Side-chain Liquid Crystalline Elastomers Research Articles

Preparation and properties of highly flexible cholesteric side-chain liquid crystalline elastomers with selective reflective properties in the visible region

ABSTRACT In this paper, we have synthesised cholesteryl 4-allyloxybenzoate (MB), crosslinking agent dodecane-1,12-diyl bis(undec-10-enoate) (MG), and A’BGF series liquid crystalline elastomer (LCE) films which take polymethylhydrosiloxane (A’) as the backbone, MB as the cholesteric liquid crystal (LC) monomer, and MG and bis-vinyl silicone oil (MF) with different reactivities as cross-linking agent for exploring the effect of the change of MF content on the properties of the elastomeric films. The experimental results show that the synthesised MB, MG, and the LCE films are all consistent with the molecular design; MB is an enantiotropic cholesteric LC. The synthesised LCE films are cholesteric LCs, have certain selective reflection characteristic, higher thermal stability, and their Tg decreased down to 3.9°C as MF increasing. During the heating and cooling, the liquid crystalline properties of the LCE films have a good reversibility. Among the synthesised LCE films, A’BGF-2 has stretching radio of 264.8% at room temperature, with the best elastic properties. By adjusting the ratio of MF, the content of small-molecule MB remained in the films can be changed, which will affect the optical properties and elasticity of the LCE films. The experiments provide a method for the preparation of highly elastic side chain LCE film.

Mechanical Manipulation of Diffractive Properties of Optical Holographic Gratings from Liquid Crystalline Elastomers

An appealing property of optical diffractive structures from elastomeric materials is a possibility to regulate their optical patterns and consequently also their diffractive features with mechanical straining. We investigated the effect of strain on diffraction characteristics of holographic gratings recorded in a monodomain side-chain liquid crystalline elastomer. The strain was imposed either parallel or perpendicular to the initial alignment direction of the material. At temperatures far below the nematic–paranematic phase transition, straining along the initial alignment affects mainly the diffraction pattern, while the diffraction efficiency remains almost constant. In contrast, at temperatures close to the nematic–paranematic phase transition, the diffraction efficiency is also significantly affected. Straining in the direction perpendicular to the initial alignment strongly and diversely influences both the diffraction pattern and the diffraction efficiency. The difference between the two cases is attributed to shear–stripe domains, which form only during straining perpendicular to the initial alignment and cause optical diffraction that competes with the diffraction from the holographic grating structure.

Synthesis and characterisation of novel side-chain chiral liquid crystalline elastomers with long dimer mesogens

ABSTRACTIn this work, novel nematic monomer (M1), chiral monomer (M2) and crosslinker (CL) have been synthesised to form a series of side chain chiral liquid crystalline elastomers (ChLCEs) LCE1-LCE7 by graft copolymerisation with poly(methylhydrogeno)siloxane backbone. M1 and M2 were both dimer type with great length, and the chiral 2-octyl terminal group was seldom used in liquid crystal elastomer system. The chemical structures, thermal stability and LC phase behaviours of monomers, crosslinker and ChLCEs were examined by different experimental techniques. The helical structure of combining the chiral and achiral liquid crystalline side chains endowed the obtained ChLCEs with chiral nematic liquid crystalline properties. By changing the dosages of the crosslinker, the mesomorphic properties of ChLCEs could be adjusted. The mesophase-isotropic phase range of the ChLCEs were slightly broadened with increasing addition of CL according to the differential scanning calorimetry (DSC). The synthesised ChLCEs exhibited excellent thermal stability from thermal analysis (TG) results. The effective crosslink density (c) of the ChLCEs was characterised by swelling experiments showing the molecular weight between the crosslinking points decreasing with the increase of the CL. The FT-IR imaging system indicates the functional groups of all the ChLCEs are well distributed.

Novel cholesteric liquid crystalline elastomers containing dimer type nematic and chiral liquid crystalline side-chains

A new set of cholesteric side chain liquid crystalline elastomers (ChLCEs) E1–E7 were graft copolymerized by hydrosilylation reaction with poly(methylhydrogeno)siloxane, nematic monomer (M1), chiral monomer (M2), and crosslinking agent (CL).

Synthesis and characterization of chiral smectic side-chain liquid crystalline elastomers containing nematic and chiral mesogens

A novel series of siloxane-based chiral smectic side-chain liquid crystalline elastomers containing nematic and chiral mesogens were fabricated through synthesis involving a one-step hydrosilication reaction via a liquid crystalline crosslinking agent containing smectic and nematic phases.

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.

Synthesis and characterisation of chiral liquid crystalline elastomers containing four-arm crosslinking agent

A new series of side-chain chiral liquid crystalline elastomers derived from M1 (cholest-5-3-ol(3β)-4-(2-propen-yloxy)]benzoate) and MC(2,5-[3,5-bis(4-(3-(4-(allyloxy)phenyl)propanoyloxy)benzoyloxy)benzoic acid]isosorbide diester). The structures of monomers and elastomers measured by using Proton Nuclear Magnetic Resonance Spectra (1H-NMR) and Fourier transform infrared spectroscopy (FTIR) separately are consistent with our design. IIP~VIP all appeared blue Grandjean (GJ) texture on the heating cycle or cooling cycle. The glass sheets of IIP~VIP were made under 150°C and measured its ultraviolet–visible spectrophotometry by PerkinElmer Lambda 950 instrument (Shelton, CT, USA). IIP~VIP all have absorptions at about 481~483 and 561~562 nm. The optical activities were measured at different temperatures on heating and cooling cycles. And the blue selective reflection of IIP~VIP on the round glass sheet can be seen. The elastomers containing less than 6 mol% of the crosslinking units displayed elasticity, reversible phase transition and high thermal stability. The glass transition temperatures reduced first and then increased, the isotropisation temperatures and the mesophase temperature ranges increased first and then decreased with increasing content of crosslinking unit. The thermogravimetric analysis (TGA) results showed that the temperatures at which 5% weight loss occurred (Td) were greater than 310°C for all the polymers.

Effect of co-monomers' relative concentration on self-assembling behaviour of side-chain liquid crystalline elastomers

A new series of liquid single crystal elastomers having a nematic–SmA and a direct isotropic–SmA phase transition.

Chiral Liquid-Crystalline Elastomers Bearing Side-Chain Fluorinated Groups and Chiral Crosslinking Units

Chiral fluorinated side-chain liquid-crystalline elastomers (LCEs)IP-IVPwere graft copolymerized by hydrosilylation reaction. The chemical structure, liquid-crystalline behavior and polarization property were characterized by use of various experimental techniques. The effective crosslink density of the chiral LCEs was studied by swelling experiments. All the samplesIP-IVPdisplayed chiral smectic C mesophase (SC*) on heating and cooling cycles. With increasing chiral crosslinking units in the elastomer systems, the glass transition temperature and chiral smectic C mesophase-isotropic phase transition temperature of fluorinated elastomers increased slightly, indicating that the temperature range of SC*mesophase became narrow with increase of chiral crosslinking agents for all the chiral fluorinated elastomers. All the samplesIP-IVPshowed 0.12-0.16 μC/cm2of spontaneous polarization.

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.

Elaboration of Side-Chain Liquid-Crystalline Elastomers and Study of Their Swelling Behavior in Anisotropic Solvents

A poly [6-(4′-cyanophenyl-4″-phenoxy) alkyl acrylate] side chain liquid crystalline elastomer and a poly [n-butylacrylate] network were synthesized via photopolymerization using 1,6-hexanediol diacrylate as a crosslinking agent. The materials obtained were characterized by differential scanning calorimetry and polarized optical microscopy. In particular, the latter method allowed to follow the swelling behavior of the polymer networks in a low molar weight liquid crystal (LC) and to observe the formation of LC gels. In the case of the anisotropic network, a stable nematic gel phase and a miscibility gap were observed, in relationship with the nematic-isotropic transition temperatures of both LC-solvent and LC-gel. The experimental phase diagrams were well reproduced by a mean field model combining the Flory-Rehner model for isotropic mixing and the Maier Saupe theory for nematic ordering.

Influence of Interim Alkyl Chain Length on Phase Transitions and Wide-Band Reflective Behaviors of Side-Chain Liquid Crystalline Elastomers with Binaphthalene Crosslinkings

A series of side-chain polysiloxane liquid crystalline elastomers, E-Cm (where m is the number of carbon atoms in the interim alkyl groups, and m = 4, 6, 8, 10), with binaphthalene derivatives as crosslinkings, were designed and synthesized. The mesophase structures of these elastomers were dependent on the value of m. The elastomers with m ≤ 8 could form cholesteric (Ch) phases, while both smectic A and Ch phases could be formed for the elastomer with m = 10. Blue phases can be achieved when m ≥ 8. Besides, the helical twisting power (HTP) of all the elastomers exhibits a turning point with changing temperature. Both the turning temperature and HTP have an apparent dependence on the value of m. By adjusting the interim alkyl chain length in the elastomers, both visible and near-infrared light wide-band reflective films can be obtained. The broadband reflection mechanism was confirmed by SEM investigations.

Preparation and Characterization of Chiral Cyclosiloxane-Based Liquid-Crystalline Elastomers Bearing Menthyl Groups

Chiral side-chain liquid-crystalline elastomers (LCEs) IP~IVP containig menthyl groups were synthesized by a one-step hydrosilylation reaction. The chemical structures and LC properties of the monomers and polymers were characterized by use of various experimental techniques. The effect of crosslinking mesogens on mesomorphic properties of the chiral LCEs was studied by swelling experiments. All the samples IP~IVP showed cholesteric mesophase when they were heated and cooled, proved by visual observation and X-ray measurements. The glass transition temperature (Tg) of elastomers increased slightly with increase of crosslinking mesogens in the polymer systems, but mesophase-isotropic phase transition temperature (Ti) decreased slightly, suggesting that the temperature range of mesophase became narrow with increase of crosslinking mesogens for all the elastomers. 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.

Volume phase transitions of biaxial nematic elastomers

We present a mean-field theory to describe biaxial nematic phases of side-chain liquid crystalline elastomers. Novel biaxial nematic phases are theoretically predicted in a side-chain liquid crystalline polymer and gel, where side chains (mesogens) and rigid-backbone chains favor mutually perpendicular orientations. We calculate uniaxial and biaxial orientational order parameters and examine deformations of the gel and stable biaxial nematic phases of the liquid crystalline gel dissolved in isotropic solvents. We predict first-order uniaxial-biaxial nematic phase transitions of the gel and the volume of the gel is discontinuously changed at the phase transition temperature.

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.

Reviewing the Mechanisms That Affect the Critical Behaviour of Nematic Side-Chain and Main-Chain Elastomers

A review of the mechanisms that influence the critical behaviour of the nematic monodomain main-chain and side-chain liquid-crystalline elastomers is presented. The crosslinking density, the applied mechanical load and the temperature at which the second crosslinking step takes place are examples of such mechanisms. This knowledge is a powerful tool when the tuning of the critical behaviour and, thus, of the thermo-mechanical response of the elastomers is needed.

Opto‐Mechanical Effect in Photoactive Nematic Side‐Chain Liquid‐Crystalline Elastomers

The mechanical behaviour of monodomain nematic side-chain liquid-crystalline elastomers containing azoderivatives as pendant groups or crosslinkers has been studied under UV irradiation and in the darkness at different temperatures. From the evaluation of the opto-mechanical experiments, the mechanical efficiency, kinetic rates, activation energies and the isomerization mechanism of the azocompounds in the liquid-crystalline matrix could be determined, as well as the effect of the chemical constitution of the azobenzene derivatives and their role in the elastomeric network.

Calorimetric study of the Paranematic-to-Nematic transition of polydomain side-chain liquid-crystalline elastomers with different mesogen composition

The phase transition behaviour of various nematic side-chain liquid-crystalline elastomers with different mesogen composition has been explored by means of high-resolution ac calorimetry. Polydomain samples of the same crosslinking density and different type of mesogens have been investigated. The results show a strong dependence of the phase transition features upon the composition of the mesogen. The distance from the critical point, reflected in the sharpness of the heat capacity anomalies, increases when adding a shorter-length mesogen. The results provide new insight for the impact of mesogens on the thermodynamic behaviour and, thus, on the thermomechanical response of nematic liquid-crystalline elastomers.

Three stage-volume phase transitions of a side-chain liquid crystalline elastomer immersed in nematic solvents

We theoretically study volume phase transitions of a side-chain liquid crystalline elastomer (LCE) immersed in nematic solvents. Six different uniaxial nematic phases (N(1 parallel), N(1 perpendicular), N(2 parallel), N(2 perpendicular), N(3 parallel), and N(3 perpendicular)) are defined by using orientational order parameter S(m) of a side-chain mesogen, S(b) of a backbone chain, and S(0) of a nematic solvent inside the gel. We derive the free energy of a side-chain LCE dissolved in nematic solvents and calculate the swelling behavior of the LCE, the order parameters, and the deformations of the LCE as a function of temperature. We find various phase transitions of the LCE, such as the isotropic (I)-N(1 parallel)-N(3 perpendicular) and the I-N(2 parallel)-N(3 parallel), etc., accompanied by volume phase transitions. These results present new concepts in the structure of a side-chain LCE dispersed in nematic solvent molecules.

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.