Side-chain Liquid Crystalline Elastomers Research Articles

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.

Nonlinear relative rotations in liquid crystalline elastomers

Relative rotations between the coupled subsystems of a complex material can become crucial in continuum modeling. In this paper the authors focus on the macroscopic description of side-chain liquid crystalline elastomers, where relative rotations between the polymer network and the director orientation associated with the liquid crystalline component are decisive. They extend the known expression for relative rotations to the nonlinear regime, within the framework of a continuum characterization of the materials. This allows the investigation of qualitatively different nonlinear effects determined by relative rotations, and they give an illustrative example. The formalism can easily be transferred to the macroscopic description of magnetic gels and will certainly be helpful in the characterization of other complex systems.

Molecular Dynamic Simulation of Side‐Chain Liquid Crystalline Elastomer Under Load

AbstractSummary: We present a molecular dynamic simulation of a side chain liquid crystalline elastomer (LCE) under load. The LCE is composed of a flexible tetrafunctional diamond like network with rod‐like mesogens attached to the network. As a precursor of the LC elastomer a flexible polymer network in a low molecular liquid‐crystal (LC) solvent was used. The phase behavior of the LCE under uniaxial stretching up to the deformations of λ = 1.5 and 2.0 at different densities was studied. As in the non‐stretched case upon density increase an isotropic to nematic phase transition occurs. However, in contrast to thermotropic side chain LC elastomers the stress induced shift transition is not observed. The stretching slightly increases the anisotropy of translational diffusion of mesogens in the nematic state. The stress‐strain dependence for LCE both in the isotropic and the nematic states is obtained. Elastic modulus increases at high values of order parameter.

Viscoelasticity of main chain liquid crystalline elastomers

Time–temperature superposition (TTS) principle was applied to dynamic mechanical analysis performed on two main-chain polydomain elastomers exhibiting a nematic and a smectic A phase. It was found that TTS did not hold neither across the nematic–isotropic nor the smectic–isotropic transitions. The nematic elastomer showed an increase in the storage modulus in the isotropic phase with respect to the nematic phase: this could be explained by means of dynamic soft elasticity, which has been claimed in some literature for side-chain liquid crystalline elastomers (SCLCEs), or in terms of the de Gennes model by a macroscopic/hydrodynamic description. The presence of the mesogen directly incorporated into the main chain increases the lifetimes of the elastic modes both in the isotropic and in the liquid crystalline (LC) phases, with respect to the SCLCEs. In the case of the smectic A elastomer, lifetimes on the order of 109s could be estimated.

Molecular Dynamic Simulation of Side‐Chain Liquid Crystalline Elastomer

AbstractSummary: Molecular dynamic simulation of side chain liquid crystalline elastomer has been carried out. As an initial state a flexible polymer network in a low molecular liquid‐crystal (LC) solvent was used. The LC solvent comprises of anisotropic rod‐like semiflexible linear molecules (mesogens) composed of particles bonded into the chain by FENE potential. Rigidity of LC molecules was induced by a bending potential. All interactions between nonbonded particles are described by a repulsive Lennard‐Jones potential. For the systems with different values of density and order parameter obtained after sufficiently long trajectory the attachment of ends of mesogens to the polymer network was simulated. The kinetic of the process of mesogens attachment to network was studied as well as morphology of attachment. The structural and dynamical behaviour of side chain LC elastomer was studied and compared with systems of polymer network in low molecular LC solvent.

Selected macroscopic properties of liquid crystalline elastomers.

In this short review we give an overview of selected macroscopic properties of sidechain liquid crystalline elastomers (LCEs) focusing on three closely related topics (a) the influence of relative rotations between the director and the strain field on various reorientation instabilities, (b) the nonlinear stress-strain curves for the polydomain-monodomain transition and for the reorientation transition in LCE monodomains and (c) the shear mechanical response of LCEs in the linear regime. We consider only already existing real materials and do not discuss hypothetical "ideal" systems. We conclude that all observations reported to date can be accounted for without invoking the concept of soft elasticity, but instead relying on macroscopic dynamics in the linear and the nonlinear domain.

Structures and properties of cholesteric liquid crystalline elastomers

The synthesis of new side-chain cholesteric liquid crystalline elastomers containing mesomorphic crosslinking agent M-1 and cholesteric monomer M-2 was described by a one-step hydrosilylation reaction. The chemical structures of the obtained network polymers were confirmed by FT-IR spectroscopy. The mesogenic properties and phase behavior were investigated by differential scanning calorimetry, polarizing optical microscopy, and X-ray diffraction measurements. The mesomorphic crosslinking agent was adopted to diminish the disturbance of the non-mesogenic crosslinking agent on liquid crystalline elastomers. The influence of the crosslinking units on the phase behavior was discussed. The liquid crystalline elastomers P-2–P-6 containing less than 10.8 mol% crosslinking agent showed elasticity, reversible phase transition, and cholesteric grandjean texture. The experimental results demonstrated that the glass transition temperatures and the isotropization temperatures of the networks polymers decreased a little with increasing concentration of crosslinking units but the cholesteric phase was not disturbed.

Mechanical properties of mono-domain side chain nematic elastomers.

We investigate the behavior of the shear rigidity modulus G = G' + iG" of three mono-domain side chain liquid-crystalline elastomers composed of side chain polysiloxanes cross-linked by either flexible or rigid cross-linkers. The measurements were taken in a frequency domain ranging from approximately 0.02 Hz to approximately 10(4) Hz applying the shear in a plane perpendicular to or containing the director. The measurements as a function of temperature show an anisotropy of G' which appears around T(NI), when decreasing the temperature, and which is due to the expected lowering of G'(//) coming from the coupling between the shear and the director. The measurements as a function of frequency show that G has two components for both geometries, in both the isotropic phase and in the nematic phase around the phase transition. One reflects the network behavior in its hydrodynamic regime ( G' is constant and G'' is approximately f, where f is the frequency), the other which appears at higher frequencies is characterized by a scaling law behavior (G' is approximately G" is approximately f(0.5)) of the Rouse type. We discuss the results in the framework of available theories and show that the three elastomers present a non-soft behavior, even for the elastomer for which the contrary was claimed, and that there is no separation of time scales between the director and the network. We also present data on a poly-domain sample and a non-mesomorphic one which complement these results.

Reply to the Commentary by E.M. Terentjev and M. Warner on “Mechanical properties of mono-domain side-chain nematic elastomers”

We argue that all dynamic mechanical experiments on side-chain liquid-crystalline elastomers performed up to now can be described without invoking the picture of dynamic soft or semi-soft elasticity.

Preparation and properties of siloxane liquid crystalline elastomers with a mesogenic crosslinking agent

A mesogenic crosslinking agent M-1 was synthesized to minimize the perturbations of non-mesogenic crosslinking agents in liquid crystalline elastomers. The synthesis of new side chain liquid crystalline elastomers containing the rigid mesogenic crosslinking agent M-1 and nematic monomer M-2 by a one-step hydrosilylation reaction is described. The chemical structures of the monomers and network polymers obtained were confirmed by FTIR and 1H NMR spectroscopy. The mesomorphic properties and phase behaviour were investigated by differential scanning calorimetry, polarizing optical microscopy, and X-ray diffraction. The influence of the crosslinking units on phase behaviour is discussed. Liquid crystalline elastomers containing less than 15 mol % of the crosslinking units showed elasticity, reversible phase transitions and a threaded texture. The experimental results demonstrated that the glass transition temperature of polymers P-1–7 increased with increasing concentration of crosslinking agent M-1; but the isotropic temperature and liquid crystalline range decreased slightly.

Synthesis and properties of cholesteric liquid crystalline elastomers

The synthesis of new side-chain cholesteric liquid crystalline elastomers, containing the flexible non-mesogenic crosslinking agent M-1 and the cholesteric monomer M-2, is described by a one-step hydrosilication reaction. The chemical structures of the monomers and network polymers obtained were confirmed by FT-IR spectroscopy. Their mesogenic properties and phase behavior were investigated by differential scanning calorimetry, polarizing optical microscopy, and x-ray diffraction measurements. The influence of the crosslinking units on the phase behavior is discussed. The network polymers showed elasticity, reversible phase transitions, and cholesteric Grandjean texture. The experimental results demonstrated that the glass transition temperatures and isotropization temperatures of network elastomers decreased as the concentration of crosslinking units was increased, but the cholesteric phase was not disturbed.

Side-Chain Cholesteric Liquid Crystalline Elastomers Derived from a Mesogenic Cross-Linking Agent

The synthesis of the two monomers M1 and M2 and a series of new side-chain cholesteric liquid crystalline elastomers P2−P8 is presented. The chemical structures of the monomers and elastomers obtained were confirmed by FTIR or 1H NMR spectroscopy. The cross-link density of the elastomers was determined by swelling experiments. The mesomorphic properties were investigated by differential scanning calorimetry (DSC), thermogravimetric analyses (TGA), polarizing optical microscopy (POM), and X-ray diffraction (XRD) measurements. Monomer M1 showed a cholesteric phase, and M2 revealed smectic and nematic phases. The effect of the cross-link density on the phase behavior of P2−P8 is discussed. Elastomers P2−P6 containing less than 12 mol % of the cross-linking units displayed elasticity, reversible liquid crystalline phase transition, wide mesophase temperature ranges, and high thermal stability. Elastomer P7 displayed stress-induced birefringence, and P8 showed only elasticity with no other texture. Experimental results demonstrated that the glass transition temperatures increased, and the isotropization temperatures and the mesophase temperature ranges of P2−P6 decreased with increasing cross-link density.

Synthesis and Properties of Side-Chain Cholesteric Liquid Crystalline Elastomers Containing Tetra-Vinyl Crosslinking Units

The synthesis of side-chain cholesteric liquid crystalline elastomers containing crosslinking agent pentaerythriol tetra-p-allyloxybenzoate (M-1) and mesogenic monomer 4-cholesteryl-4’-undecenyloyloxybenzoate (M-2) is presented by hydrosilication reaction. The chemical structures of the monomers and liquid crystalline networks were confirmed by Fourier transform infrared (FT-IR) and proton nuclear magnetic resonance (1H NMR) spectroscopy. Mesomorphic properties and phase behavior were investigated by differential scanning calorimetry, polarizing optical microscopy, and X-ray diffraction measurements. The influence of crosslinking units on phase behavior was discussed. The networks containing less than 8 mol% of crosslinking units showed elasticity, reversible phase transition and cholesteric grandjean texture. The glass transition temperatures increased and clearing temperatures decreased with concentration of crosslinking units.

Influence of Molecular Parameters on the Elastic and Viscoelastic Properties of Side-Chain Liquid Crystalline Elastomers

We investigated the influence of molecular parameters (amount of mesogen, cross-linking density, cross-linker length) on the complex shear modulus G* = G‘ + iG‘ ‘ of polydomain smectic-A side-chain liquid crystalline elastomers composed of side-chain polysiloxane cross-linked by aliphatic chains. G‘ presents two components in the isotropic phase. One is independent of the frequency and reflects the presence of the permanent network; the other is frequency-dependent and is characterized by a scaling law behavior G‘ = G‘ ‘ ∼ f 0.5 of the Rouse type, where f is the frequency. We show that this scaling law is independent of the molecular parameters except when the amount of mesogen becomes very small. In contrast, the crossover frequency between the elastic and the Rouse regime is a function of the cross-linking density and the cross-linker length but is independent of the amount of mesogen, except when the latter is very small. The elastic plateau follows the same behavior as the crossover frequency. It is no longer visible in the smectic-A phase, and the dynamics of this phase are essentially governed by a transient network characterized by a f 0.3 scaling law which is independent of the cross-linking density.

Liquid Crystal Elastomers with Mechanical Properties of a Muscle

Free-standing anisotropic side chain liquid crystalline elastomer films have been prepared using mesogens with laterally affixed polymerizable side chains. We present data on two networks: one containing the monomer of 4‘-acryloyloxybutyl 2,5-(4‘-butyloxybenzoyloxy)benzoate and another from a 50/50 mol % mixture of the above with 4‘-acryloyloxybutyl 2,5-di(4‘-pentylcyclohexyloyloxy)benzoate. The cross-linking was achieved using 10 mol % of 1,6-hexanediol diacrylate. The calculated cross-linking density, as determined from the Young's modulus, was in the 10 -5 mol/cm3 range. Thermoelastic responses show strain changes through the nematic−isotropic phase transition to be 30−45%. The order parameters of the oriented films were determined from the dichroic ratio of IR absorption at 3343 cm-1 to the in-plane aromatic stretching overtone of the LC mesogen core. The variation of the order parameter with temperature scales similar to the strain changes at constant stress. Isostrain studies, conducted through the nematic to isotropic phase transition, show that the two networks behave as true elastomers with significant differences in the force developed. Dynamic shear measurements near the nematic to isotropic phase transition region show that the mechanical relaxation peak appears above 100 Hz, and that viscoelastic relaxations are minimal in the nematic to isotropic phase transition region below 5−10 Hz.

Side chain liquid crystalline elastomers II. Thermal properties and orientational behaviour of the LCE based on unsaturated copolyesters

The effect of crosslinking density on the phase behaviour of smectic liquid crystalline networks was studied; the results showed that crosslinking in their smectic phases can greatly enhance the stability of the liquid crystalline phase. The higher the crosslinking density, the higher the smectic-isotropic transition temperature. The mechanically-induced orientation was studied by polarized FTIR spectroscopy. The smectic liquid crystalline network could be oriented parallel to the mechanical field at higher draw ratio λ, while at lower λ the mesogenic groups are oriented perpendicular to the field for the networks with higher crosslinking density. The observed mechanically-induced orientation is interpreted by a proposed mechanism.

Rheology of Liquid Crystalline Elastomers in Their Isotropic and Smectic A State

The dynamics of two different side-chain liquid crystalline elastomers (SCLCE) exhibiting a smectic A phase is investigated by low-frequency shear and compression experiments. We find that both the isotropic and the smectic A phase have their own characteristic viscoelastic behavior, which seems to be independent of the compound under investigation and thus universal. The relaxation in the isotropic phase shows a distribution of relaxation times, which gives rise to a scaling law of the elastic moduli with an exponent of 0.5. The viscoelastic response in the smectic A phase, however, exhibits a much broader spectrum of relaxation times containing very long-lived modes. It appears that there exists a low-frequency scaling law with an exponent of 0.3 characterizing the smectic A phase. We propose that this result stems from a transient smectic network with a very long lifetime that was set up by the smectic domains. At the phase transition, this transient smectic network disappears, which leads to a sharp decrease of the dynamic shear as well as the compression modulus.

Side-chain liquid crystalline elastomers: the relationship between the orientational ordering of the polymer backbone and the length of the coupling chain

The influence of cross-linking on the phase behaviour of a series of side-chain liquid crystalline elastomers has been studied. For samples cross-linked in the temperature range corresponding to the nematic phase, the phase transition was shifted compared to that observed when an identical sample was cross-linked in the isotropic phase. This shift represented a stabilisation of the nematic phase in the former case, in line with theoretical expectations. By utilising a novel, slow cross-linking method, which allows the polymer backbone to take up an equilibrium conformation prior to network formation, it proved possible to monitor the shifts in phase transition temperature as a function of the length of the methylene chain coupling the mesogenic units to the polymer backbone. The results obtained are related to the backbone anisotropy and indicate that the level of orientational order of the polymer in the nematic phase backbone increases with a reduction in the length of the coupling chain.

Side chain liquid crystalline elastomers I

Side chain liquid crystalline polyesters ( PMx ) were synthesized through the melt polymerization between N- 6 - ( 4- (( 4'- nitrophenyl )azo) phenyl )oxy )hexyl diethanolamine (C6), maleic anhydride and succinic anhydride. The effect of the polymer backbone on the phase behavior of PMx was studied by DSC, polarized optical microscopy ( POM ) and wide angle X-ray diffraction (WAXD) and the results showed that smectic -isotropic transition temperature (TSI) increased largely, while melting temperatures ( Tm ) decreased slightly and Tg increased slightly with the increase of the content of maleic anhydride in the copolymers. The results also indicated that introduction of maleic anhydride to polymer backbone was unfavorite of the molten crystallization of the side chains. The IR-dichroism study on the side chain liquid crystalline elastomers prepared from PMx showed that the elastomers exhibited anisotropic behavior after stretching under the mechanical fields.

Influence of Network Topology on Polydomain−Monodomain Transition in Side Chain Liquid Crystalline Elastomers with Cyanobiphenyl Mesogens

Orientation behavior of acrylate-based liquid crystalline (LC) networks containing different types of cross-links is studied and discussed. Cross-links incorporated by chemical reaction with cross-...