Anomalous sorption kinetics in the methanol vapor–poly(methyl methacrylate) system

Abstract

ABSTRACT:The non-Fickian sorption kinetics of methanol vapor in poly(methyl methacrylate) films 8 and 51 m thick at 25°C are presented. The behavior of the system was studied in series of interval and integral absorption runs. The relevant diffusion coefficient and viscous relaxation processes were studied separately by kinetic analysis of the first and second stages of sorption kinetic curves. The sorption isotherm concaved upward at high activities, this being typical of Flory-Huggins behavior, whereas it exhibited a convex-upward curvature at low methanol vapor activities, this indicating sorption in the excess free volume of the polymer matrix. After excess free-volume fill-up, the concentration dependence of the diffusion coefficient was found to be well represented by the free-volume theory of Vrentas and Duda. Relaxation frequencies calculated from the second stage of two-stage curves exhibited a weak dependence on the concentration. Integral sorption experiments indicated that the system exhibited nearly case II kinetics at high methanol vapor activities.

Experimental and theoretical investigation of anomalous sorption kinetics in the methanol vapor–poly(methyl methacrylate) system at 35 °C

“…Analytical reagent methanol was used as vapor source. A detailed description of the experimental set‐up and procedure can be found elsewhere 1. Briefly, the sorption apparatus includes a temperature‐controlled liquid methanol flask for generating the desired vapor methanol pressure and a mercury manometer for vapor pressure measurements.…”

“…Methanol saturation pressure ( p sat ) was calculated to be 201.47 torr at 35 °C 13. The sample under study was the same one used in previous sorption study1 at 25 °C, and so it can be considered as an annealed polymer sample.…”

“…Sorption kinetic experiments consist the most common way of studying diffusion of various vapor or gas penetrants in polymeric materials. In such experiments,1, 2 a polymer film of thickness 2 l is pre‐equilibrated at a constant vapor pressure p i and temperature by bringing both surfaces of the film in contact with the vapor. Then, at time t = 0, p is raised (in the case of absorption) or lowered (in the case of desorption) to a new constant pressure p f .…”

“…The above kinetic behavior is usually interpreted in terms of the viscoelastic response of the glassy polymer to the osmotic stress induced by the penetrant 1, 8. The viscoelastic response is clearly manifested in two‐stage sorption kinetics, where the first stage corresponds to limited sorption and subsequent diffusion into the “elastically” swelling or “unrelaxed” polymer up to a quasi‐equilibrium, while the second stage is the result of long‐range chain rearrangements, which permit further (“delayed” or “viscous”) swelling and are slow on the diffusion timescale.…”

“…In this work, we present an experimental study of the sorption kinetic behavior in the system PMMA–methanol vapor (PMMA–MeOH), at 35 °C. The system has been extensively studied at 25 °C1 during a previous work. The main features of non‐Fickian kinetics are studied in series of interval absorption runs and the transport kinetic parameters are extracted by application of a model accounting for non‐Fickian sorption kinetics 5.…”