renaturation.

cond-mat.soft↗

arXiv2006

Mechanically induced helix-coil transition in biopolymer networks

The quasi-equilibrium evolution of the helical fraction occurring in a biopolymer network (gelatin gel) under an applied stress has been investigated by observing modulation in its optical activity. Its variation with the imposed chain extension is distinctly non-monotonic and corresponds to the transition of initially coiled strands to induced left-handed helices. The experimental results are in qualitative agreement with theoretical predictions of helices induced on chain extension. This new effect of mechanically stimulated helix-coil transition has been studied further as a function of the elastic properties of the polymer network: crosslink density and network aging.

cond-mat.soft↗

arXiv2006

Rheology of carbon nanotube dispersions

We report on rheological properties of a dispersion of multi-walled carbon nanotubes in a viscous polymer matrix. Particular attention is paid to the process of nanotubes mixing and dispersion, which we monitor by the rheological signature of the composite. The response of the composite as a function of the dispersion mixing time and conditions indicates that a critical mixing time t* needs to be exceeded to achieve satisfactory dispersion of aggregates, this time being a function of nanotube concentration and the mixing shear stress. At shorter times of shear mixing, t t*, has several universal rheological features; at nanotube concentration above a characteristic value ~2-3wt% the effective elastic gel network is formed, while the low-concentration composite remains a viscous liquid. We use this rheological approach to determine the effects of aging and re-aggregation.

cond-mat.mtrl-sci↗

arXiv2006

Infrared actuation in aligned polymer-nanotube composites

Rubber composites containing multi-walled carbon nanotubes have been irradiated with near infrared light to study their reversible photo-mechanical actuation response. We demonstrate that the actuation is reproducible across differing polymer systems. The response is directly related to the degree of uniaxial alignment of the nanotubes in the matrix, contracting the samples along the alignment axis. The actuation stroke depends on the specific polymer being tested, however, the general response is universal for all composites tested. We conduct a detailed study of tube alignment induced by stress and propose a model for the reversible actuation behavior, based on the orientational averaging of the local response. The single phenomenological parameter of this model describes the response of an individual tube to adsorption of low-energy photons; its experimentally determined value may suggest some ideas about such a response.

cond-mat.mtrl-sci↗

arXiv2004

Photonic gaps in cholesteric elastomers under deformation

Cholesteric liquid crystal elastomers have interesting and potentially very useful photonic properties. In an ideal monodomain configuration of these materials, one finds a Bragg-reflection of light in a narrow wavelength range and a particular circular polarization. This is due to the periodic structure of the material along one dimension. In many practical cases, the cholesteric rubber possesses a sufficient degree of quenched disorder, which makes the selective reflection broadband. We investigate experimentally the problem of how the transmittance of light is affected by mechanical deformation of the elastomer, and the relation to changes in liquid crystalline structure. We explore a series of samples which have been synthesized with photonic stop-gaps across the visible range. This allows us to compare results with detailed theoretical predictions regarding the evolution of stop-gaps in cholesteric elastomers.

cond-mat.soft↗

arXiv2003

Stereo-selective swelling of imprinted cholesteric networks

Molecular chirality, and the chiral symmetry breaking of resulting macroscopic phases, can be topologically imprinted and manipulated by crosslinking and swelling of polymer networks. We present a new experimental approach to stereo-specific separation of chiral isomers by using a cholesteric elastomer in which a helical director distribution has been topological imprinted by crosslinking. This makes the material unusual in that is has a strong phase chirality, but no molecular chirality at all; we study the nature and parameters controlling the twist-untwist transition. Adding a racemic mixture to the imprinted network results in selective swelling by only the component of ``correct'' handedness. We investigate the capacity of demixing in a racemic environment, which depends on network parameters and the underlying nematic order.

cond-mat.soft↗

arXiv2003

Phase chirality and stereo-selective swelling of cholesteric elastomers

Cholesteric elastomers possess a macroscopic ``phase chirality'' as the director n rotates in a helical fashion along an optical axis $z$ and can be described by a chiral order parameter. This parameter can be tuned by changing the helix pitch p and/or the elastic properties of the network. The cholesterics also possess a local nematic order, changing with temperature or during solvent swelling. In this paper, by measuring the power of optical rotation, we discover how these two parameters vary as functions of temperature or solvent adsorbed by the network. The main result is a finding of pronounced stereo-selectivity of cholesteric elastomers, demonstrating itself in the retention of the ``correct'' chirality component of a racemic solvent. It has been possible to quantify the amount of such stereo-separation, as the basic dynamics of the effect.

cond-mat.soft↗

arXiv2003

Nematic elastomers with aligned carbon nanotubes: new electromechanical actuators

We demonstrate, for the first time, the large electromechanical response in nematic liquid crystalline elastomers filled with a very low (~0.01%) concentration of carbon nanotubes, aligned along the nematic director at preparation. The nanotubes create a very large effective dielectric anisotropy of the composite. Their local field-induced torque is transmitted to the rubber-elastic network and is registered as the exerted uniaxial stress of order ~1kPa in response to a constant field of order ~1MV/m. We investigate the dependence of the effect on field strength, nanotube concentration and reproducibility under multiple field-on and -off cycles. The results indicate the potential of the nanotube-nematic elastomer composites as electrically driven actuators.

cond-mat.soft↗

arXiv2002

Networks of helix-forming polymers

Biological molecules can form hydrogen bonds between nearby residues, leading to helical secondary structures. The associated reduction of configurational entropy leads to a temperature dependence of this effect: the "helix-coil transition". Since the formation of helices implies a dramatic shortening of the polymer dimensions, an externally imposed end-to-end distance R affects the equilibrium helical fraction of the polymer and the resulting force- extension curves show anomalous plateau regimes. In this article, we investigate the behaviour of a cross-linked network of such helicogenic molecules, particularly, focusing on the coupling of the (average) helical content present in a network to the externally imposed strain. We show that both an elongation and compression can lead to an increase in helical domains under appropriate conditions.

cond-mat.soft↗

arXiv2002

UV-isomerisation in nematic elastomers as a route to photo-mechanical transducer

The macroscopic shape of liquid crystalline elastomers strongly depends on the order parameter of the mesogenic groups. This order can be manipulated if photoisomerisable groups, e.g. containing N=N bonds, are introduced into the material. We have explored the large photo-mechanical response of such an azobenzene-containing nematic elastomer at different temperatures, using force and optical birefringence measurements, and focusing on fundamental aspects of population dynamics and the related speed and repeatability of the response. The characteristic time of ``on'' and ``off'' regimes strongly depends on temperature, but is generally found to be very long. We were able to verify that the macroscopic relaxation of the elastomer is determined by the nematic order dynamics and not, for instance, by the polymer network relaxation.

cond-mat.mtrl-sci↗

arXiv2002

Dynamic Soft Elasticity in Monodomain Nematic Elastomers

We study the linear dynamic mechanical response of monodomain nematic liquid crystalline elastomers under shear in the geometry that allows the director rotation. The aspects of time-temperature superposition are discussed at some length and Master Curves are obtained between the glassy state and the nematic transition temperature Tni. However, the time-temperature superposition did not work through the clearing point Tni, due to change from the ``soft-elasticity'' nematic regime to the ordinary isotropic rubber response. We focus on the low-frequency region of the Master Curves and establish the power-law dependence of the modulus G' ~ omega^a. This law agrees very well with the results of static stress relaxation, where each relaxation curve obeys the analogous power law G' ~ t^{-a} in the corresponding region of long times and temperatures.

cond-mat.mtrl-sci↗

arXiv2001

UV-Manipulation of Order and Macroscopic Shape in Nematic Elastomers

A range of monodomain nematic liquid crystal elastomers containing differing proportions of photo-isomerisable mesogenic moieties, which turn from a rod-like to a kinked shape upon ultraviolet (UV) irradiation, was studied. Depending on the proportion and positional role of the photo-sensitive groups in the crosslinked polymer network, different types and magnitudes of response were found. The principle consequence of such photo-isomerisation is the destabilisation of the nematic phase, whose order parameter depends on temperature in a near-critical fashion. Accordingly, the effect of UV-irradiation is dramatically enhanced near the critical temperature, with the associated reduction in the nematic order parameter manifesting as a change in the macroscopic shape of the elastomer samples, producing a large uniaxial contraction. Theoretical analysis of this phenomenon gives a good quantitative agreement with experiment.

cond-mat.soft↗

arXiv2001

Evolution of photonic structure on deformation of cholesteric elastomers

We subject a monodomain cholesteric liquid crystal elastomer to uniaxial strain perpendicular to its helical axis and study the response of its texture to deformation. A combination of mechanical, optical and X-ray scattering measurements confirms the prediction for the director rotation, coarsening and then unwinding the cholesteric helix. The study of optical absorption of circularly polarised light quantifies the complex dependence of the photonic bandgap structure on strain and directly relates to the microscopic deformation of elastomer. Agreement is found with the recently proposed theoretical prediction of the photonic structure of cholesteric elastomers.

cond-mat.soft↗

arXiv2001

A Tube Model of Rubber Elasticity

Polymer entanglements lead to complicated topological constraints and interactions between neighbouring chains in a dense solution or melt. Entanglements can be treated in a mean field approach, within the famous reptation model, since they effectively confine each individual chain in a tube-like geometry. In polymer networks, due to crosslinks preventing the reptation constraint release, entanglements acquire a different topological meaning and have a much stronger effect on the resulting mechanical response. We apply the classical ideas of reptation dynamics to calculate the effective rubber-elastic free energy of an entangled rubbery network. We then compare the results with other theoretical approaches and establish a particularly close mapping with the hoop-model, with equally good description of experimental data. The present consistent reptation theory allows further development of dynamic theory of stress relaxation.

cond-mat.soft↗

arXiv2001

Tube Model for the Elasticity of Entangled Nematic Rubbers

Dense rubbery networks are highly entangled polymer systems, with significant topological restrictions for the mobility of neighbouring chains and crosslinks preventing the reptation constraint release. In a mean field approach, entanglements are treated within the famous reptation approach, since they effectively confine each individual chain in a tube-like geometry. We apply the classical ideas of reptation dynamics to calculate the effective rubber-elastic free energy of anisotropic networks, nematic liquid crystal elastomers, and present the first theory of entanglements for such a material.

cond-mat.soft↗

arXiv2001

Spontaneous thermal expansion of nematic elastomers

We study the monodomain (single-crystal) nematic elastomer materials, all side-chain siloxane polymers with the same mesogenic groups and crosslinking density, but differing in the type of crosslinking. Increasing the proportion of long di-functional segments of main-chain nematic polymer, acting as network crosslinking, results in dramatic changes in the uniaxial equilibrium thermal expansion on cooling from isotropic phase. At higher concentration of main chains their behaviour dominates the elastomer properties. At low concentration of main-chain material, we detect two distinct transitions at different temperatures, one attributed to the main-chain, the other to the side-chain component. The effective uniaxial anisotropy of nematic rubber, r(T) proportional to the effective nematic order parameter Q(T), is given by the average of the two components and thus reflects the two-transition nature of thermal expansion. The experimental data is compared with the theoretical model of ideal nematic elastomers; applications in high-amplitude thermal actuators are discussed in the end.

cond-mat.soft↗

arXiv2001

Linear hydrodynamics and viscoelasticity of nematic elastomers

We develop a continuum theory of linear viscoelastic response in oriented monodomain nematic elastomers. The expression for dissipation function is analogous to the Leslie-Ericksen version of anisotropic nematic viscosity; we propose the relations between the anisotropic rubber moduli and new viscous coefficients. A new dimensionless number is introduced, which describes the relative magnitude of viscous and rubber-elastic torques. In an elastic medium with an independently mobile internal degree of freedom, the nematic director with its own relaxation dynamics, the model shows a dramatic decrease in the dynamic modulus in certain deformation geometries. The degree to which the storage modulus does not altogether drop to zero is shown to be both dependent on frequency and to be proportional to the semi-softness, the non-ideality of a nematic network. We consider the most interesting geometry for the implementation of the theory, calculating the dynamic response to an imposed simple shear and making predictions for effective moduli and (exceptionally high) loss factors.

cond-mat.soft↗