Side-chain Liquid Crystal Elastomers Research Articles

Thermomechanical response of liquid crystal elastomers: role of crosslinker density

In this work thermomechanical properties of main-chain liquid crystal elastomers (MC-LCEs) with different degrees of crosslinking were investigated, and gradual loss of thermomechanical response was observed on repetitive measurements. Specifically, six samples of MC-LCEs were prepared, with crosslinker-to-mesogen relative concentration ranging from 5% to 10% in steps of 1%. The obtained results were then compared to thermomechanical response of side-chain liquid crystal elastomers (SC-LCEs). Additionally, thermomechanical response of polymer dispersed main-chain liquid crystal elastomers (MC-PDLCEs) was investigated. Results indicate that in MC-LCEs the concentration of crosslinker defines thermomechanical response and affects stability of the system. The loss of thermomechanical response is negligible in the case of crosslinker to mesogen ratio being the smallest, namely in 5% sample, and it is unaffected by glasslike to nematic phase transition. SC-LCEs do not show any sign of such behaviour and remain stable after several cycles of thermomechanical measurements.

Effect of Spacer and Mesogen in Side-Chain Liquid Crystal Elastomer Structure on Reversible Actuation Behavior

Effect of Spacer and Mesogen in Side-Chain Liquid Crystal Elastomer Structure on Reversible Actuation Behavior

Impact of molecular architectures on mesogen reorientation relaxation and post-relaxation stress of liquid crystal elastomers under electric fields

Applying electric fields to liquid crystal elastomers (LCEs) has attracted attention as a fast and energy-efficient actuation method for soft robotics. It is helpful to predict the minimum electric field strength that can drive the mesogen reorientation and the time required for the reorientation of LCEs with specific molecular architecture to optimize LCE electrical actuation. In this study, molecular dynamics (MD) simulations of main- and side-chain LCE systems under an applied electric field were performed, and the relaxations of stress and orientational order were observed. The minimum electric field strength that can drive the reorientation for each LCE molecular system was estimated. The time required for reorientation follows a power law of the electric field strength, and the reorientation time for side-chain LCEs is much shorter than that for main-chain LCEs. The behavior of the relaxation value of stress with respect to the electric field strength differed markedly between the main- and side-chain LCEs. These results suggest that designing LCEs at the molecular architecture level and using different LCE molecules in different actuation sites are essential to more precise soft robotics.

New Liquid Crystalline Elastomeric Films Containing a Smectic Crosslinker: Chemical and Physical Properties

Side-chain liquid crystal elastomers (SC-LCEs) have been designed by using a new smectic crosslinker. Two types of monodomain films were prepared based on polysiloxane chains, with a different relative concentration of both crosslinker and mesogenic comonomers. The mesomorphic behavior of the two SC-LCE systems was investigated by differential scanning calorimetry and polarized optical microscopy showing a different mesomorphic behavior: in one case, we obtained a nematic SC-LCE film, in the other case, a Smectic A SC-LCE film. In both systems, the mesophases were stable in a wide temperature range. Moreover, the SC-LCE films possess a relatively high orientation at room temperature. The physical-chemical properties, such as the local orientational ordering, structural organization, and dynamics of SC-LCEs’ constituents were studied by means of static and dynamic 2H NMR experiments, small-angle X-ray, and wide-angle X-ray diffractions. The relevant physical properties, such as the thermo-elastic and thermo-mechanic behaviors, are reported and discussed in view of the practical applications.

Siloxane-based side-chain liquid crystal elastomers containing allyl 4-((4-decyloxy)benzoyl)oxy)benzoate as a monomer

Siloxane-based side-chain liquid crystal elastomers containing allyl 4-((4-decyloxy)benzoyl)oxy)benzoate as a monomer

Main-chain liquid-crystal elastomers versus side-chain liquid-crystal elastomers: similarities and differences in their mechanical properties.

After a general introduction on the main aspects of the mechanical properties of main-chain liquid-crystal elastomers (MCLCEs) and side-chain liquid-crystal elastomers (SCLCEs), new results will be presented dealing with several MCLCEs with a cross-linker density C = 8%, 6% and 4% and with a SCLCE with C = 10%, all prepared by the two-step cross-linking process. A non-SCLCE with bulky side-groups similar in shape to the mesogens was also synthesized for comparison with the SCLCE. Most of the experiments were performed with a piezorheometer allowing the determination of the shear anisotropy of the samples by applying shear in a direction parallel or perpendicular to the director, and with a thermo-elastic device for the E measurements. The main results concern: (a) the influence of the supercritical nature of SCLCE and the subcritical nature of MCLCEs on the mechanical properties of these elastomers, as well as that of SmC domains present in MCLCEs; (b) the relationship between the degrees of elongation and of anisotropy deduced from the variations of and during the poly-domain to mono-domain transition of the 10% SCLCE and the 8% MCLCE; (c) the determination of the Poisson's ratio showing that it is isotropic for the non-SCLCE, with a crossover between 0.5 (classical value for rubbers) for small strains and 0.38 for high strains, and anisotropic for the 10% SCLCE and 8% MCLCE, with values <0.5. The particular behaviors of the Poisson's ratios can be explained by confinement effects occurring when stretching increases.

Synthesis of vinyl-functionalized azobenzene mesogens and study of their liquid-crystalline behavior

ABSTRACTFour vinyl-functionalized azobenzene-based liquid crystals (LCs) were synthesized and characterized by 1H and 13C nuclear magnetic resonance, and Fourier transform infrared spectroscopies. Their thermal stability was measured by thermogravimetric analysis and their thermotropic liquid-crystalline properties were determined by differential scanning calorimetry (DSC), polarized optical microscopy (POM), and X-ray diffraction (XRD). The azobenzene-based LCs displayed a nematic phase whereas those mesogens having an extra phenyl group developed lamellar mesophases of the SmA and SmC types. Combined with non-photoactive monomers, these vinyl-functionalized azobenzene-based mesogens can be used to prepare photo-active side-chain liquid crystal elastomers having potential use in optical actuators.

Biocompatible 3D Liquid Crystal Elastomer Cell Scaffolds and Foams with Primary and Secondary Porous Architecture.

3D biodegradable and highly regular foamlike cell scaffolds based on biocompatible side-chain liquid crystal elastomers have been prepared. Scaffolds with a primary porosity characterized by spatially interlaced, interconnected microchannels or an additional secondary porosity featuring interconnected microchannel networks define the novel elastomeric scaffolds. The macroscale morphology of the dual porosity 3D scaffold resembles vascular networks observed in tissue. 3D elastomer foams show four times higher cell proliferation capability compared to conventional porous templated films and within the channels guide spontaneous cell alignment enabling the possibility of tissue construct fabrication toward more clinically complex environments.

Experimental Studies of Thermo-Induced Mechanical Effects in the Main-Chain Liquid Crystal Elastomers

Liquid crystal elastomers (LCEs), either side-chain LCEs (SCLCEs) or main-chain LCEs (MCLCEs), possess a combination of LC and elastic properties, and are expected to be used as artificial muscles. We experimentally investigated the thermo-induced mechanical effects showed by MCLCEs with four different crosslinker concentrations, i.e., 8%, 12%, 14% and 16%. The samples were heated up to the critical temperature and the images were recorded. The samples made the contraction in direction parallel to the director and the expansion in direction perpendicular to the director. Drastic changes occured when approaching the critical temperature, the greater the crosslinkers concentration the bigger the maximum contraction and expansion. The shape anisotropy expression showed that heating up to the critical temperature caused the system no longer in anisotropic state.

Nematic fluctuations and semisoft elasticity in liquid-crystal elastomers

We give a detailed theory of nematic fluctuations in liquid-crystal elastomers (LCEs) and calculate relaxation rates as obtained by dynamic light scattering (DLS). In ideal LCEs, a nematic state is formed by a spontaneous orientational symmetry breaking of an isotropic state, manifesting itself in an existence of a coupled director-shear soft mode (Goldstone mode). The relaxation rate of the soft mode (a pure bend and a pure splay mode) goes to zero in a long-wavelength limit. In a real, nonideal sample with a locked-in anisotropy, on the other hand, the relaxation rates of these modes become finite. Nonideal elastomers are characterized by a plateau in the stress-strain curve, and the soft mode can be detected only upon stretching to the point of elastic instability at which the director starts to rotate. We use the semisoft model of Gaussian elasticity to derive relaxation rates as a function of deformation for different scattering geometries. We show that the bend-mode relaxation rate goes to zero at the threshold strain, so it is the soft mode. The splay mode, on the other hand, is not soft because the relaxation rate is finite at the threshold strain. We provide experimental evidence and compare DLS measurements of splay and bend modes of two side-chain LCE samples differing in crosslinking densities. Results of both samples are in complete agreement with the predictions of the semisoft model, which indicates that director relaxation properties are not influenced much by the crosslinking conditions.

Optical properties of light-sensitive liquid-crystal elastomers in the vicinity of the nematic-paranematic phase transition

We investigate light-induced patterning of a monodomain side-chain liquid crystal elastomer (SC-LCE) doped with light-sensitive azobenzene moiety in the temperature region close to the nematic-paranematic phase transition. We show that a strongly nonlinear relationship between the concentration of the cis isomers of the azomesogens and the refractive index modification of the material, which is characteristic for the phase transition region, results in nonmonotonous time dependence of the diffraction efficiency of a probe beam. From this effect we determine the sensitivity of the nematic transition temperature on the molar fraction of the cis isomers. The relation between the cis isomer molar fraction and nematic order also provides a possibility for recording hidden holograms, which can be made visible by cooling the sample from the paranematic to the nematic phase.

Mechanic and electromechanic effects in biaxially stretched liquid crystal elastomers

The effect of combined electromechanic force fields in nematic side chain liquid crystal elastomers will be analyzed. A biaxially stretched plate in the x- and y-directions under an electric field applied in the perpendicular direction to the plate will be considered. A neo-Hookean model is chosen, which implies Gaussian behaviour. Results are obtained for both a soft and semisoft case showing the effect of the electric field on the rotation of the director and the free energy density function.

Direct preparation of nematic liquid crystalline elastomer actuators by electron beam irradiation polymerization

Oriented side chain liquid crystal elastomer (LCE) films with thermo- and photo-mechanical properties were prepared via electron beam (EB) irradiation. A magnetic field was applied to the nematic monomers during the polymerization to obtain aligned LCEs. The effects of the crosslinker concentration in feed, delivered dose and direction of the magnetic field during the polymerization on the properties of the LCEs were studied, respectively.

Mechanical properties of monodomain nematic side-chain liquid-crystalline elastomers with homeotropic and in-plane orientation of the director

We present the first study of the shear mechanical properties of monodomain nematic side-chain liquid-crystal elastomers (SCLCEs) prepared by cross-linking with UV irradiation a nematic side-chain liquid-crystal polymer oriented with an electric or a magnetic field. Their elastic behavior was studied in the dry, swollen and stretched states, in order to check the various theoretical descriptions of these systems. The shear measurements taken on the dry samples show that the shear anisotropy is much smaller than that of the usual twice cross-linked samples oriented by a mechanical stretching of the network formed after the first cross-linking step, demonstrating that the elasticity of the networks strongly depends on the preparation procedure used. The shear experiments performed on the swollen state of these two different types of elastomers reveal that the elasticity of the network is Gaussian for the elastomers oriented with the electric or the magnetic field, and non-Gaussian for the elastomers oriented with the usual stretching procedure. The analysis of the stress-strain curves of both types of elastomers with the neoclassical model based on Gaussian rubber elasticity confirms the Gaussian and non-Gaussian nature of their elasticity. The shear experiments performed as a function of the elongation of the homeotropically oriented elastomer when the shear is applied in a direction parallel to the elongation, do not show the decrease of the associated shear modulus, which is theoretically expected when the strain approaches the threshold value marking the beginning of the elastic plateau. However, the observation of this effect could be prevented by possible small misalignments of the director, as suggested by a calculation presented in one of the theories describing this effect.

Domains In Nematic Elastomers

ABSTRACTA theory based on an elastic model and including coupling between elastic deformation and nematic order parameter as well as an interaction between crosslinks and orientation is given. The nematic order parameter and the stress tensor have been determined on elongation and as a function of temperature as well as he anisotropy of the conformation of a side chain liquid crystal elastomer on application of a mechanical stress. A transition from a turbid, low ordered polydomain phase to a transparent, Monodomain is shown to occur.

Liquid crystal elastomers based upon cellulose derivatives

Crosslinked networks displaying both thermotropic and lyotropic liquid crystalline phases have been prepared from hydroxypropyl cellulose. The phase transitions from the liquid crystal to isotropic state are reversible for materials with moderate levels of crosslinking. These materials show characteristic optical textures similar to uncrosslinked cellulose-based liquid crystal polymers although well defined patterns typical of cholesteric structures were not observed. The samples were insoluble in water, but high levels of swelling with acetone and benzene were observed and in the steady state condition a lyotropic phase was present even though the level of swelling was in excess of 500%. A reversible liquid crystalline to isotropic phase was observed with the swollen materials as the fraction of swellant was adjusted. The elastomers show marked global orientation effects when mechanically deformed but surprisingly the response was less dramatic than in some side-chain liquid crystal elastomers.