Evaristo Riande

Polymer viscoelasticity : stress and strain in practice

Structure of polymers crystalline and amorphous states in polymers rubber elasticity stress-strain relations for ideal solids and ideal liquids linear viscoelasticity - viscoelastic functions dynamic viscoelastic functions experimental determination of viscoelastic properties viscoelastic behaviour of polymers above the glass-transition retardation and relaxation spectra viscoelastic models molecular models of the viscoelastic polymers viscoelasticity of glassy and semicrystalline polymers flow behaviour of polymer melts and solutions yield crazing and fracture reinforced polymers multiaxial analysis of linear viscoelastic stress flexion and torsion of viscoelastic rods.

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.

On electromechanical stability of dielectric elastomers

This paper studies the competition between electric and mechanical force fields simultaneously applied to a polar elastomer that can lead to electric breakdown. The analysis of the system, performed assuming that the free energy of the elastomer is simply the addition of polarizing and stretching energies leads to the classical “thermodynamic” (in this case “electromechanical”) stability.

Effect of physical aging on the gas transport properties of PVC and PVC modified with pyridine groups

Abstract The gas transport coefficients of polyvinyl chloride (PVC) and PVC modified with pyridine groups have been studied. It has been observed that there is a strong time dependence of the permeability and diffusivity of oxygen, nitrogen, carbon dioxide and methane in membranes prepared by solvent casting of PVC and pyridine modified PVC. For PVC there is a two-fold reduction of the diffusion coefficients during the first two days, and about one order of magnitude, a month after the membranes are prepared, and no stabilisation of the trend is seen after a month. Membranes prepared from modified PVC show a short-term diffusion rate reduction which is similar to that found in PVC, while at longer times the diffusion rate decrease levels off quickly, attaining constant values after about ten days. The time dependence of the transport coefficients is attributed to the samples’ physical aging and an attempt is made to fit the experimental data by considering a stretched exponential time dependence of the volume contraction on aging.

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.

Synthesis and polymerization of acrylic monomers with hydrophilic long side groups. Oxygen transport through water swollen membranes prepared from these polymers

The synthesis and characterization of tetraethyleneglycol acrylate (TTEGA) and tetraethyleneglycol diacrylate are described. Radical polymerization reactions of TTEGA were carried out at different temperatures and the curves of conversion against time, obtained by using dilatometric techniques, allowed the determination of kp/kt12, where kp and kt are, respectively, the propagation and termination rate constants. The values found for this ratio were comparatively much higher than those reported in the literature for other acrylic monomers. The polymer, poly(tetraethyleneglycol acrylate), is soluble in water, exhibits low glass transition temperature (−45°C) and the percentage of syndiotactic dyads in the chains lies in the vicinity of 65 ± 5%, a value normally found in similar polymers. Both poly(tetraethyleneglycol acrylate-co-tetraethyleneglycol diacrylate) and poly(triethyleneglycol acrylate-co-triethyleneglycol diacrylate) membranes were prepared by radical polymerization of the corresponding monomers and a small quantity of diacrylic esters. Electrochemical techniques were used to evaluate oxygen transport through these membranes swollen in water. The apparent values of both the permeability and diffusion coefficients are unusually large as a consequence of a high swelling degree of these membranes. Although the solubility coefficient of oxygen in the swollen hydrogels is larger than in water, restrictions in the diffusion path caused by the polymer matrix decrease the diffusion coefficient of the gas to ca one-third of its value in water.

Effect of annealing on the permeation characteristics of gases of coextruded LLDPE films

The temperature dependence of both the permeability and diffusion coefficients of carbon dioxide, oxygen and nitrogen in annealed LLDPE films are studied. It is found that the values of the permeability coefficient through the annealed membranes are nearly four times larger than those through the non-annealed ones. The fact that annealing slightly diminishes the values of the diffusion coefficient leads to the conclusion that the rise in permeability detected in the films by effect of annealing should be attributed to an increase in solubility. The permeability characteristics of the films are interpreted in terms of the free volume theory.

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.

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)).

Permeability of co-extruded linear low-density polyethylene films to oxygen and carbon dioxide as determined by electrochemical techniques

Abstract The mechanical relaxation spectra of co-extruded linear low-density polyethylene (LLDPE) films, prepared from copolymers of ethylene and 1-octene, were measured in parallel and transverse directions to the processing orientation. Both the γ- and β-relaxations do not show a noticeable dependence on the direction in which the measurements were performed. However, whereas the α-relaxation in the measurements performed in the parallel direction appears as two peaks, in order of increasing temperature, which were denoted as α′ and α″, the measurements carried out in the transverse direction only exhibit the α′-peak. The influence of tensile drawing on the permeability of co-extruded LLDPE films to oxygen and carbon dioxide was investigated by electrochemical techniques over the range of temperatures where the α′-relaxation process is located. In general, the permeability coefficients do not show a significant dependence on the drawing direction in the temperature interval corresponding to the low-temperature region of the α′-peak. In this high-temperature zone, the values of the permeability coefficient for O 2 and CO 2 through the oriented films after tensile drawing are significantly lower than those obtained for these gases through undrawn co-extruded LLDPE films. The diffusion coefficients do not show a definite dependence on tensile drawing.