Cholesteric Elastomers Research Articles

Volume phase transitions of cholesteric liquid crystalline gels.

We present a mean field theory to describe anisotropic deformations of a cholesteric elastomer without solvent molecules and a cholesteric liquid crystalline gel immersed in isotropic solvents at a thermal equilibrium state. Based on the neoclassical rubber theory of nematic elastomers, we derive an elastic energy and a twist distortion energy, which are important to determine the shape of a cholesteric elastomer (or gel). We demonstrate that when the elastic energy dominates in the free energy, the cholesteric elastomer causes a spontaneous compression in the pitch axis and elongates along the director on the plane perpendicular to the pitch axis. Our theory can qualitatively describe the experimental results of a cholesteric elastomer. We also predict the first-order volume phase transitions and anisotropic deformations of a gel at the cholesteric-isotropic phase transition temperature. Depending on a chirality of a gel, we find a prolate or oblate shape of cholesteric gels.

Study on the Optical and Mesomorphic Properties of Cholesteric Side-Chain Liquid Crystalline Elastomers Containing a Nematic Cross-Linking Agent

To study the effect of the content of the nematic cross-linking units on the mesophase behaviors, the optical and mesomorphic properties of liquid crystalline elastomers (LCEs), aseries of LCEs have been synthesized by hydrosilylation reaction with poly(methylhydrogeno)siloxane, a cholesteric liquid crystalline monomer, and a nematic cross-linking agent. The chemical structures and properties of the synthesized LCEs have been investigated by use of various techniques. Homopolymer P0 bearing only cholesteric component displays a smectic A phase, but elastomers P1-P5 containing different content of nematic cross-linking units show a cholesteric mesophase. The reflection wavelengths of theLCEs show a weak temperature dependence at lower temperatures but a strong temperature dependence at higher temperatures. Furthermore, the maximum reflection wavelengths of the LCEs can be stabilized over a wide temperature range when the LCEs are heated, suggesting that the helical structure and pitch of the cholesteric phase can be stabilized with a polymer network.

New chiral liquid crystalline monomers, polymers, and elastomers derived from menthol derivatives: synthesis and mesomorphism

To investigate liquid crystalline properties and structure relationships of chiral compounds based on menthol, a series of new chiral monomers derived from menthol derivatives, a mesogenic crosslinking agent, the corresponding side chain homopolymers with siloxane backbone, and cholesteric elastomers were synthesized. The structures and purity of these chiral compounds obtained in this study were characterized using FTIR, 1H NMR, and elemental analyses. The mesomorphism and thermal stabilities were investigated using differential scanning calorimetry, polarizing optical microscopy, thermogravimetric analysis, and X-ray diffraction. The selective reflection of light for the chiral monomers was studied with UV/visible/near IR. The effect of the crosslinking agent content on the phase transition temperatures of the elastomers was discussed. It was found that these chiral monomers seemed beneficial for the formation of the mesophases when a flexible spacer was inserted between the mesogenic core and the terminal menthyl groups. All obtained chiral monomers showed a cholesteric phase, chiral smectic C phase, and cubic blue phase. The crosslinking agent exhibited a smectic A (SA) phase and nematic phase. However, their corresponding homopolymers with siloxane chains tended to form a lower order SA phase. With an increase of the mesogenic rigidity, the melting temperatures, glass transition temperatures (T g), and isotropic temperatures (T i) of chiral monomers or homopolymers all increased. For the elastomers, general tendency was toward increased T g and T i with increasing the crosslinking agent content.

Precise determination of the Poisson ratio in soft materials with 2D digital image correlation

In this article, we demonstrate the application of digital image correlation (DIC) in evaluating the strains and Poisson ratio of a range of soft materials in terms of their spatial and temporal resolutions. Homogeneous samples of polydimethylsiloxane (PDMS) elastomer were used as control substance and were measured to have Poisson ratios of 0.498, 0.503, 0.500, and 0.499, in agreement with the reported incompressible value of 0.50. Two carbon nanotube (CNT) elastomer composites of identical composition, but one of a homogeneous and the other of a highly inhomogeneous CNT distribution, were used to determine the spatial resolution with good results. The relaxation of a polydomain liquid crystal elastomer (3D-LCE), a cholesteric liquid crystal elastomer (CLCE), and a polyacrylamide gel (PAAm) in water, were used to determine the temporal resolution of the technique. A video at 25 fps was used to evaluate the time dependence of the 3D-LCE over which time an increase in the Poisson ratio was observed. The 3D-LCE relaxes from its initial state at 0.42 to 0.50, converging towards incompressibility at equilibrium. The CLCE was found to have a similar initial value of 0.44 but converged to ∼0.60, a consequence of its anisotropic elastic nature. PAAm gel relaxation in water was studied over a time period of 7 hours with digital images taken periodically every minute. Its Poisson ratio was found to decrease smoothly from 0.50 to 0.26, with an accompanying reduction in force. The equilibrium result compares well to the 0.25 value predicted by a theory of the strain-induced swelling of dilute gels. In summary, we find DIC to be a powerful and easy to implement method of accurately measuring local strains in a range of soft materials.

Effect of cholesteric liquid crystalline elastomer with binaphthalene crosslinkings on thermal and optical properties of a liquid crystal that show smectic A‐cholesteric phase transition

A side‐chain polysiloxane cholesteric liquid crystalline elastomer (ChLCE) with binaphthalene derivate as crosslinkings and cholesterol derivate as liquid crystalline monomers was designed and synthesized. A binaphthyl chiral dopant (CD) was synthesized as well. The chemical structures and liquid crystalline properties of the ChLCE and the CD were characterized by Fourier transform infrared spectroscopy, proton nuclear magnetic resonance spectroscopy, element analyses, differential scanning calorimetry and polarizing optical microscopy measurements. The helical twisting power of the ChLCE exhibited a turning point with changing temperature and was smaller than that of the CD. In addition, the effect of the ChLCE on phase transition temperatures and thermal‐optical properties of a liquid crystal that show smectic A (SmA)‐cholesteric (Ch) phase transition was studied. Worthily, the testing of the reflection wavelength with changing temperature suggested that the adding of the ChLCE in liquid crystals that show SmA‐Ch phase transition can expedite their SmA‐Ch transition. In addition, the network structure of the ChLCE may play a significant role in the accelerating of the transition. These properties provided theoretical and experimental foundations for applying ChLCE in thermally sensitive liquid crystal devices requiring fast response, such as thermally controllable windows, materials and displays. Copyright © 2012 John Wiley & Sons, Ltd.

Piezochromic Polymer Materials Displaying Pressure Changes in Bar-Ranges

A piezochromic material exhibiting a pressure dependent reversible shift of the selective reflection wavelength via the entire visible range is developed. The material consists of a cholesteric mixture embedded in a cholesteric elastomer matrix bearing mesogenic side chains with similar molecular structure. The change of the selective reflection has its origin in a compressible helix structure. Thus, the pitch length is switched by pressure changes. To trigger the reversible piezochromic effect already pressure changes in bar-range are sufficient. The pressure controlled colour changes are excellent detectable by the human eye and still appear even after 100 cycles.

Synthesis and Phase Behavior of Chiral Liquid Crystal Elastomers Containing Cholesteryl Group

The synthesis of new side chain cholesteric liquid crystalline elastomers (ChLCEs) containing the cholesteric monomer M and the flexible non-mesogenic crosslinking agent C, is described. The selective reflection of light for M was characterized with UV/Visible/NIR. The phase behavior and mesomorphism were investigated by differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and X-ray diffraction (XRD). The effect of the content of crosslinking units on the phase behavior and mesomorphism of elastomers P1– P8is discussed. The ChLCEs exhibit elasticity, reversible phase transitions, and cholesteric Grandjean texture. The experimental results demonstrate that the glass transition temperature and isotropic temperature of ChLCEs decrease with increasing the content of crosslinking unit, but the cholesteric phase is not disturbed.

Liquid crystalline elastomers based on cyclic cyclosiloxane and mesogenic crosslinking units

Cholesteric liquid crystalline elastomers (LCEs) IP–VIP were graft copolymerized by a one-step hydrosilylation reaction with cyclo(pentamethylhydrogeno)siloxane, a cholesteric liquid crystalline (LC) monomer cholesteryl 3-(4-(undec-10-enoyloxy)phenyl)acrylate and crosslinking LC monomer biphenyl-4,4′-diyl bis(4-(allyloxy)benzoate). The effect of crosslinking mesogens on mesomorphic properties of the chiral LCEs was studied by swelling experiments. All the samples IP–VIP showed cholesteric mesophase when they were heated and cooled. The glass transition temperature (T g) of elastomers increased slightly with increase of crosslinking mesogens in the polymer systems, but mesophase–isotropic phase transition temperature (T i) decreased slightly, suggesting that the temperature range of mesophase became narrow with increase of crosslinking mesogens for all the elastomers. In XRD curves, all the samples IP–VIP exhibited diffuse reflections at 2θ ≈ 17° suggesting lack of the smectic-layered packing arrangement, and the intensity of these diffuse diffraction peaks decreases with increase of mesogenic crosslinking units, suggesting that the order between two neighbor LC molecules disturbed by the mesogenic crosslinking agents. The maximum reflection bands shift slightly to long wavelength and become broad at the same temperature, indicating that the helical structure is partially disrupted because of both the constraint of chemical crosslinking agents and the different mesogenic units.

Liquid crystalline properties of cholesteric elastomers based on a mesogenic monomer containing menthyl groups

A new chiral mesogenic monomer (MLC) based on menthyl groups and the corresponding cholesteric elastomers (LCE1–LCE3) were prepared. Their chemical structures and purity were characterized by Fourier transform infrared, nuclear magnetic resonance and elemental analyses. The liquid crystalline properties were investigated by differential scanning calorimetry, polarizing optical microscopy, thermogravimetric analysis and X-ray diffraction. The selective reflection of light for MLC was characterized with ultraviolet/visible/near infrared. The effect of the content of crosslinking units on the liquid crystalline behaviour of LCE1–LCE3 is discussed. The experimental results show that the chiral monomer and the corresponding elastomers containing menthyl groups can show mesomorphism when a flexible linkage chain is inserted between the mesogenic core and the bulky terminal menthyl fragments to reduce the steric effect. MLC exhibited a chiral smectic C phase, a cholesteric phase and a cubic blue phase. LCE1–LCE3 with a low content of crosslinking units showed a cholesteric phase because of the introduction of the nematic crosslinking unit. With increasing content of crosslinking units, the corresponding glass transition temperature increased, while the clearing temperature decreased. Thermogravimetric analysis showed that LCE1–LCE3 have good thermal stability. Copyright © 2012 Society of Chemical Industry

Cholesteric monomer and elastomers containing menthyl groups: Synthesis and phase behavior

AbstractThe synthesis of a new cholesteric monomer (MLC) containing menthyl groups and a series of cholesteric elastomers (LCE1−LCE4) is described. Their chemical structures and purity were characterized by FTIR, 1H‐NMR, and elemental analyses. The phase behavior and thermal stability were investigated by differential scanning calorimetry, polarizing optical microscopy, X‐ray diffraction, and thermogravimetric analysis. By inserting a flexible spacer between the mesogenic core and the terminal menthyl groups, mesomorphism of MLC was realized. LCE1−LCE4 with low content of crosslinking unit exhibited cholesteric phase because of the introduction of the nematic crosslinking unit. This indicates that low levels of chemical crosslinking do not significantly affect the phase behavior and mesomorphism of the elastomers, and reversible mesophase transitions can be observed. In addition, with increasing the content of crosslinking unit, the corresponding Tg decreased for LCE1−LCE4, whereas their Ti did not remarkable change. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Mechanically tuned defect-mode multiplet for cholesteric photonic elastomers

The multiplet structure of the optical defect modes generated within the bandgap are analyzed by a finite number of identical equispaced twist defects in axially elongated cholesteric elastomers. It is shown that n(0) defect modes induced by n(0) twist defects can be mechanically tuned. The transfer matrix is obtained and the defect frequencies when n(0)=1,2 are expressed in terms of twist angle, fractional shape anisotropy, and longitudinal deformation. For n(0)=2, the coupling between modes is analytically studied and it is shown that when the separation between defects is much larger than the line width, the defect modes do not interact and become degenerated.

Synthesis and phase behaviour of chiral liquid crystalline monomers based on menthyl groups, smectic polymers and cholesteric elastomers

The synthesis of new chiral liquid crystal monomers (M1 −M4 ) derived from menthyl groups, the corresponding smectic polymers (P1 −P4 ), and cholesteric elastomers (E1 −E4 ) based on M2 and crosslinking agent (CA ) is presented. The chemical structures and purity were characterised by Fourier transform infrared, 1H nuclear magnetic and elemental analyses. The phase behaviour and mesomorphism were investigated by differential scanning calorimetry, polarising optical microscopy, thermal gravimetric analysis and X-ray diffraction. The selective reflection of light for M1 −M4 was characterised with ultraviolet/visible/near infrared. The structure–property relationships of M1 −M4 and P1 −P4 were studied. The effect of the content of mesogenic crosslinking units on phase behaviour and mesomorphism of E1 −E4 was discussed. By inserting a flexible spacer between the mesogenic core and the terminal menthyl groups, M1 −M4 all showed strong mesomorphism and exhibited an oily streak and focal conic textures of the cholesteric phase, or the broken fan-shaped texture of the smectic C* phase and the platelet texture of a cubic blue phase. P1 −P4 exhibited batonnet textures of the smectic A phase, indicating that the liquid crystal polymers with siloxane chains tended to form a lower order smectic phase. For M1 −M4 or P1 −P4 , the melting temperature Tm , the isotropic temperature Ti and the glass transition temperature Tg increased, and the mesophase temperature range widened with increasing rigidity of the mesogenic units or decreasing spacer length. E1 −E4 exhibited a cholesteric texture because of the introduction of the nematic crosslinking unit. By increasing the content of the crosslinking unit, the general tendency is toward increased Tg and decreased Ti .

Synthesis and Mesophase Properties of a Series of Chiral Rod-Like Polysiloxanes

Cholesteric liquid crystals elastomers have been extensively studied during the last two decades. They showed Bragg-reflection in visible, Infrared and even UV wavelength Range and a particular circular polarization. Their special helical structure and the adjustment of pitch made them suitable for using as optical thin films. In this study, two kinds of mesogenic monomers undecylenate cholesteryl ester (M1) and p-phenylene di-4-(allyloxyl) benzoate (M2) were synthesized. Then the two monomers were fixed onto poly- (methylsiloxane) backbone by grafting polymerization using Dicyclopentadienylplatinum (II) Chloride as catalyst. A series of side-chain polymers with different monomer densities produced by the reaction of hydropolysiloxane and M1/M2with different molar ratios were obtained. The synthesized polymers were characterized by IR, H1NMR and GPC. The mesophases were characterized by using differential scanning calorimetry (DSC), polarizing optical microscopy (POM).

Wide-band reflective films produced by side-chain cholesteric liquid-crystalline elastomers derived from a binaphthalene crosslinking agent

A series of novel side-chain cholesteric liquid-crystalline elastomers based on polysiloxane and cholesterol derivate monomer were prepared by adopting a crosslinking agent containing binaphthalene group. The chemical structures and mesomorphic properties of the monomers and elastomers were confirmed by FT-IR, 1H NMR, DSC and POM measurements. Worthily, the elastomers exhibited unusual temperature dependence of the helical twisting power (HTP) which is demonstrated resulting from coordination of the crosslinking agent and the mesogenic units. With increase in temperature, the HTP of elastomers containing small quantity of the crosslinking unit exhibited a turning point, while that of elastomers comprising much more crosslinking unit shifted straight. Furthermore, a single layer wide-band reflective film with non-uniform pitch distribution was prepared by utilizing the HTP variety of elastomers with change in temperature. From scanning electron microscopy (SEM) investigations, the mechanism of the broadband reflection was verified.

Defect Mode in Axially Deformed Cholesteric Elastomers

We consider an axially elongated (or stretched) cholesteric elastomer, prolate or oblate, containing a spaceless twist defect. We show that the wavelength of its localized mode can be tuned by either axially compressing (or enlarging) the elastomer or by changing its pitch. We calculate the dwell time given by the inverse relative line width and exhibit that it is enhanced when the values of the deformation and pitch are near a locus in the parameter's sample that we call the pseudo-isotropic curve. We also show the density of states diverges in the pseudo-isotropic curve.

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.

Influence of different nematic crosslinking unit on mesomorphism of side-chain cholesteric elastomers containing menthyl groups

The synthesis of two cholesteric monomers (M1 and M2), nematic crosslinking agent (C1 and C2), and the corresponding side-chain elastomers containing menthyl groups (P1 and P2 series) is described. The mesomorphism was investigated by differential scanning calorimetry, polarizing optical microscopy, X-ray diffraction, and thermogravimetric analysis. The effect of the content of the different nematic crosslinking unit on the mesomorphism of the elastomers was discussed. M1 and M2 showed cholesteric and blue phases; C1 and C2 showed nematic phase. Because of the introduction of the nematic crosslinking unit, elastomers P1-1−P1-5 and P2-1−P2-5 exhibited cholesteric phase. With increasing the content of nematic crosslinking unit, T g of the obtained elastomers revealed an increased tendency, and T i of P1 series firstly increased then decreased, while T i of P2 series decreased the mesomorphism of the corresponding elastomers when the content of nematic crosslinking unit was 12 mol.%.

Mechanically controlled defect mode in cholesteric elastomers

We consider an axially elongated cholesteric elastomer having a twist defect. We show that its localized mode can be mechanically tuned, and the scaling of the inverse relative line width can be largely enhanced when the values of the deformation and shape anisotropy are near the pseudoisotropic curve. This choice causes a tremendous variation in the behavior of the photon dwell time in the defect mode, which then grows linearly versus the sample thickness. The shift of the defect wavelength, the reflection band width, and the angle between the electric and magnetic fields are also calculated.

Metastable Untwisted States in Cholesteric Elastomers

Metastable Untwisted States in Cholesteric Elastomers

Side-chain cholesteric liquid crystalline elastomers containing isosorbide as chiral agent – synthesis and characterisation

In this work the new-style nematic monomer M1 , chiral crosslinking reagent MC and a series of new side-chain cholesteric liquid crystalline elastomers derived from M1 and MC were prepared. The effect of the content of the chiral crosslinking unit on phase behaviour of the elastomers has been discussed. Polymer P1 showed nematic phase, P2 –P7 showed cholesteric phase, P3 formed Grandjean texture in the heating cycle and turned out a blue Grandjean texture in the cooling cycle, P2 –P3 with less than 6 mol% of chiral crosslinking agent gave rise to selective reflection. The elastomers containing less than 15 mol% of the crosslinking units displayed elasticity, reversible phase transition and high thermal stability. Experimental results demonstrated that the glass transition temperatures reduced first and then increased, and the isotropisation temperatures and the mesophase temperature ranges decreased with increasing content of crosslinking unit.