Biaxially stretched nematic liquid crystalline elastomers.

The biaxial stretching of sheets of liquid crystalline neo-Hookean elastomer has been studied in the isotropic case. The results suggest two types of laminate structures in the process of quasiconvexification of the free energy, a fact that implies the appearance of several shear terms in the deformation gradient matrix. More that one decomposition of the deformation gradient is possible, which is consistent with a bifurcation in the undeformed configuration (λ = 1) . This situation is similar to the well-known Rivlin's problem of the triaxial symmetric traction of a neo-Hookean cube. The problem can easily be generalized for an anisotropic material by introducing a semisoft term in the free-energy expression. In this case, the horizontal plateau corresponding to the minimal energy, characteristic of the soft elasticity, disappears, and only an equilibrium condition is obtained.

Contributions of dipolar relaxation processes and ionic transport to the response of liquids to electrical perturbation fields.

The objective of this work was to study the influence of small variations in the chemical structure on the molecular dynamics of liquids using as models bis(cyclohexylmethyl) 2-methyl- and dicyclohexyl 2-methylsuccinate. The dielectric behavior of the low molecular weight liquids was studied over a wide range of frequencies and temperatures. The results show that the temperature dependence of the dielectric strengths, relaxation times, and shape parameters of the secondary and glass-liquid relaxations are very sensitive to the slight differences in the structures of the liquids. Significant changes take place in the dielectric strength of the ß relaxation in the glass liquid transition. Moreover, the temperature dependence of the ß relaxation exhibits Arrhenius behavior in the glassy state and departs from this behavior in the liquid state. Special attention is paid to the temperature dependence of low-frequency relaxations produced by the motion of a macrodipole arising from charges located near the liquid-electrode boundaries.

Instability of incompressible cylinder rubber tubes under radial electric fields.

The possibility of instability of cylindrical tubes of elastomers is analyzed taking into account Mooney-Rivlin (M. Mooney, J. Appl. Phys. 11, 582 (1940)) and Ogden (R.W. Ogden, Proc. R. Soc. London, Ser. A 326, 565 (1972)) models. In some aspects this paper is the elasto-electric counterpart of a previous paper by Bustamante, Dorfmann and Ogden (R. Bustamante, A. Dorfmann, R.W. Ogden, J. Engn. Math. 59, 139 (2007)). However in the present case the appearance of instability is checked. The results indicate that models more complex than the Mooney-Rivlin model are necessary to predict instability zones showing minimum critical points.

Effect of an electric field on the bifurcation of a biaxially stretched incompressible slab rubber.

This paper describes the effect of an electrical field on the bifurcation phenomenon that appears in a biaxially stretched slab of Mooney-Rivlin material (M. Mooney, J. Appl. Phys. 11, 582 (1940)) subjected to equal dead loads. The main conclusion of the analysis is that the stretch ratio at which the bifurcation phenomenon appears crucially depends on the configuration of the system rubber slab-electrodes. The theoretical foundations of the present study are based on a recent formulation on this subject carried out by Dorfmann and Ogden (A. Dorfmann, R.W. Ogden, Acta Mech. 174, 167 (2005); J. Elasticity 82, 99 (2006)).

Superdipole liquid scenario for the dielectric primary relaxation in supercooled polar liquids.

We propose a dynamic structure of coupled dynamic molecular strings for supercooled small polar molecule liquids and accordingly we obtain the Hamiltonian of the rotational degrees of freedom of the system. From the Hamiltonian, the strongly correlated supercooled polar liquid state is renormalized to a normal superdipole liquid state. This scenario describes the following main features of the primary or alpha-relaxation dynamics in supercooled polar liquids: (1) the average relaxation time evolves from a high temperature Arrhenius to a low temperature non-Arrhenius or super-Arrhenius behavior; (2) the relaxation function crosses over from the high temperature exponential to low temperature nonexponential form; and (3) the temperature dependence of the relaxation strength shows non-Curie features. According to the present model, the crossover phenomena of the first two characteristics arise from the transition between the superdipole gas and the superdipole liquid. The model predictions are quantitatively compared with the experimental results of glycerol, a typical glass former.

Biological oxygen apparent transmissibility of hydrogel contact lenses with and without organosilicon moieties.

The instrument oxygen transmissibility (IOT) of organosilicon hydrogels, measured by electrochemical procedures, is 5-10 times larger than that of conventional hydrogels. A method is described that allows the estimation of the oxygen tension at the lens-cornea interface for closed- and open-eyelids situations by combining the IOT of the hydrogels and corneal parameters such as corneal thickness, corneal permeability and oxygen flux across the cornea. From these results the biological oxygen apparent transmissibility (BOAT) is obtained, an important parameter which an multiplication with the pressure of oxygen on the external part of the lens gives the oxygen flux onto the cornea. Contact lenses with oxygen transmissibility higher than 100 Dk/t units [1 Dk/t unit=10(-9) [cm(3) O(2) (STp) cm(-2)s(-1)(mmHg)(-1)] posses a large oxygen tension at the lens-cornea interface that substantially reduces the oxygen flux onto the cornea. Lenses whose oxygen transmissibility is lower than 50 Dk/t units allow a rather small oxygen flux onto the cornea under closed eyelids condition that prevent their use for extended wear.

Oxygen permeability of hydrogel contact lenses with organosilicon moieties.

Oxygen transport through two extended wear (day and night) hydrogel contact lenses that contain organosilicon moieties (balafilcon A and lotrafilcon A) was studied in the hydrate (hydrogel) and dry (xerogel) states. The water uptake increased the oxygen permeability [(Dk)app] and transmissibility [Dk/L(av)] coefficients of the dry materials by about 70%. The (Dk)app for the hydrated lenses was determined following the so-called stack procedure. The values obtained were 107 +/- 4 barrer for balafilcon A and 141 +/- 5 barrer for lotrafilcon A, about 5-10 times larger than those previously reported for conventional (without organosilicon moieties) extended wear hydrogels contact lenses. The Dk/L(av) for -3.00 diopter lenses (harmonic average thickness, L(av) = 75 +/- 2 microm for lotrafilcon, and 85 +/- 2 microm for balafilcon) was 123 +/- 6 barrer/cm for balafilcon A and 183 +/- 8 barrer/cm for lotralicon A. The minimum oxygen transmissibility 87 barrer/cm stipulated by Holden and Mertz to avoid corneal edema with extended wear contact can be easily achieved with lotrafilcon and balafilcon lenses of diverse dioptric powers if the central and peripheral thickness of the lenses are kept below the critical level of oxygen transmissibility.

Modeling dynamically heterogeneous coupled relaxations.

A model describing dynamically heterogeneous relaxation in complex coupled systems is presented. The model predicts the splitting of a high-temperature single Debye relaxation to a low-temperature bimodal relaxation. The bimodal process shows a crossover from a Debye-type relaxation to an approximately stretched-exponential relaxation. Some general features of relaxation in complex systems emerge from the predictions of the model, and a comparison of the model with experiments is reported.

A potentiostatic study of oxygen transmissibility and permeability through hydrogel membranes.

The permeability characteristics of membranes prepared from hydrogels of poly(2-hydroxyethylmethacrylate) (PHEMA) and poly(2-hydroxyethylmethacrylate-co-N,N-dimethylacrylamide) (PHNDA) are described. True values of the permeability and transmissibility coefficients of oxygen in the membranes are determined by using electrochemical procedures involving the measurement of the steady state current either in membranes with different thickness or in a single membrane in which its thickness is varied with layers of moistened paper. Comparison of the results obtained for the transport properties in these hydrogels with others obtained in other hydrogels permit to conclude that the degree of swelling rather than the chemical nature of the hydrogels affects the permeation properties. The chemical structure presumably only affects in high degree the chemical stability and flexibility of the hydrogel membranes.

Oxygen transport through methacrylate-based hydrogels with potential biological capability.

The permeability to oxygen of hydrogels prepared from copolymers of 2-hydroxymethyl methacrylate and p-methacryloxyl-oxyacetanilide have been studied by using an oxygen electrode in combination with a permeometer. The transmissibility Dk/L and the permeability Dk (where D, k and L are, respectively, the diffusion coefficient, the Henry constant and the thickness of the hydrogel) are measured by a combination of steady state and transitory state measurements. Both transport coefficients increase with the water content, which in turn depends on the copolymer composition. The values of these quantities tend toward a limiting value for water-saturated hydrogels. The ratio of the characteristic volume for diffusion of the oxygen molecule to the free volume of water per mole water is found to be in the vicinity of 0.10, and this value increases slightly as the fraction of the hydrophilic comonomer in the hydrogel increases. A detailed comparison of the biogels studied with six commercial contact lenses has also been performed.