606 research outputs found
Linear hydrodynamics and viscoelasticity of nematic elastomers
We develop a continuum theory of linear viscoelastic response in oriented
monodomain nematic elastomers. The expression for dissipation function is
analogous to the Leslie-Ericksen version of anisotropic nematic viscosity; we
propose the relations between the anisotropic rubber moduli and new viscous
coefficients. A new dimensionless number is introduced, which describes the
relative magnitude of viscous and rubber-elastic torques. In an elastic medium
with an independently mobile internal degree of freedom, the nematic director
with its own relaxation dynamics, the model shows a dramatic decrease in the
dynamic modulus in certain deformation geometries. The degree to which the
storage modulus does not altogether drop to zero is shown to be both dependent
on frequency and to be proportional to the semi-softness, the non-ideality of a
nematic network. We consider the most interesting geometry for the
implementation of the theory, calculating the dynamic response to an imposed
simple shear and making predictions for effective moduli and (exceptionally
high) loss factors.Comment: Latex 2e or PDFlatex (4 EPS or JPG figures) - to appear in
Euro.Phys.J.
Dynamic Soft Elasticity in Monodomain Nematic Elastomers
We study the linear dynamic mechanical response of monodomain nematic liquid
crystalline elastomers under shear in the geometry that allows the director
rotation. The aspects of time-temperature superposition are discussed at some
length and Master Curves are obtained between the glassy state and the nematic
transition temperature Tni. However, the time-temperature superposition did not
work through the clearing point Tni, due to change from the ``soft-elasticity''
nematic regime to the ordinary isotropic rubber response. We focus on the
low-frequency region of the Master Curves and establish the power-law
dependence of the modulus G' ~ omega^a. This law agrees very well with the
results of static stress relaxation, where each relaxation curve obeys the
analogous power law G' ~ t^{-a} in the corresponding region of long times and
temperatures.Comment: Latex, [epj]{svjour} style, 9 pages 11 figures submitted to Euro.
Phys. J.
Tube Model for the Elasticity of Entangled Nematic Rubbers
Dense rubbery networks are highly entangled polymer systems, with significant
topological restrictions for the mobility of neighbouring chains and crosslinks
preventing the reptation constraint release. In a mean field approach,
entanglements are treated within the famous reptation approach, since they
effectively confine each individual chain in a tube-like geometry. We apply the
classical ideas of reptation dynamics to calculate the effective rubber-elastic
free energy of anisotropic networks, nematic liquid crystal elastomers, and
present the first theory of entanglements for such a material.Comment: amended version (typos corrected, appendix extended
Cellular solid behaviour of liquid crystal colloids. 2. Mechanical properties
This paper presents the results of a rheological study of thermotropic
nematic colloids aggregated into cellular structures. Small sterically
stabilised PMMA particles dispersed in a liquid crystal matrix densely pack on
cell interfaces, but reversibly mix with the matrix when the system is heated
above Tni. We obtain a remarkably high elastic modulus, G'~10^5 Pa, which is a
nearly linear function of particle concentration. A characteristic yield stress
is required to disrupt the continuity of cellular structure and liquify the
response. The colloid aggregation in a ``poor nematic'' MBBA has the same
cellular morphology as in the ``good nematic'' 5CB, but the elastic strength is
at least an order of magnitude lower. These findings are supported by
theoretical arguments based on the high surface tension interfaces of a
foam-like cellular system, taking into account the local melting of nematic
liquid and the depletion locking of packed particles on interfaces.Comment: Latex 2e (EPJ style) EPS figures included (poor quality to comply
with space limitations
Some properties of membranes in nematic solvents
The fluctuation spectrum of membranes in nematic solvents is altered by the
boundary condition imposed on the bulk nematic director by the curved membrane.
We discuss some properties of single and multi-membrane systems in nematic
solvents, primarily based on the Berreman-de~Gennes model. We show that:
membranes in nematic solvents are more rigid and less rough than in their
isotropic counterparts; have a different Helfrich steric stabilization energy,
proportional to , and hence a different compression modulus in the
lamellar state; and can exhibit phase separation via unbinding during a quench
into the nematic state. We also discuss the preparation and possible
experimental effects of nematic-mediated surfactant membrane system
- …