Concentration dependence of the time lag for diffusion

ABSTRACT:The release process of three osmotically active solutes with various solubilities in water (NaCl, CsNO 3 , and CsCl) from silicone rubber matrices is presented. The kinetics of release for different initial loads of the salts were supplemented by measurements of the kinetics of concurrent water uptake. To gain insight on the relevant non-Fickian transport mechanisms, the morphology, the diffusion and sorption properties and the physicochemical state of water in the salt-depleted matrices were studied. In addition, both salt-loaded and saltdepleted matrices were characterized with respect to their mechanical properties. The combined information, derived from these techniques, supported the operation of a release mechanism carried out through the formation of microscopic cracks, interconnecting the permanently formed cavities inside the matrices. The results indicate that these microscopic cracks may have healed upon drying.

Section: Resultscontrasting

confidence: 62%

Section: Resultsmentioning

confidence: 54%

Comparative study of the release kinetics of osmotically active solutes from hydrophobic elastomeric matrices combined with the characterization of the depleted matrices

Soulas

Sanopoulou

Papadokostaki

2009

Sorption effects in the measurement of the time lag for water vapor

Barrie

Machin

1968

The time-lag methodl72 has proved to be particularly useful for the det,ermination of the diffusion constant D of a gas or vapor in a polymer membrane. Frisch3 extended the analysis to systems with concentrationdependent D and obtained a general expression for the concentration dependence of the time lag L. Usually, a constant pressure of gas is maintained at the ingoing face of an initially outgassed membrane and the increase of the pressure at the outgoing face recorded as afunction of time. Extrapolation of the linear or steady-state portion of this plot back to the time axis gives the time lag as intercept. When water is the diffusate, the sorption of vapor on the glass walls of the measuring section can affect the build-up of the pressure. Consequently there have been relatively few attempts to measure time lags for water vapor in polymers. An exception is the work of Stannett et al.,4-7 who attempted to allow for the effects of sorption by subjecting the measuring section to an equilibrium pressure of -0.02 mmHg prior to commencing a run. With this modified procedure, L and hence the diffusion coefficient for several water-polymer systems were found to be constant, whereas steady-state and sorption-desorption kinetic analyses indicated that D(c) decreased with increasing concentration C of vapor. When D(c) decreases with C the variation of L with C is relatively small and the more difficult to measure accurately.s However, for a t least some of the water-polymer systems examined the variation of L with C predicted by using steady-state data and the Frisch equation was considered to be significant even allowing for the experimental scatter of points expected of a constant L system. It is of interest, therefore, to examine the effect on L of sorption of water on the walls of the apparatus, particularly in relation to the pretreatment technique of Stannett et al.The data of Frankg and Barrett and Gaugerlo for the sorption of water by washed Pyrex glass at 25°C in the relative humidity range (1) where Q is the amount sorbed at an equilibrium pressure p , (in mm Hg). In a typical permeation run the final pressure in the measuring section may be several times the "pretreatment" pressure of 0.02 mm Hg. Clearly, 2633

Permeation of gases through modified polymer films. V. Permeation and diffusion of helium, nitrogen, methane, ethane, and propane through γ‐ray crosslinked polyethylene

Macdonald

Huang

1981

SynopsisThe permeation and diffusion of helium, nitrogen, methane, ethane, and propane through y-irradiated polyethylene films were investigated. These studies were carried out with two objectives in mind: (1) to determine the effect of crosslinking by y irradiation on permeability and diffusivity using the gas molecules as molecular probes; and (2) to study the plasticizing effects of the low hydrocarbons on the polyethylene film. The y-ray-induced crosslinking efficiency of polyethylene was investigated in the following irradiation atmospheres: vacuum, acetylene, and nitrogen-acetylene mixtures. Results showed that irradiation in acetylene decreased the crosslinking efficiency while an acetylene-nitrogen atmosphere increased the efficiency compared to irradiation in vacuum. Both the permeation constants and the diffusion coefficients were found to decrease with increasing irradiation dose while the activation energies increased. The permeation constants of the organic gases through polyethylene increased with molecular diameter while the diffusion coefficients decreased. This increase in permeability was attributed to an increase in the solubility due to solubilization of the membrane by the penetrant. For example, the molecular diameter of propane is 4.397 A compared with 2.807 8, for methane; however, propane permeated the polyethylene film a t a rate twice that of methane. Nitrogen and methane have approximately the same molecular diameters-2.7085 and 2.807 A, respectively-but owing to the plasticizing effect of methane, it permeated the film at a rate three times greater than that of nitrogen. I t is interesting to note that the stronger the plasticizing ability of the penetrant, the greater the effect of the irradiation dose. The permeability of propane decreased by 40.7%, while the permeability of helium decreased by 6.4% after an irradiation dose of 50 Mrad.