A. R. Tajbakhsh

uv manipulation of order and macroscopic shape in nematic elastomers

A range of monodomain nematic liquid-crystal elastomers containing differing proportions of photoisomerizable mesogenic moieties, which turn from a rodlike to a kinked shape upon ultraviolet (uv) irradiation, was studied. Depending on the proportion and positional role of the photosensitive groups in the crosslinked polymer network, different types and magnitudes of response were found. The principle consequence of such photoisomerization is the destabilization 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.

Spontaneous thermal expansion of nematic elastomers

Abstract: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 the 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 an 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.

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 ~ 1 kPa in response to a constant field of order ~ 1 MV/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.

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

Abstract:The macroscopic shape of liquid-crystalline elastomers strongly depends on the order parameter of the mesogenic groups. This order can be manipulated if photo-isomerisable 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.

Infrared actuation in aligned polymer-nanotube composites

Rubber composites containing multiwalled carbon nanotubes have been irradiated with near-infrared light to study their reversible photomechanical 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.

Polysiloxane surfactants for the dispersion of carbon nanotubes in nonpolar organic solvents.

We develop two new amphiphilic molecules that are shown to act as efficient surfactants for carbon nanotubes in nonpolar organic solvents. The active conjugated groups, which are highly attracted to the graphene nanotube surface, are based on pyrene and porphyrin. We show that relatively short (C18) carbon tails are insufficient to provide stabilization. As our ultimate aim is to disperse and stabilize nanotubes in siloxane matrix (polymer and cross-linked elastomer), both surfactant molecules were made with long siloxane tails to facilitate solubility and steric stabilization. We show that the pyrene-siloxane surfactant is very effective in dispersing multiwall nanotubes, while the porphyrin-siloxane makes single-wall nanotubes soluble, both in petroleum ether and in siloxane matrix.

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 polarized light quantifies the complex dependence of the photonic band-gap 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.

Effect of crosslinking on the photonic bandgap in deformable cholesteric elastomers

Cholesteric liquid crystal (CLC) application for tunable lasing devices has become a subject of study for many research groups. In particular, embedding the liquid crystal in an elastomer allows tunability by simple mechanical stretching. Here we report a study on the dependence of the selective reflection band on the stretching together with measurements of film relaxation after stretching, and we try to discuss and elucidate the role of crosslinking in the polymer matrix. We obtained laser devices made with cholesteric liquid crystal elastomers in a three-layer configuration, where an isotropic layer containing a laser dye is sandwiched between two CLC elastomers: in this work we show some preliminary but quantitative results on laser tunability.

Soft elasticity and mechanical damping in liquid crystalline elastomers

The dynamic soft response of polydomain liquid crystalline elastomers to simple shear is reported. Significantly, these materials also show extremely large loss behavior with tan δ exceeding 1 or even 1.5 over very wide temperature ranges, with clear implications for damping applications. By comparing materials that exhibit different types of liquid crystalline phases, we identify the nematic state as a better damping phase than that in materials with smectic phases. Additionally, we provide experimental evidence for directions which should be explored for further improvements in the damping behavior of liquid crystalline elastomers.

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.