(797g) Thermodynamic Analysis of the Solubility of Gases and Vapors in Swelling Glassy Polymers: A Predictive Modeling Approach | AIChE

(797g) Thermodynamic Analysis of the Solubility of Gases and Vapors in Swelling Glassy Polymers: A Predictive Modeling Approach

Authors 

Minelli, M. - Presenter, University of Bologna
Doghieri, F., University of Bologna



The determination of the solubility of gases and vapors in polymeric systems is of remarkable interest for many different applications, ranging from the fabrication of gas separation membranes to the development of barrier materials for packaging or sensors.

Equation of state approaches have been developed in the past following different routes, which provide reliable means for the calculation of the solubility of gases and vapors in rubbers or polymer melts. On the other hand, no comparable tools are available for the case of glassy polymers. The general approach called Non Equilibrium Thermodynamics of Glassy Polymers (NET-GP) coupled to Lattice Fluid (LF) equation of state was shown to be able to predict the solubility of gases and vapors in glassy polymers at various pressures based on pure component parameters. The corresponding NELF model relies on the use of polymer mass density as an internal state variable for the description of the out-of-equilibrium conditions for the penetrant-polymer system. The model application requires some information on the volumetric behavior of the polymer in sorption conditions, which might be a serious limitation to its predictive ability.

In the present work, a simple and effective rheological model has been considered in order to evaluate a priori the swelling induced by the penetrating species. The main assumption is in considering that the polymer relaxation occurs involving two characteristic times on fairly different scales. If temperature is appreciably below the glass transition of the polymer, only the short-time relaxation phenomena are involved during sorption within the usual experimental times.

This model allows the estimation of the polymer swelling at increasing penetrant activity and for the determination of the complete sorption isotherm by the NELF model, in a purely predictive way.

This tool was satisfactorily applied for many sorption isotherms of swelling gases (such as CO2 or C2H4) and organic vapors (xylene, toluene, or hydrocarbons) in a variety of glassy polymers such as polystyrene, polycarbonate, polysulfone or amorphous Teflon. The model was also able to capture complex behaviors such as those given by non linear trends of dilation or when a change in concavity of the sorption isotherm is observed due to a penetrant-induced glass transitions of the polymer. The experimental dilation isotherms in sorption conditions, for those systems for which they were available, confirmed the reliability of the present model.

Furthermore, the model was employed to describe the solubility behavior of CO2 in glassy polymers exposed to different pretreatment. The effect of thermal annealing, or the exposure of the polymer to a swelling agent, on the polymer sorptive capacity has been also analyzed. The present model was thus able to describe such differences simply by considering different initial polymer density, which is indeed assumed in the model as the only order parameter.

In conclusion, the NELF model, endowed with a rheological tool for the swelling prediction, is able to describe the solubility of gases and vapors in glassy polymer systems predictively. The modeling results, as obtained from a variety of different polymer/penetrant systems in different conditions, are compared with the experimental data retrieved from the literature showing the remarkable accuracy of this approach.

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