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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.

Proton Transport in Membranes Prepared from Sulfonated Polystyrene-Poly(vinylidene fluoride) Blends

The preparation of new cation-exchange membranes from polymer composites based on poly(vinylidene fluoride), sulfonated polystyrene-co-divinylbenzene, and antimonic acid is reported. The thermal properties of the composites have been characterized by differential scanning calorimetry. Values of the transport number of protons in the membranes were obtained from the observable electric potential. It is defined from the potential difference measured between the electrodes reversible to one of the constituent ions in equilibrium with the system. When compared with Nafion cation-exchange membranes, the membranes described in this work exhibit interesting proton transport properties that could make them suitable as polymer electrolytes in fuel cells.

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.

Influence of cellulose reinforcers on gas transport through natural rubber

This work reports the effect of temperature and pressure on the permeability, diffusion and solubility coefficients of N 2 ,O 2, and CO2 in natural rubber reinforced with different amounts of cellulose. The values of the permeability coefficient of carbon dioxide, oxygen and nitrogen in these materials are significantly smaller than in natural rubber. The curves representing the dependence of the permeability coefficient on the pressure of the upstream-chamber, exhibit an upturn in the low pressure region as the pressure decreases. The isotherms showing the dependence of the diffusion coefficient on the pressure of the upstream chamber exhibit the same pattern as those of the permeability coefficient. The experimental results are interpreted in terms of changes in the free volume in the polymer membranes.

Determination of the Oxygen Transmissibility and Permeability of Hydrogel Contact Lenses

To test the validity of the method of stacked hydrogel contact lenses to obtain the oxygen permeability and transmissibility coefficients of the lenses, the coefficients of one low hydration (38% water) and two high hydration (55 and 58% water) hydrogel contact lenses stacked one to five on an oxygen electrode were deter- mined. From the oxygen diffusion through the lenses, the current intensity in the stationary state was determined, and from this the instrument the oxygen transmis- sibility was obtained. The permeability coefficients of the lenses, corrected for edge effects, were obtained from the slope of the plot of the reciprocal of the transmissibility coefficients versus the lens thicknesses. The oxygen permeability and transmissibility coefficients of the lenses obtained neglected the boundary layers resistance between the stacked lenses and, therefore, these are not the true coefficients. This article compares the apparent oxygen permeability coefficients of the hydrogel contact lenses, obtained by others, with the true oxygen permeability coefficients obtained with a corrected equation that takes into account the boundary layers between the stacked lenses.

Relaxation behavior of acrylate and methacrylate polymers containing dioxacyclopentane rings in the side chains

The synthesis of poly[(2,2-dimethyl-1,3-dioxolan-4-yl) methyl acrylate)] (PACGA) and poly[(2,2-dimethyl-1,3-dioxolan-4-yl) methyl methacrylate] (PMCGA) is reported. Both polymers present dielectric and mechanical β subglass absorptions at −128 and −115 °C, respectively, at 1 Hz, followed by ostensible glass–rubber or α relaxations centered in the vicinity of 0 and 67 °C, respectively, at the same frequency. The values of the activation energy of both the mechanical and dielectric β absorptions lie in the vicinity of 10 kcal mol−1. The critical interpretation of the relaxation behavior of PMCGA suggests that dipolar intramolecular correlations play a dominant role in the response of the polymer to an electric field. The subglass relaxations of PACGA and PMCGA are further compared with the relaxation behavior of poly(1,3-dioxane acrylate), poly(1,3-dioxane methacrylate), and other polymers in the glassy state. The strong conductive processes observed in PMCGA at low frequencies and high temperatures were studied under the assumption that that these processes arise from Maxwell–Wagner–Sillars effects occurring in the bulk combined with Nernst–Planckian electrodynamic effects caused by interfacial polarization in the films.

New method to determine the true transmissibilities and permeabilities of oxygen in hydrogel membranes

In the present paper, an electrochemical method to obtain the true transmissibilities and permeabilities of hydrogels to oxygen is proposed. The method involves the measurement of the electric current arising from the flow of oxygen through a potentiostat cell integrated by a top water layer, the hydrogel membrane, and a thin water layer between the membrane and the cathode, where the oxygen is reduced. Varying the thickness of the top water layer, the true permeation characteristics of the hydrogels are obtained. A parallel study was carried out in which the transmissibilities and permeation coefficients were obtained from the variation of the steady state current with the thickness of the membranes. The permeation results obtained by the two methods agree very satisfactorily. The method also permits to determine the values of the transmissibility of the water layers of the experimental set-up.

Gas Transport in Vulcanized Natural Rubber-Cellulose. II. Composites

This work reports the transport of carbon dioxide, oxygen, and nitrogen in amorphous membranes of vulcanized natural rubber reinforced with regenerated cellulose. The values of the permeability coefficient of carbon dioxide, oxygen, and nitrogen in the composites with 25% of cellulose, measured at 25 °C and 15 cmHg of pressure, are roughly one-third of those measured in the same conditions for these gases in natural rubber. The isotherms representing the variation of both the permeability and diffusion coefficients of the gases with pressure present a relatively sharp increase in the region of low pressures, attributed to changes in the free volume. The analysis of the permeability characteristics of the membranes in terms of the free-volume theory suggests that gas transport is severely hindered in both the cellulose phase and the cellulose–rubber interphase of the composites.

Thermomechanical and diffusive studies in films prepared from copolymers of ethylene-1-octene subject to longitudinal and transversal induced stretching

Abstract The effects of induced stretching on the relaxation behaviour and on gas transport properties in coextruded linear low-density polyethylene (LLDPE) films, prepared from copolymers of ethylene-1-octene, are reported. The spectra show that the stretching, regardless of the direction of its application, does not produce significant changes in the γ and β relaxation processes. However, in the α region, there are some differences in the relaxation response depending on the direction of the induced stretching. The longitudinal stretching gives rise to a displacement of the temperature at which the α″ process occurs in LLDPE2 film with respect to LLDPE1 film. On the other hand, the transverse stretching does not produce a displacement of the α″ relaxation, but it does produce a decrease of intensity of the α′ relaxation peak in both films. Transport coefficients have been measured for CO 2 and O 2 in stretched polyethylene samples over temperatures ranging from 25 to 85°C. In all cases, a linear plot of the logarithm of the gas transport coefficients as a function of the reciprocal absolute temperature is observed, indicating that activated diffusion is occurring. Similarly, a break in these Arrhenius plots can be noted at nearly the same temperature range as reported in the thermomechanical study. The activation energies associated with the transport mechanism are reported. On the other hand, the results reveal that the films are dependent on thermal history. In order to obtain reproducible results, a suitable control of the thermal conditioning of the films is advisable.

Relaxation behavior of methacrylic polymers with bulky hydrophilic groups in their structures

The isochrones showing the temperature dependence of the loss relaxation modulus of poly(neopentyl glycol methacrylate) present an ostensible subglass absorption called β relaxation that roughly has the same intensity as the glass–rubber relaxation, or α process. The dielectric relaxation spectrum of this polymer also exhibits a well-developed β process followed at higher temperatures by the glass–rubber, or α relaxation, which strong conductive effects only permit to be detected at high frequencies. A detailed study of the conductive contributions to the dielectric loss above Tg was carried out using a theory that assumes that the dispersion observed in tan δ in the frequency domain arises from the Maxwell–Wagner–Sillars effect combined with Nernst–Planck electrodynamic effects caused by interfacial polarizations in the interface polymer electrodes. Attempts were made to evaluate the equivalent salt concentration that would produce the conductive effects experimentally observed.