Modelling the solubility of CO2 in rubbery and glassy amorphous PS and PMMA with the SAFT-γ Mie group-contribution EoS and the NET-GP approach

Abstract

The sorption of fluids in polymers is important for their industrial application in, e.g., separation membranes, corrosion protection liners and permeation barriers. The solubility of carbon dioxide (CO₂) in polymers is of interest for its transport and storage. In this work, CO₂ solubility in polystyrene (PS) and polymethyl methacrylate (PMMA), above and below the glass transition temperature $T_g$ of the pure polymers (both ∼105 °C), is modelled by combining the SAFT-γ Mie group-contribution equation of state (EoS), together with the framework of nonequilibrium thermodynamics for glassy polymers (NET-GP). Selected parameters of the EoS are optimised to fit pure polymer density and CO₂ solubility measurements from the literature at pressures up to 20 MPa, and temperatures from 150 to 200 °C (although the default parameters already give good agreement for CO₂ solubility in PS). Good agreement with solubility measurements at lower temperatures, though still above $T_g$, is obtained at up to 40 MPa. Below the $T_g$ of the pure polymers, between 35 °C and 81 °C, application of NET-GP results in only a small increase in the predicted CO₂ solubility at < 2–3 MPa, when the amount of sorbed CO₂ is small, over that determined from the EoS alone. Thus CO₂ appears to readily plasticise both polymers, inducing an equilibrium rubbery state. The methodology presented is a computationally efficient tool for the investigation of fluid sorption in amorphous polymers, which can be readily extended to other fluid+polymer pairs, including ones with novel chemistries.