Liquid crystal elastomers (original) (raw)
Related papers
1999
Recent experimental and theoretical work shows that liquid-crystalline elastomers and gels have a highly mobile axis of anisotropy. Despite being nominally elastic solids, they also show features of fluids, such as the effect of soft elasticity. Work over the last few years is reviewed and some of the most important discoveries, as well as the outstanding problems in this field, are highlighted. We examine the unusual mechanical properties of nematic and smectic rubbers, their randomly disordered equilibrium textures, some aspects of dynamics and mechanical relaxation and the effect of uniform chiral piezoelectricity in amorphous polymer networks.
Order and Disorder in Liquid-Crystalline Elastomers
2011
Order and frustration play an important role in liquid-crystalline polymer networks (elastomers). The first part of this review is concerned with elastomers in the nematic state and starts with a discussion of nematic polymers, the properties of which are strongly determined by the anisotropy of the polymer backbone. Neutron scattering and X-ray measurements provide the basis for a description of their conformation and chain anisotropy. In nematic elastomers, the macroscopic shape is determined by the anisotropy of the polymer backbone in combination with the elastic response of elastomer network. The second part of the review concentrates on smectic liquid-crystalline systems that show quasi-long-range order of the smectic layers (positional correlations that decay algebraically). In smectic elastomers, the smectic layers cannot move easily across the crosslinking points where the polymer backbone is attached. Consequently, layer displacement fluctuations are suppressed, which effectively stabilizes the one-dimensional periodic layer structure and under certain circumstances can reinstate true long-range order. On the other hand, the crosslinks provide a random network of defects that could destroy the smectic order. Thus, in smectic elastomers there exist two opposing tendencies: the suppression of layer displacement fluctuations that enhances translational order, and the effect of random disorder that leads to a highly frustrated equilibrium state. These effects can be investigated with high-resolution X-ray diffraction and are discussed in some detail for smectic elastomers of different topology.
Liquid crystalline elastomers: dynamics and relaxation of microstructure
Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 2003
The equilibrium mechanical response of nematic elastomers can be soft or hard depending on the relation between the imposed strains and the nematic director, in particular, if the local nematic director is able to respond by rotating. The dynamical response proves to be equally unusual. We examine the linear dynamic mechanical response of monodomain nematic elastomers under shear and the aspects of time{ temperature superposition of the dynamical data across phase-transition regions. In the low-frequency region of the master curves, one nds a dramatic reduction of rubber plateau modulus and the rise in internal dissipation: in the shear geometries compatible with dynamic soft elasticity. Power-law variation of the storage modulus with frequency G 0 / ! a agrees very well with the results of static stress relaxation, where each relaxation curve obeys the analogous power law G 0 / t ¡a in the corresponding region of long times and temperatures.
Viscoelasticity of main chain liquid crystalline elastomers
Polymer, 2006
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 smecticisotropic 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 10 9 s could be estimated. q
Soft Elasticity in Main Chain Liquid Crystal Elastomers
Crystals, 2013
Main chain liquid crystal elastomers exhibit several interesting phenomena, such as three different regimes of elastic response, unconventional stress-strain relationship in one of these regimes, and the shape memory effect. Investigations are beginning to reveal relationships between their macroscopic behavior and the nature of domain structure, microscopic smectic phase structure, relaxation mechanism, and sample history. These aspects of liquid crystal elastomers are briefly reviewed followed by a summary of the results of recent elastic and high-resolution X-ray diffraction studies of the shape memory effect and the dynamics of the formation of the smectic-C chevron-like layer structure. A possible route to realizing auxetic effect at molecular level is also discussed.
Friction tensor of flexible polymer chains in nematic liquid crystals
Macromolecules, 1991
Hydrodynamic properties of a flexible polymer chain in low molecular weight liquid crystals (LC) have been studied. The starting point is the derivation of the hydrodynamic Oseen tensor for anisotropic fluids that satisfy a modified Navier-Stokes equation. The nematics of LC breaks the spatial isotropy and leads to a necessary introduction of the friction tensor Z for polymers. The general expression for 2: is obtained as a formal perturbation expansion within the Kirkwood-Riseman theory, and the component corresponding to the polymer diffusive motion along the nematic direction is evaluated explicitly by using the renormalization group method and by considering the smallness of one of the Leslie coefficients of the nematic solvent. The result depends on the LC Miesowicz viscosities in a nontrivial way, yet the molecular weight dependence is still universal.
Linear Viscoelasticity of Leslie-Ericksen Liquid Crystal Polymers
2004
The linear viscoleasticity of seven lyotropic and thermotropic liquid crystalline polymers is characterized using the Leslie-Ericksen equations of defect-free nematodynamics for small amplitude oscillatory capillary Poiseuille flow, and using analytical, numerical and scaling methods. The experimental data sets used in this study correspond to the six Leslie viscosities coefficients for seven nematics liquid crystal polymers, that include shear flow-aligning and non-aligning materials. The predicted equivalent rheological responses between the shear flow-aligning and shear non-aligning polymers demonstrate the universality of nematodynamics. In this work the principles of superposition are developed, applied, and shown to be accurate in collapsing the data sets for both classes of polymers. The scaled resonance peak in the loss tangent (tan δ=G”/G’) as a function of the oscillation frequency shown to be a universal constant for monodomain nematics. Introduction The Leslie-Ericksen l...