Sorption kinetics and equilibrium uptake for water vapor in soft‐contact‐lens hydrogels

Abstract

A gravimetric-sorption technique was used to obtain kinetic and equilibrium adsorption/desorption data for water vapor in four different soft-contact-lens (SCL) polymers at 35 degrees C. The SCL materials are a conventional hydrogel (polymacon) with a low water content at saturation (<50 wt %); two conventional hydrogels (hilafilcon A and alphafilcon A) with a high water content at saturation (>50 wt %); and a siloxane hydrogel (balafilcon A). Absorption and desorption equilibrium isotherms (water activity versus water weight fraction) overlap at high water contents, whereas significant hysteresis is observed at low water contents. The hysteresis loop is likely due to trapping of water in the polymer during the desorption process because of a rubber-to-glass transition of the SCL-film surfaces. Sorption data were interpreted using Flory-Rehner theory. The positive Zimm and Lundberg cluster function suggests that water tends to cluster in these SCL materials, except at very low water content. For polymacon and hilafilcon A, Fickian diffusion is observed for all activities for both water sorption and desorption. However, for alphafilcon A and balafilcon A, non-Fickian features appear at intermediate/low activities, in particular during water desorption, suggesting coupling of the diffusion process with polymer-matrix relaxation. The diffusion coefficient increases significantly with water concentration for polymacon and hilafilcon A (from approximately 0.3 x 10(-8) to 4.0 x 10(-8) cm2/s) because of augmented mixture free volume induced by water sorption, whereas a more complex composition dependence is observed for alphafilcon A and balafilcon A probably as consequence of a combined effect of polymer relaxation, plasticization, and water clustering.

Relaxational study of poly(vinylpyrrolidone-co-butyl acrylate) membrane by dielectric and dynamic mechanical spectroscopy

“…water content and some contain over 70% [35]. Moreover, the water swollen membrane was a highly tractable material with fairly the same mechanical properties that the dried membranes.…”

Water Transport in Resin-modified Glass-ionomer Dental Cement

“…Water uptake has been studied in pure polyHEMA and in HEMA copolymers, using both exposure to water vapor [15] and immersion in liquid water [16][17][18]. The process was Fickian in all cases, and the diffusion coefficient has been determined twice as 1.7 Â 10 À7 cm 2 /s [16,17] and once as 1.96 Â 10 À7 cm 2 /s [18].…”

Osmotic Pressure and Phase Boundary Determination of Multiphase Systems by Analytical Ultracentrifugation

“…[4] With more complex systems such as reactive gels or clays, hysteresis effects must also be clearly identified. [5] Exciting modern aspects of chemical thermodynamics make use of a membrane osmometer and a point-by-point determination of osmotic pressure to investigate complex solvents including salt or added alcohols. [6] In surfactants and lipids, the packing of molecules in the form of an equilibrium film leads to constraint in local curvature and hence self-assembly of these amphiphiles into one of the canonical phases, where local geometry can be lamellar, cylindrical or simple droplets.…”

“…(&) represents AUC experimental data and (*) represents literature experimental data. [24] (c) in the high concentration regime represents theoretical osmotic pressure calculated using Equation (4), (c, grey) in the low concentration regime represents theoretical osmotic pressure using Equation (5). Vertical lines delineate the literature-determined phase boundaries at l w = 67 and 230 nm, and in (a) the Debye length for 10 À3 m salt, of~10 nm.…”