(336c) Modelling the Solubility of Gases in High Free Volume Glassy Polymers with the Nelf Model

High free volume glassy polymers such as polyimides, substituted polyacetylenes, polynorbornenes are very promising for the gas separation or sensor field, due to extremely high gas/vapour solubility and permeability.

In this work we present an experimental characterization of sorption and diffusion of organic n-alkane vapours in poly (trimethyl-silyl norbornene), as well as the volume dilation isotherm associated to the polymer swelling during sorption. The polymer examined is endowed with a high fractional free volume and exhibits a very high vapour uptake.

The experimental data have then been modelled with the Non Equilibrium Lattice Fluid (NELF) approach. The model provides an extension of the Lattice Fluid equation of state, valid above the glass transition, to the non equilibrium state typical of glassy polymers below Tg. The gas or vapour solubility isotherm can be reliably predicted by the model on the basis of the knowledge of the polymer density in the glass and of the pure component parameters, typically obtained from information on the volumetric behaviour of the pure penetrant, e.g. along the Liquid-Vapour saturation curve, and of the pure polymer, in terms of Pressure-Volume-Temperature data above Tg.

In the case of polynorbornenes, however, there is no experimental access to the equilibrium volumetric behavior above the glass transition, since due to their rigid structure they do not exhibit a glass transition before degradation, and thus the determination of the pure polymer parameters cannot be obtained through the usual procedure. To overcome this difficulty we have adjusted the polymer parameters on experimental gas solubility data at infinite dilution, obtained by inverse gas chromatography. The results have been compared successfully to the experimental vapour solubility and dilation isotherms of normal pentane and hexane; the same parameters have been used to evaluate a priori the solubility of other penetrants.