Liquid Single Crystal Elastomers Research Articles

On the nonlinear stress-strain behavior of nematic elastomers – materials of two coupled preferred directions

We present a nonlinear macroscopic model in which nematic side-chain liquid single crystal elastomers are understood as materials that show two preferred directions. One of the two directions is connected to the director of the liquid crystalline phase and the other one becomes anchored in the polymer network during the procedure of synthesis. The specific properties of the materials arise from the coupling between these two preferred directions. We take into account this coupling via the variables of relative rotations between the two directions. For this purpose, we have extended the variables of relative rotations to the nonlinear regime. In addition, we generalize the concept in such a way that it can also be used for the description of other systems coupling two preferred directions. In order to test our picture, we compare its predictions to the experimental observations on nematic monodomain elastomers. As a result, we find that our model describes the nonlinear strain-induced director reorientation and the related plateaulike behavior in the stress-strain relation, which are characteristic of these materials. In addition, our model avoids the unphysical notion of a vanishing or small linear elastic shear modulus. Finally, we demonstrate that ordinary nonlinear elastic behavior of the materials, i.e., not connected to any reorientation of the director field, also plays an important role in the appearance of the stress-strain curves and must be taken into account.

Instabilities in nematic elastomers in external electric and magnetic fields

In this paper we study the macroscopic behavior of nematic side-chain liquid single crystal elastomers exposed to an external electric or magnetic field. For this purpose we use the framework of a continuum model. The geometries investigated comprise the bend and the twist geometry known from the classical Frederiks transition in low molecular weight liquid crystals. For the bend geometry we find a laterally homogeneous and a two-dimensional undulatory instability, which may compete at onset. In the case of the twist geometry three instabilities can occur at onset, two of which are two dimensional and clearly show undulations. As a major result we propose how the values of the twist coefficient K(2) and the values of the material parameters D(1) and D(2) connected to relative rotations between the director field and the polymer network can be determined from experimental observations. In addition, we explain why a twist experiment is probably the most suitable set-up in order to measure the material parameter D(1).

Criticality Controlled by Cross-Linking Density in Liquid Single-Crystal Elastomers

A high-resolution calorimetry and deuteron-nuclear magnetic resonance study of a paranematic-nematic phase transition was performed on liquid single-crystal elastomers. We show that density variations of both rodlike and pointlike cross-links strongly affect the mean value and the dispersion of local mechanical fields. The system exhibits an inherent weakly disordered orientational state composed of regions with the temperature profile of the nematic order parameter ranging from first order to supercritical. On increasing the cross-linking density the predominantly first order thermodynamic response transforms into a predominantly supercritical one.

Shape Anisotropy and Optical Birefringence Measurements of Dry and Swollen Liquid Single Crystal Elastomers

We study the relation between the shape anisotropy changes, Δα, and the optical birefringence, Δ n , of dry and swollen rigid bi-functionally (8.0 mol %, 8A2) and flexible tri-functionally [5 mol % (5V3) and 7 mol % (7V3)] cross-linked liquid single crystal elastomers (LSCEs) as a function of temperature. 4- n -pentyl-4-cyanobiphenyl (5CB) is used as the solvent. Plotting Δα as a function of Δ n , a linear dependence is observed for all LSCE samples. This indicates that Δα results from the liquid crystal ordering. The largest Δα (∼0.2 at T =30 °C) is observed in dry 8A2, which has a large frozen-in orientational order (FOO). The swollen samples show much smaller Δα, ∼0.03–0.05, because of a smaller modulus elasticity after the swelling process.

Trifunctionally Cross-Linked Liquid Single Crystal Elastomers: Swelling Dynamics and Electromechanical Effects

Here, we study the influence of cross-linking density, X, on the swelling, thermal and electric field response of trifunctionally cross-linked liquid single crystal elastomers (V3 LSCEs) swollen with low molecular weight liquid crystals, 4-n-pentyl-4-cyanobiphenyl (5CB). The cross-linker concentrations are X3V3=3.3 mol %, X5V3=5 mol %, and X7V3=7 mol %. LSCE networks are characterized by frozen-in orientation order, P. Xc is a constitutional critical point for LSCE networks. When X<Xc (P=0), there are no shape changes and a nematic-isotropic transition takes place at TNI. When X>Xc (P>0), supercritical behavior prevails over a range of temperatures, ΔTs. Taking ΔTs∝(X-Xc)1/2, we find a critical cross-linking density, Xc≈3%, for V3 LSCEs. 5CB diffuses ⊥n into V3 similar to isotropic gels with typical times, τ⊥n in minutes, that decreases with increasing (X-Xc). Swelling with reorientation effects is initiated by 5CB propagating into V3 followed by relaxation of the front profile with τ⊥(2) (twist), and τ|| (13) (splay-bend) that both increase nearly parabolically with increasing (X-Xc). Front speeds are 50% faster in 5V3 than in 7V3 but no fronts were observed in 3V3. Compared to dry V3 LSCE volumes, Vd, the swollen volumes, Vs, increased as X→Xc: Vs/Vd∼(X-Xc)-0.27 reducing the frozen-in orientational order at X to \ ildeX=X(Vd/Vs). The maximum electromechanical effect found in swollen V3 was δMAX∼(\ ildeX-\ ildeXc) with \ ildeXc∼0.25%. The size of the effect is much smaller than observed in 8A2 which has a larger P. For 7V3 the effect is about 4 times smaller and for 5V3, it is an order of magnitude smaller.

Cholesteric elastomers in external mechanical and electric fields

In our studies, we focus on the reaction of cholesteric side-chain liquid single-crystal elastomers (SCLSCEs) to static external mechanical and electric fields. By means of linearized continuum theory, different geometries are investigated: The mechanical forces are oriented in a direction either parallel or perpendicular to the axis of the cholesteric helix such that they lead to a compression or dilation of the elastomer. Whereas only a homogeneous deformation of the system is found for the parallel case, perpendicularly applied mechanical forces cause either twisting or untwisting of the cholesteric helix. This predominantly depends on the direction in which the director of the cholesteric phase is anchored at the boundaries of the elastomer, and on the sign of a material parameter that describes how deformations of the elastomer couple to the relative rotations between the elastomer and the director. It is also this material parameter that leads to an anisotropy of the mechanical reaction of the system to compression and dilation, due to the liquid crystalline order. The effect of an external electric field is studied when applied parallel to the helix axis of a perfect electric insulator. Here an instability arises at a threshold value of the field amplitude, where the latter results from a competition between the effects of the external electric field on the one hand and the influences of the boundaries of the system, the cholesteric order, and the coupling between the director and the polymer network on the other hand. The instability is either homogeneous in space in the directions perpendicular to the external electric field and includes homogeneous shearing, or, for certain values of the material parameters, there arise undulations of the elastomer and the director orientation perpendicular to the direction of the external electric field at onset. This describes a qualitatively new phenomenon not observed in cholesteric systems yet, as these undulations are due to the coupling of the director orientation to the deformations of the elastomer. Furthermore, we find that theoretically it should be possible to synthesize cholesteric and nematic SCLSCEs in which the director can be reoriented by an external electric field without macroscopically distorting the sample; the latter applies for one specific set of values for the material parameters involved. We also propose ways of experimental access to the material parameters in the general situation.

A New Type of Macroscopically Oriented Smectic‐A Liquid Crystal Elastomer

AbstractSmectic‐A liquid single crystal elastomers (SA‐LSCE) are materials where the rubber elasticity of the polymer network is combined with the one‐dimensional positional long‐range order of mesogenic groups which are covalently attached to the network. In the systems investigated so far, a mechanical deformation of the network causes significant reorientation processes of the layered structure. We present a new type of SA‐LSCE in which this structure remains unaffected on mechanical deformation both parallel and perpendicular to the director. The thermoelastic behavior, macroscopic dimensions, and stress–strain measurements parallel as well as perpendicular to the director are investigated. X‐ray studies confirm that a deformation parallel and perpendicular to the layer normal does not alter the macroscopically ordered lamellar structure with respect to the order parameter and the layer correlation length. We propose a simple picture where defects within the lamellar structure might account for these findings.magnified image

Rotatoelectricity in cholesteric side-chain liquid single crystal elastomers

We analyze the phenomenon of rotatoelectricity which is characteristic of cholesteric side-chain liquid single crystal elastomers. Using a linearized macroscopic continuum description and our previous work we show that if such a material is exposed to a static external electric field oriented parallel to the cholesteric helical axis, the director of the liquid crystalline phase will rotate around the helical axis. The material considered is assumed to be a perfect electric insulator. We propose an experiment in which the effect of rotatoelectricity should be directly observable and from which the ratio of the material parameters involved will be accessible.

Thermo-mechanical properties of tri-functionally crosslinked liquid single crystal elastomers

We have investigated thermo-mechanical properties and optical birefringence of the V3 cross-linker liquid single crystal elastomers (V3LSCEs) with different concentrations of the flexible tri-functional cross-linker as a function of temperature. The length of V3LSCEs along the director n ˆ shrinks on heating while it expands perpendicular to n ˆ . Drastic length changes as well as large variations in the optical birefringence, Δ n, are observed near T c. The maximum contraction increases with increasing cross-linking concentration. When the anisotropy of the shape change is plotted as a function of Δ n, a linear dependence is observed only deep in the nematic phase for larger cross-linking concentrations.

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.

A Versatile Preparation Route for Thin Free‐Standing Liquid Single Crystal Elastomers

AbstractSummary: Thin films of nematic and cholesteric side‐chain polymers containing reactive benzophenone units can be macroscopically oriented by electric/magnetic fields or surface interactions. After UV‐irradiation, liquid single crystal elastomers (LSCEs) are formed. With this simple, new pathway, macroscopically ordered free‐standing LSCE films are easily accessible having a thickness in the range of about 100 nm to 100 μm as outlined in Figure. magnified image

Hystereses of volume changes in liquid single crystal elastomers swollen with low molecular weight liquid crystal

The hystereses of volume changes in liquid single crystal elastomers (LSCEs) swollen with a low molecular weight liquid crystal (LMWLC), 5CB, are studied as a function of temperature. The swollen LSCE shows significant hystereses in the volume changes at temperatures T NI and T A during the processes on heating and on cooling, where the temperature T NI is the apparent nematic-isotropic transition for outside LMWLC and T A the nematic-isotropic transition for LMWLC inside the LSCE. No significant hysteresis at T B however can be observed which is the nematic-isotropic phase transition temperature for equilibrium shape of the swollen LSCE (networks).

Cover Picture: Macromol. Rapid Commun. 5/2004

Cover: The picture displays the X‐ray pattern of a highly ordered smectic A Liquid Single Crystal Elastomer (LSCE). The higher order reflexes indicate a nearly solidlike state of order.Further details can be found in the Communication by E. Nishikawa, J. Yamamoto, H. Yokoyama, and H. Finkelmann* on page 611.

Birefringence measurement of liquid single crystal elastomer swollen with low molecular weight liquid crystal

We experimentally measured the birefringence of a liquid single crystal elastomer (LSCE) swollen with a low molecular weight liquid crystal (LMWLC), 5CB, by polarizing microscopy as a function of temperature. The optical intensity of swollen LSCE shows significant intensity changes at temperatures T NI, T A and T B that indicates a variety of different transitions. A temperature T NI is known as the apparent nematic-isotropic transition for outside LMWLC, T A and T B are interpreted as the nematic-isotropic transition for LMWLC inside the LSCE and for the swollen LSCE, respectively.

Cholesteric Liquid Single‐Crystal Elastomers (LSCE) Obtained by the Anisotropic Deswelling Method

Cholesteric liquid single-crystal elastomers (LSCE) were obtained by an anisotropic deswelling method. The resulting network is investigated by means of optical microscopy and X-ray measurements, which reveals that this orientation method is suitable to obtain a highly ordered cholesteric LSCE with the helical axis being perpendicular to the biaxial extension axis.

A Smectic A Liquid Single Crystal Elastomer (LSCE): Phase Behavior and Mechanical Anisotropy

The analysis of the phase behavior of a smectic A (SA) elastomer reveals a nematic phase existing within a small temperature range below the isotropic state. Stress-optical measurements in the pretransformational regime of the isotropic state indicate smectic as well as nematic fluctuations yielding a critical exponent of γ = 0.65. The formation of the liquid single crystal elastomer (LSCE) at the isotropic to liquid crystalline phase transformation equals a nematic LSCE. At the nematic to SA phase transformation, the orientation of the director remains constant while the tendency of the network strands towards an oblate equilibrium conformation is suppressed by the high modulus parallel to the smectic layer normal. The mechanical anisotropy of the SA-LSCE as a function of the temperature is characterized by entropy elasticity perpendicular to the smectic layer normal. Parallel to the layer normal the mechanical response is determined by the enthalpy elastic response of the smectic layers having a modulus larger by about two orders of magnitude. In this direction the modulus decreases linearly with increasing temperature and reflects the falling stability of the layers. Accordingly, above a deformation of about 2% the homogeneous layered structure breaks down at a threshold stress that also falls linearly with increasing temperature while the threshold strain remains constant at about 2% elongation.

Cholesteric Liquid Single-Crystal Elastomers (LSCE) Obtained by the Anisotropic Deswelling Method

Cholesteric Liquid Single-Crystal Elastomers (LSCE) Obtained by the Anisotropic Deswelling Method

Dynamics of Backbone Chain and Side Groups in a Nematic Liquid Single Crystal Elastomer Studied by Combined Mechanical and Stress-Optical Measurements

Dynamic mechanical and stress-optical measurements on nematic liquid single elastomers have been carried out both in the nematic and in the isotropic phase. In case of shear stress and in case of tensile stress applied parallel to the director the compliances were not affected by the nematic-isotropic transition. For tensile stress oriented perpendicular an increased compliance was observed in the vicinity of the phase transition. The stress-optical coefficients for tensile stresses applied pararallel and perpendicular to the director both increase on approaching the nematic-isotropic transition. In the isotropic phase its temperature dependence is well described by the Landau-de Gennes theory. Far from the transition temperature in the nematic phase other mechanisms superpose this behavior, leading to negative stress-optical coeffients for stress parallel, and positive coefficients perpendicular to the director.

Smectic-A liquid single crystal elastomers - strain induced break-down of smectic layers

The mechanical response of a smectic-A liquid-crystalline side-chain polymer network with macroscopically ordered layers (smectic-A liquid single crystal elastomer) to extensive stress applied parallel to the layer normal is described. By stress-strain measurements a characteristic threshold strain of about 3% is observed, at which the elastic modulus changes dramatically. In the regime of small deformation the elastomer remains optically transparent possessing a large Young modulus. The macroscopically ordered smectic-A structure is conserved, probably showing some layer undulation. Above the threshold strain, however, the monodomain structure breaks down and the elastomer becomes completely opaque showing a much smaller elastic modulus. By strain dependent X-ray scattering measurements the structure of the polydomain is analysed. A splitting of the small angle reflection into four maxima is observed, indicating a layer rotation. The intensities of these reflections, however, decrease drastically with strain suggesting a partial distortion of the smectic layers toward a nematic like structure.

Smectic-A liquid single crystal elastomers – strain induced break-down of smectic layers

Smectic-A liquid single crystal elastomers – strain induced break-down of smectic layers