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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.

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.

Complex permittivity of a conducting, dielectric layer containing arbitrary binary Nernst–Planck electrolytes with applications to polymer films and cellulose acetate membranes

The theory of Trukhan [Sov. Phys. Solid State (Engl. Transl.), 1963, 4, 2560] for the calculation of the complex permittivity of a conducting dielectric film containing a binary 1 : 1 electrolyte is critically reviewed. It is shown that the final result of Trukhan is correct, in spite of some errors and odd procedures in the original derivation. The method of Trukhan is changed to a more concise procedure which is better suited to generalisations, and the theory is generalised to binary electrolytes of any charge type. The dimensionless excess impedance (over and above the Maxwell–Wagner–Sillars impedance) and the complex relative permittivity are functions of the type of electrolyte, the dimensionless frequency, the ratio of the ionic diffusion coefficients and the film thickness scaled by the Debye length. The complex relative permittivity has a Debye-like relaxation, but for very different values of the two diffusion coefficients and for films which are thin compared with the Debye length, one can clearly distinguish two dielectric relaxations, one for each ion. The peak in the dielectric loss corresponding to the ion of higher valency, is dominating in thin films. When the film is thicker, (or when the electrolyte concentration is increased) the low-frequency peak diminishes relative to the high-frequency peak. The maximum in the loss tangent is positioned at higher frequencies than the maximum in the dielectric loss. Therefore it is often an advantage to fit the loss tangents to actual measurements, since measurements at low frequencies are generally more difficult. Two applications of the theory are treated: the low-frequency dielectric relaxation of a dry copolymer of vinylidene cyanide and vinyl acetate with conducting ions (T. Furukawa, M. Date, K. Nakajima, T. Kosaka and I. Seo, Jpn. J. Appl. Phys., 1986, 25, 1178) and a wet membrane of dense cellulose acetate (I. W. Plesner, B. Malmgren-Hansen and T. S. Sorensen, J. Chem. Soc., Faraday Trans., 1994, 90, 2381). The use of the so-called ‘constant-phase element’ is discussed and criticised. The constant-phase element leads to infinite dissipation at zero frequency in contrast with the generalised Trukhan theory (and common sense). The constant-phase element is just superficially fitting the data in a limited range of frequency and no physical information (e.g. diffusion coefficients) is obtained from the fitted parameters of such an element.

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.

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.

Permeability of oxygen through membranes of poly(cyclohexyl acrylate)

Abstract The permeability of oxygen through membranes of poly(cyclohexyl acrylate) at temperatures in the vicinity of the glass transition temperature ( T g ) is determined using an electrochemical experiment. The diffusion coefficient of the gas follows Arrhenius behaviour with an activation energy of 22 kcal mol −1 , a surprisingly high value if it is considered that the range of activation energies observed in most systems is 1–20 kcal mol −1 . The interpretation of the free volume of the membrane at T g suggests that the critical volume necessary for permeation of the gas to take place is nearly four times lower than the value of this parameter for the mechanical and dielectric processes. It is suggested that bulky side groups may aid the transport of gas in the glassy state but presumably hinder it in the rubbery state.

Electrochemical properties of ion-exchange membranes based on sulfonated EPDM-polypropylene blends

The preparation of cation-exchange membranes by compression molding of sulfonated EPDM-polypropylene blends is described. Concentration potentials measured in a two-compartmental cell with configuration Ag/AgCl HCl solution (C 1 )/membrane/ HCl solution (C 2 ) AgCl/Ag., are a linear function of log a 2 (=C 2 γ 2 , where C 2 and γ 2 are, respectively, the concentration and activity coefficient of the solution in compartment 2 keeping constant the concentration in compartment 1, C 1 . The values of the proton transport numbers, t + , decrease from 0.99 to 0.94 when the ratio C 2 /C 1 increases from 0.1 to 5, nearly the same decrease reported for Nafion 117 in the same variation of concentration ratios. Proton conductivities at 25°C are 9.84 × 10 -4 and 7.25 X 10 -4 S/cm, respectively, for water-saturated membranes containing 20 and 10% of polypropylene, more than one decade below the value reported in the literature for the conductivity of Nafion membranes at the same temperature. These conductivities at 90°C amount to 8.35 X 10 -2 and 1.03 X 10 -2 S/cm, respectively.

Complex permittivity of a film of poly[4-(acryloxy)phenyl-(4-chlorophenyl)methanone] containing free ion impurities and the separation of the contributions from interfacial polarization, Maxwell–Wagner–Sillars effects and dielectric relaxations of the polymer chains

The previous theoretical study of measured complex permittivities of the amorphous copolymer of vinylidene cyanide and vinyl acetate (T. S. Sorensen and V. Compan, J. Chem. Soc., Faraday Trans., 1995, 91, 4235) is refined and used to explain measurements of the complex permittivity of the amorphous polymer poly[4-(acryloxy)phenyl(4-chlorophenyl)methanone] at different temperatures and frequencies. Throughout the paper we stress the use of physical models rather than the traditional use of equivalent circuits without much informational content. The data exhibit a maximum in the loss tangent at low frequencies (below 0.1 Hz) which can be explained by interfacial polarization near the two condensor plates, and from which information concerning the diffusion and the concentration of free charge (mobile ion) carriers may be estimated. The diffusion coefficient for the fast ion varies from 1.9 × 10–14 m2 s–1 at 120 °C to 1.8 × 10–13 m2 s–1 at 140 °C. The activation energy for diffusion is Ea/R≈ 12 800 K. At frequencies in the range 0.1–10 Hz, the Maxwell–Wagner–Sillars contribution dominates. This contribution is due to the bulk conductivity of the free charge carriers. From the value of this conductivity and the information obtained from the interfacial polarization, the value of the static permittivity of the polymer may be found. Experimentally, this value is completely obscured by the effects of the free charge carriers. At higher frequencies, the dielectric relaxations connected with the motion of segments of the polymer chains are seen if corrections are made for conductivity. Two dielectric loss peaks are observed moving towards higher frequencies when the temperature increases. The low frequency relaxation has an activation energy Ea/R≈ 11 200 K. For the high frequency relaxation Ea/R≈ 24 500 K. The approximate identity of the activation energy of the low frequency relaxation and the activation energy of diffusion might indicate that the same molecular reorientation is involved in both cases.

Simulations of diffusive and sorption processes of gases in polyimide membranes: Comparison with experiments

The trajectories of carbon dioxide, oxygen and nitrogen in membranes prepared from poly(bisphenol A-co-4-nitrophthalic anhydride-co-1,3-phenylene diamine) were simulated by assuming that the molecules of gas migrate through the glassy polymer in a sequence of hops between local minima of the potential energy. The values of the diffusion coefficients in units of 10−9 cm2 s−1 were 1.8, 7.3 and 7.8, in fair agreement with the experimental results, 1.3, 5.4 and 1.6, respectively, in the same units. The solubility coefficient was simulated by using Monte Carlo techniques in which the probabilities of inserting/removing a molecule of diffusant in a matrix containing n molecules were evaluated as the product of three factors depending, respectively, on: (a) the pressure p and temperature T, (b) the interaction energy between the diffusant particle and the polymer host matrix, and (c) the volume of the diffusant. The simulated pressure dependence of the concentration of carbon dioxide in the membrane displays the same pattern as that experimentally found. The simulated values of the solubility coefficients of CO2, O2 and N2, at 30°C and 76 cmm Hg of pressure, are in rather good agreement with the experimental results obtained from both permeation and sorption experiments.