Nematic elastomers: The influence of external mechanical stress on the liquid‐crystalline phase behavior

Die Makromolekulare Chemie 03/2003; 190(12):3269 - 3284. DOI: 10.1002/macp.1989.021901224


The influence of external mechanical stress on the nematic-isotropic phase transformation of nematic elastomers was investigated. The experimental results of IR-dichroism measurements in the nematic phase and stress-optical measurements in the isotropic phase are in good agreement with the theoretical predicitions of the phenomenological Landau-de Gennes theory. This is for the first time that a significant influence of an external field on the nematic-isotropic phase transformation temperature and on the nematic order parameter S has been proved.

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    • "Circularly polarized light with the same handedness as the CLC will be reflected according to Bragg equation * n p λ = , where the λ is central wavelength of the photonic bandgap (PBG), n is the average refractive index of liquid crystal, and p is the helical pitch of CLC. The periodic helical structure of a CLC is susceptible to external stimulus, such as temperature[6], mechanical stress[7], optical radiation[8], electric filed[9], and magnetic field[10], which enables dynamic Bragg grating[11], bi-stable reflective display device[12], and broad-band polarizer[13]. Reactive mesogen (RM) is a kind of polymerizable liquid crystals, in which the liquid crystal molecules could be " frozen " during polymerization process to form polymers with optically birefringence[14]. "

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    ABSTRACT: We develop a continuum theory for the mechanical behavior of rubber-like solids that are formed by the cross-linking of polymeric fluids that include nematic molecules as elements of their main-chains and/or as pendant side-groups. The basic kinematic ingredients of this theory are identical to those arising in continuum-level theories for nematic fluids: in addition to the deformation, which describes the trajectories of material particles, an orientation, which delineates the evolution of the nematic microstructure, is introduced. The kinetic structure of our theory relies on the precept that a complete reckoning of the power expended during the evolution of a continuum requires the introduction of forces that act conjugate to each operative kinematic variable and that to each such force system there should correspond a distinct momentum balance. In addition to conventional deformational forces, which expend power over the time-rate of the deformation and enter the deformational (or linear) momentum balance, we, therefore, introduce a system of orientational forces, which expend power over the time-rate of the orientation and enter an additional orientational momentum balance. We restrict our attention to a purely mechanical setting, so that the thermodynamic structure of our theory rests on an energy imbalance that serves in lieu of the first and second laws of thermodynamics. We consider only nematic elastomers that are incompressible and microstructurally inextensible, and a novel aspect of our approach concerns our treatment of these material constraints. We refrain both from an a priori decomposition of fields into active and reactive components and an introduction of Lagrange multipliers; rather, we start with a mathematical decomposition of the dependent fields such as the deformational stress based on the geometry of the constraint manifold. This naturally gives rise to active and reactive components, where only the former enter into the energy imbalance because the latter automatically expend zero power in processes consistent with the constraints. The reactive components are scaled by multipliers which we take to be constitutively indeterminate. We assume constitutive equations for the active components, and the requirement that these equations be consistent with the energy imbalance in all processes leads to the active components being determined by an energy density response function of the deformation gradient, the orientation, and the orientation gradient. We formulate the requirements of observer independence and material symmetry for such a function and provide, as a specialization, an expression that encompasses the energy densities used in the Mooney-Rivlin description of rubber and the Oseen-Zöcher-Frank description of nematic fluids.
    No preview · Article · Jul 1999 · Journal of Elasticity
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    ABSTRACT: Structural changes through successive phase transformations of a chiral smectic liquid-crystalline elastomer are investigated by X-ray scattering technique. In uniaxially deformed elastomers, the smectic layer seemingly tilts even in the SmA phase, in which an in-plane chevron structure formed in the tilted smectic phase. On the basis of an analysis of the layer reflection peaks, the layer correlation length in the tilted smectic phases is shorter than that in the non-tilted SmA phase, though smectic layers in the tilted smectic phases are better ordered than those in SmA.
    No preview · Article · Nov 2002 · Macromolecular Chemistry and Physics
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