Smectic Elastomers Research Articles

Effect of cross-link concentration on the isotropic to smectic-A phase transition in smectic elastomers

We describe the isotropic to smectic-A phase transition in smectic elastomers within Landau phenomenological theory. We analyze the effects of external mechanical stress and cross-link concentration on the isotropic to smectic-A phase transition. Both the orientational and translational order parameters decrease with the increase of cross-link concentration. The first order isotropic to smectic-A transition in low-molecular-weight materials, can become weak first order transition in smectic elastomers. The effect of cross-link concentration on the strain birefringence and correlation length in the isotropic phase of the transition is discussed.

Synthesis and characterisation of novel chiral smectic liquid crystalline elastomers containing menthol dimer-type side chain

ABSTRACTA series of siloxane-based chiral smectic liquid crystalline elastomers (ChSLCEs) formed by a dimer-type smectic monomer (M1), a dimer-type chiral smectic monomer (M2) and a smectic cross-linker (CL) were reported. M1 and M2 were both new liquid crystals with dimer structures, and their chemical structures and purities were characterised by Fourier transform infrared spectra and 1H NMR. The thermal stability and mesomorphism of monomers, cross-linker and ChSLCEs were examined by different analytical techniques. The results showed both monomers exhibited excellent smectic properties. The combination of different smectic phases from the two liquid crystalline side chains endowed the obtained ChSLCEs with good chiral smectic properties. The effect of different dosages of cross-linkers on the mesomorphic properties of the elastomers was also examined. The temperature ranges of mesophase-isotropic phases was narrowed with increasing addition of the cross-linkers according to the differential scanning calorimetry. All of the ChSLCEs exhibited good thermal stability from thermal gravity analysis results. The effective cross-link densities of ChSLCEs were calculated according to the swelling experiments to give the fact that the molecular weights between the cross-linking points decrease with the increase of the cross-linkers. Novel achiral smectic monomer (M1), chiral smectic monomer (M2), and an achiral smectic crosslinking agent (CL) have been synthesized to form a series of side chain chiral smectic liquid crystalline elastomers (ChSLCEs) LCE1-LCE7 by graft copolymerization with the poly(methylhydrogeno)siloxane backbone

PB12 ナノダイヤモンドを充填したキラル液晶エラストマーにおけるメソゲンの配向と相転移挙動(化学・材料,ポスター発表,2012年日本液晶学会討論会)

PB12 ナノダイヤモンドを充填したキラル液晶エラストマーにおけるメソゲンの配向と相転移挙動(化学・材料,ポスター発表,2012年日本液晶学会討論会)

Что общего между смектическими эластомерами и металлами, подвергнутыми холодной обработке?

Что общего между смектическими эластомерами и металлами, подвергнутыми холодной обработке?

Electrically induced deformation in chiral smectic elastomers with different domain structures.

Electrical actuation is investigated in two kinds of chiral smectic liquid-crystal elastomers (LCEs) with different domain structures LCE1 and LCE2: The latter is better than the former in orientational order. Tracking fluorescent beads dispersed on the samples enables us to measure the two-dimensional strain tensors in ferroelectric elastomer films. It turns out that the electric-field-induced strain is polarity dependent and the type of molecular orientation responsible for the strain is specified. In LCE1 the shear strain is dominant, whereas in LCE2 it is comparable to the elongation strain, which is explained by the rotation of the principal axes. The essential differences of the two elastomers are observed in the eigenvalues of the strain tensors. The absolute values for LCE1 are larger than those for LCE2. The difference is discussed on the basis of the domain structures.

Calorimetric Investigation of the Isotropic to Smectic-A Phase Transition of Smectic Liquid-Crystalline Elastomers

A high-resolution calorimetric study has been performed in the vicinity of the isotropic to smectic-A phase transition of smectic liquid crystalline elastomers. The transition is weakly first order for both investigated samples. Nevertheless, different features have been observed when the crosslinking density is changed, such as substantial broadening of the transition and decrease of the involved latent heat. The latter is sluggishly released in a range of few K.

Mesomorphic Properties of Main-Chain Chiral-Smectic Elastomers Cross-Linked in Different Phases

We investigate the mesomorphic properties of main-chain chiral-smectic elastomers cross-linked in different phases, analyzing three samples prepared by photo-cross-linking a chiral smectic C (SmC*) polymer in the SmC*, SmA, and isotropic (Iso) phases. The SmC*-cross-linked elastomer exhibits successive SmC*-SmA-Iso transformations similarly to the precursor polymer. However, only the orthogonal molecular alignment due to the SmA structure appears in the SmA-cross-linked elastomer. Moreover, isotropic alignment remains in most of the Iso-cross-linked elastomer even at room temperature. Since the effect of the fixed orientation is not localized in the vicinity of cross-links because of the direct coupling between the mesogenic orientation and the polymeric conformation in main-chain smectic elastomers, the transformation from the cross-linked phase to a higher-order phase might be forbidden.

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.

Measurement of electrically induced shear strain in a chiral smectic liquid-crystal elastomer

The mechanical response to electrical stimulation was investigated in a chiral smectic elastomer. The two-dimensional strain tensor in an elastomer film was precisely measured by tracking fluorescent beads dispersed on the film. Shear deformation in the film was clearly observed when an electric field was applied perpendicular to the film surface. The temperature dependence of the strain tensor was also investigated, and the origin of the electric-field-induced shear strain in the chiral smectic-C phase was mainly attributed to the Nambu-Goldstone mode.

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.

Large electromechanical effect of isotropic-genesis polydomain nematic elastomers

We demonstrate that an isotropic-genesis polydomain nematic elastomer (I-PNE) exhibits a large strain reaching 35% as well as a polydomain-to-monodomain transition at a nominal electric-field strength of 25 MV m−1. The I-PNE has no ferroelectricity, and the maximum electrical strain observed is larger than the reported values for the ferroelectric chiral smectic elastomers. The I-PNE was obtained by cross-linking the mesogens in the high-temperature isotropic state and then by cooling the elastomer to the temperatures in the nematic state. The pronounced E-field responsivity originates from the softness regarding the director rotation specific to the isotropic-genesis polydomains.

The effect of mesogenic and non-mesogenic crosslinking units on the phase behaviour of side-chain smectic and cholesteric elastomers

Chiral monomer (M1 ), mesogenic and non-mesogenic crosslinking agents (C1 and C2 ), and the corresponding liquid crystalline elastomers (P1 and P2 series), have been synthesised. Their chemical structures have been characterised by Fourier transform infrared or 1H nuclear magnetic resonance and their phase behaviour investigated by differential scanning calorimetry, polarising optical miscoscopy, thermo-gravimetric analysis (TGA) and X-ray diffraction. The effect of the crosslinking unit on the phase behaviour of the elastomers has been studied. M1 showed a cholesteric oily streak and focal conic texture. C2 exhibited a nematic enantiotropic thread-like and schlieren texture, and a monotropic fan-shaped texture in the SA phase. Due to the introduction of the mesogenic crosslinking unit, elastomers, P2-1 −P2-5 , exhibited a cholesteric phase, while elastomers, P1-1 −P1-4 , derived from a non-mesogenic crosslinking unit, exhibit a SA phase. As the content of the crosslinking unit increased, the T g of the P1 series initially decreased and then increased, and the T i of the series decreased. In the P2 series the T g increased, but the T i initially increased and then decreased. TGA confirmed that all the elastomers had improved thermal stability.

Helical Mechanoclinic Deformation of Main-Chain Chiral Smectic Elastomers

The helical mechanoclinic deformation of a main-chain chiral smectic elastomer, which is prepared by a crosslinking reaction under twist deformation, is investigated. The twist deformation induces a layer tilt angle that depends on the handedness of twist. The layer tilt angle in the right-handedly twisted elastomer, of which the handedness is consistent with that of the helix in the SmC* phase of the non-crosslinked backbone polymer, is estimated to be up to 16° at room temperature, although that in the left-handedly twisted elastomer is less than several degrees. The experiments provide evidence of chiral coupling between tilt and twist for helical mechanoclinic deformation in the chiral smectic system.

Quenched random disorder and x-ray scattering in smectic elastomers

In this work we examine layer fluctuations in a smectic elastomer with quenched random disorder induced by crosslinks. The system is analyzed in a continuum model and crosslinks are introduced as a random field in a macroscopic picture. In the case of small deformations and replica symmetry the intensity profile for x-ray scattering along the layer normal was determined for layer displacements smaller than the layer separation. In this regime it is predicted that for large enough crosslink densities the first-order diffraction pattern of the solid assumes a characteristic squared-Lorentzian form, showing a decay of short-range order over a length scale of 20 nm. Crosslinks are observed to disorder the system by decreasing the correlation length, which we show not to be a consequence of the random field. The coupling to random crosslinks is predicted to retard the decrease in the correlation length and hence found to stabilize the one-dimensional periodic layer structure against thermal fluctuations. The dependence of the correlation length on the crosslink density leads us to propose an estimate for the percolation limit of a smectic elastomer network.

Electromechanics of Monodomain Chiral Smectic C Elastomer: Mechanical Response to Electric Stimulation

So far, we have succeeded in obtaining monodomain SmC* elastomers with macroscopic C2 symmetry of the unwound SmC* state by biaxial mechanical deformation, such as two successive deformation processes and more perfectly mechanical shear deformation. Moreover, we confirmed these samples exhibits a biaxial shape change under thermal stimulation, which means spontaneous and reversible shear deformation in a heating and cooling process where successive phase transitions occur. The purpose of this paper is to show a macroscopic mechanical response to electric excitation of the monodomain SmC* elastomers obtained by the shear deformation. We estimate an electric-field-induced deformation ΔL = L(E) – L(0), which is defined as shrinkage and/or elongation in the direction of the layer normal. ΔL is plotted as a function of an electric field strength in Fig.1 which is measured just above the SmC*-to-SmA phase transition temperature at 80oC. The elastomer shrinks by application of the positive electric field, where ΔL decreases with increase of the field strength and reaches about -5 μm under an electric field at +2.5 V/μm. On the other hand, the elastomer elongates by application of the negative field, namely the electric field in the opposite direction; ΔL is estimated at about +3.5 μm under an electric field of -2.5 V/μm. Because a coupling between polarization and electric field contributes to the electric-field-induced deformation in the monodomain chiral smectic elastomer, the direction of deformation depends on the polarity of applied electric field in the field dependence of ΔL. Response and Characterization 7: 4:50 5:10 p.m.

Mechanical switching of ferroelectric rubber

At the A to C transition, smectic elastomers have recently been observed to undergo approximately 35% spontaneous shear strains. We first explicitly describe how strains of up to twice this value could be mechanically or electrically induced in Sm-C elastomers by rotation of the director on a cone around the layer normal with an elastic cost dependent on constraints. Second, for typical sample geometries, we give the various microstructures in Sm-C akin to those seen in nematic elastomers under distortions with constraints. It is possible to give explicit results for the nature of the textures. Chiral Sm-C elastomers are ferroelectric. We calculate how the polarization could be mechanically reversed by large, hard, or soft strains of the rubber depending upon sample geometry.

Crosslinking Topology and Chain Anisotropy of Smectic Liquid-Crystalline Elastomers

We have investigated the molecular mechanism of the spontaneous and reversible deformation of smectic elastomers during the isotropic-to-smectic phase transformation, by observing the shape change as well as an X-ray image of the elastomers for two different types of crosslinker, namely, a rod-like crosslinker and a hydroquinone-type crosslinker. The elastomer crosslinked by the rod-like crosslinker exhibits elongation corresponding to the change in the order parameter S during the isotropic-to-smectic phase transformation. As for another elastomer crosslinked by the hydroquinone-type crosslinker, which is too flexible to work as mesogen, the shape change is weakly coupled with the orientational order of mesogens. We conclude that the crosslinking topology attributed to the chemical constitution of the crosslinker predominantly controls the chain anisotropy of polymer network as well as the deformation behavior of smectic elastomers during the isotropic-to-smectic phase transformation.

Order, disorder and stretching of a smectic elastomer with ‘side-on’ mesogenic side groups

We report an X-ray study of the order in a ‘side-on’ smectic-A elastomer in which both the polymer backbone and the mesogenic side groups are, on average, parallel to the smectic layer normal. The present system shows in all phases locally correlated structures as evidenced by systematic diffuse scattering at small angles, attributed to the fluorinated end groups of the mesogens. The elastomer shows order of the smectic layers over a limited range in spite of the relatively low crosslink concentration (4%). Upon stretching along the layer normal we find a linear increase of the smectic layer period for small loading (elastic regime). These changes parallel the stress variation for the elastic part of the macroscopic stress–strain curve and saturate at larger strains. The attendant increase in the width of the X-ray peaks indicates a decrease of the average domain dimensions. In the plastic regime no distortion of the smectic monodomain structure or layer rotation is observed. Instead the stress is relieved by ‘melting’ of the layer structure giving rise to a nematic-like state with short-range layer correlations: at the final stage about four smectic layers.

Anomalous elasticity in nematic and smectic elastomer tubule phases

We study anomalous elasticity in the tubule phases of nematic and smectic elastomer membranes, which are flat in one direction and crumpled in another. These phases share the same macroscopic symmetry properties including spontaneously broken in-plane isotropy and hence belong to the same universality class. Below an upper critical value D_{c}=3 of the membranes' intrinsic dimension D , thermal fluctuations renormalize the elasticity with respect to elastic displacements along the tubule axis so that elastic moduli for compression along the tubule axis and for bending the tubule axis become length-scale dependent. This anomalous elasticity belongs to the same universality class as that of d -dimensional conventional smectic liquid crystals with D taking on the role of d . For physical tubule phases, D=2 , this anomaly is of power-law type and thus might by easier to detect experimentally than the logarithmic anomaly in conventional smectics.

Smectic-Ctilt under shear in smectic-Aelastomers

Stenull and Lubensky [Phys. Rev. E 76, 011706 (2007)] have argued that shear strain and tilt of the director relative to the layer normal are coupled in smectic elastomers and that the imposition of one necessarily leads to the development of the other. This means, in particular, that a smectic-A elastomer subjected to a simple shear will develop smectic-C-like tilt of the director. Recently, Kramer and Finkelmann [e-print arXiv:0708.2024; Phys. Rev. E 78, 021704 (2008)], performed shear experiments on smectic-A elastomers using two different shear geometries. One of the experiments, which implements simple shear, produces clear evidence for the development of smectic-C-like tilt. Here, we generalize a model for smectic elastomers introduced by Adams and Warner [Phys. Rev. E 71, 021708 (2005)] and use it to study the magnitude of SmC-like tilt under shear for the two geometries investigated by Kramer and Finkelmann. Using reasonable estimates of model parameters, we estimate the tilt angle for both geometries, and we compare our estimates to the experimental results. The other shear geometry is problematic since it introduces additional in-plane compressions in a sheetlike sample, thus inducing instabilities that we discuss.