Heat effects in sorption of organic vapors in rubbery polymers

Abstract

SYNOPSISDuring supposedly isothermal sorption/desorption of gases or vapors by solid polymers, latent heat effects alter local temperatures with the result that diffusion behavior may appear to be non-Fickian. Even when sorption curves are seemingly Fickian, spurious values of the diffusion coefficient, D, may still be inferred. These phenomena are examined in an experimental and theoretical study of incremental sorption/desorption of acetone vapor in the fluoroelastomer, vinylidene fluoride/hexafluoropropylene copolymer. A theoretical analysis developed earlier to model water vapor sorption in wool and cellulose is shown to successfully reconcile measured mass transfer rates and temperature changes. Because diffusion time scales typical of organic vapors dissolved in rubbery polymers are often comparable to or smaller than those of heat transfer between the solid and vapor phases, heat effects accompanying sorption are slowly dissipated, and neglecting them may produce large errors in D.The primary effect of a perturbation in solid temperature is on vapor solubility. In sorption, elevated temperatures reduce solubility, which slows mass uptake. In desorption, reduced temperatures increase solubility, slowing devolatilization. In both cases, the result is artifactual lowering of the apparent D.A secondary effect is associated with the dependence of D on temperature. The increase in apparent D manifests itself primarily at the start of sorption/ desorption. Its significance decreases as the increment in vapor activity decreases.' Because of the small activity increments in the experimental study, D was treated as a constant in the theoretical analysis outlined below. The phenomena investigated here were first analyzed rigorously in a study of water vapor sorption in wool and polymer^.^^^ Because of water's large heat of vaporization, its heat of sorption, A H S , is much greater than those characteristic of organic vapors in polymers, which is probably why heat effects in the latter systems have generally been neglected. However, our recent studies of organic vapor sorption in rubbery polymers495 have demonstrated significant heat effects and correspondingly abnormal sorption behavior, especially at high vapor activities.Investigations of heat effect^^^^^^^ have heretofore focussed on the determination of correct D values from sorption/desorption data, and calculation of time courses of solid-phase temperature. Large temperature excursions were measured during recently reported integral desorption measurement^.^ Apparently no previous investigation has reconciled model predictions with both experimentally measured mass transfer rates and temperature data, as we do here. In addition, we explore the utility of a single dimensionless parameter that emerges from 937

Kinetics of Water Vapor Sorption in Wood Cell Walls: State of the Art and Research Needs

“…Therefore, the driving potential for bulk transport of water molecules from the air to the wood is decreased locally. The effect of temperature changes during sorption has been suggested from several studies of sorption in various materials [64,65], where the approach to equilibrium was slower than predicted by a Fickian diffusion model, see Section 5.1. Moreover, the shape of the sorption curves did not correspond to the Fickian diffusion model.…”

“…Moreover, the shape of the sorption curves did not correspond to the Fickian diffusion model. The experimental data could be explained by taking into account the increase in temperature during sorption and subsequent thermal equilibration [64][65][66]. Importantly, the sorption process was retarded by the temperature changes occurring but followed the thermal equilibration, meaning that equilibrium for both solvent uptake and temperature with the surrounding atmosphere was completed within the same timeframe.…”

“…A recent model for explaining sorption kinetics in wood is the thermally limited moisture transfer (TLMT) model developed by Willems [124]. This model includes effects on the sorption kinetics from both external mass transfer and temperature increase/decrease from sorption at the wood surface, similar to other previously developed theoretical models [64][65][66]. In the TLMT model, the sorption process is described by two components, see Equation 5, which both relate to coupled moisture and heat transfer between a wood specimen and its surroundings.…”

Sorption and diffusion of organic vapors in two fluoroelastomers

“…However, it is only recently that similar non‐Fickian effects in the sorption of organic vapors in polymer films have been attributed to nonisothermal diffusion conditions 2. The direct observation of the temperature increase accompanying the sorption of acetone in Fluorel, determined as part of this study, was reported in a prior publication 3. The present report is concerned with an attempt to interpret the sorption and diffusion results as a key to understanding the differences in the properties of the two fluoroelastomer samples.…”

Solvent sorption in a polymer–solvent system – Importance of swelling and heat effects

“…Further, the model involved a linear approximation to determine solubility change with temperature. Another study has been conducted to assess these effects [8], and the results of this study indicate that heat effects are extremely important during sorption in polymeresolvent systems. Further, they concluded that sigmoidal uptake curves commonly result from heat effects alone in typical gravimetric experiments.…”