Exponential Decay of Correlations in a Model for Strongly Disordered 2D Nematic Elastomers

Lattice Monte-Carlo simulations were performed to study the equilibrium ordering in a two-dimensional nematic system with quenched random disorder. When the disordering field, which competes against the aligning effect of the Frank elasticity, is sufficiently strong, the long-range correlation of the director orientation is found to decay as a simple exponential, Exp[-r/x]. The correlation length {x} itself also decays exponentially with increasing strength of the disordering field. This result represents a new type of behavior, distinct from the Gaussian and power-law decays predicted by some theories.Comment: Latex file (4 pages) + 2 EPS figure

Slow stress relaxation in randomly disordered nematic elastomers and gels

Randomly disordered (polydomain) liquid crystalline elastomers align under stress. We study the dynamics of stress relaxation before, during and after the Polydomain-Monodomain transition. The results for different materials show the universal ultra-slow logarithmic behaviour, especially pronounced in the region of the transition. The data is approximated very well by an equation Sigma(t) ~ Sigma_{eq} + A/(1+ Alpha Log[t]). We propose a theoretical model based on the concept of cooperative mechanical resistance for the re-orientation of each domain, attempting to follow the soft-deformation pathway. The exact model solution can be approximated by compact analytical expressions valid at short and at long times of relaxation, with two model parameters determined from the data.Comment: 4 pages (two-column), 5 EPS figures (included via epsfig

Quasi-long range order in the random anisotropy Heisenberg model

The large distance behaviors of the random field and random anisotropy Heisenberg models are studied with the functional renormalization group in $4-\epsilon$ dimensions. The random anisotropy model is found to have a phase with the infinite correlation radius at low temperatures and weak disorder. The correlation function of the magnetization obeys a power law $<{\bf m}({\bf r}_1) {\bf m}({\bf r}_2)>\sim| {\bf r}_1-{\bf r}_2|^{-0.62\epsilon}$. The magnetic susceptibility diverges at low fields as $\chi\sim H^{-1+0.15\epsilon}$. In the random field model the correlation radius is found to be finite at the arbitrarily weak disorder.Comment: 4 pages, REVTe

Soft and non-soft structural transitions in disordered nematic networks

Properties of disordered nematic elastomers and gels are theoretically investigated with emphasis on the roles of non-local elastic interactions and crosslinking conditions. Networks originally crosslinked in the isotropic phase lose their long-range orientational order by the action of quenched random stresses, which we incorporate into the affine-deformation model of nematic rubber elasticity. We present a detailed picture of mechanical quasi-Goldstone modes, which accounts for an almost completely soft polydomain-monodomain (P-M) transition under strain as well as a ``four-leaf clover'' pattern in depolarized light scattering intensity. Dynamical relaxation of the domain structure is studied using a simple model. The peak wavenumber of the structure factor obeys a power-law-type slow kinetics and goes to zero in true mechanical equilibrium. The effect of quenched disorder on director fluctuation in the monodomain state is analyzed. The random frozen contribution to the fluctuation amplitude dominates the thermal one, at long wavelengths and near the P-M transition threshold. We also study networks obtained by crosslinking polydomain nematic polymer melts. The memory of initial director configuration acts as correlated and strong quenched disorder, which renders the P-M transition non-soft. The spatial distribution of the elastic free energy is strongly dehomogenized by external strain, in contrast to the case of isotropically crosslinked networks.Comment: 19 pages, 15 EPS figure

Electro-Mechanical Fredericks Effects in Nematic Gels

The solid nematic equivalent of the Fredericks transition is found to depend on a critical field rather than a critical voltage as in the classical case. This arises because director anchoring is principally to the solid rubbery matrix of the nematic gel rather than to the sample surfaces. Moreover, above the threshold field, we find a competition between quartic (soft) and conventional harmonic elasticity which dictates the director response. By including a small degree of initial director misorientation, the calculated field variation of optical anisotropy agrees well with the conoscopy measurements of Chang et al (Phys.Rev.E56, 595, 1997) of the electro-optical response of nematic gels.Comment: Latex (revtex style), 5 EPS figures, submitted to PRE, corrections to discussion of fig.3, cosmetic change

Quasi-long-range order in nematics confined in random porous media

We study the effect of random porous matrices on the ordering in nematic liquid crystals. The randomness destroys orientational lang-range order and drives the liquid crystal into a glass state. We predict two glass phases one of which possesses quasi-long-range order. In this state the correlation length is infinite and the correlation function of the order parameter obeys a power dependence on the distance. The small-angle light-scattering amplitude diverges but slower than in the bulk nematic. In the uniaxially strained porous matrices two new phases emerge. One type of strain induces an anisotropic quasi-long-range-ordered state while the other stabilizes nematic long-range order.Comment: 4 pages, Revte

Electrical Conductivity of Electrospun Polyaniline and Polyaniline-Blend Fibers and Mats

Submicrometer fibers of polyaniline (PAni) doped with (+)-camphor-10-sulfonic acid (HCSA) and blended with poly(methyl methacrylate) (PMMA) or poly(ethylene oxide) were electrospun over a range of compositions. Continuous, pure PAni fibers doped with HCSA were also produced by coaxial electrospinning and subsequent removal of the PMMA shell polymer. The electrical conductivities of both the fibers and the mats were characterized. The electrical conductivities of the fibers were found to increase exponentially with the weight percent of doped PAni in the fibers, with values as high as 50 ± 30 S/cm for as-electrospun fibers of 100% doped PAni and as high as 130 ± 40 S/cm upon further solid state drawing. These high electrical conductivities are attributed to the enhanced molecular orientation arising from extensional deformation in the electrospinning process and afterward during solid state drawing. A model is proposed that permits the calculation of mat conductivity as a function of fiber conductivity, mat porosity, and fiber orientation distribution; the results agree quantitatively with the independently measured mat conductivities.United States. Army Research Office (Institute for Soldier Nanotechnologies, Contract ARO W911NF-07-D- 0004

Rheological behaviour of concentrated suspensions of soft spheres

Rheological properties of concentrated suspensions of soft spheres have been studied in a concentration range where all the volume is filled by more or less swollen particles. The suspensions appear as soft gels and show a dynamic modulus which is nearly frequency independent. Under shear, an orientation of the deformed spheres at 45° from the velocity lines is observed. Since above a critical concentration, the elastic modulus appears related to the intrinsic softness of the particles, the modulus of the suspension has been calculated theoretically from a simple core-shell model of a swollen individual particle through a minimization of the free energy. A satisfactory agreement is observed with experimental data, using reasonable values of the fitting parameters

Magnetic Nanoparticles Aggregation in Magnetic Gel Studied by Electron Magnetic Resonance (EMR)

Aggregation of magnetic nanoparticles immobilized in polymer gels was studied by ferromagnetic resonance and paramagnetic sensor techniques. Ferromagnetic resonance spectra of magnetic gels prepared in the presence of external magnetic field of 1.5 kG were compared to the spectra of gels synthesized in the absence of a magnetic field. Application of a magnetic field led to formation of linear aggregates of magnetic particles in the polymer matrix. The aggregates did not come apart after the field was switched off. The fraction of aggregated particles (of 62(6)%) and aspect ratio (elongation) of the aggregates (12.6(1.3)) was determined using paramagnetic sensor technique