Sorption of hydrocarbons and alcohols in addition-type poly(trimethyl silyl norbornene) and other high free volume glassy polymers. II: NELF model predictions

Sorption and swelling of alkane and alcohol vapors in addition-type poly(trimethyl silyl norbornene) (PTMSN) obtained in part I of this work were analyzed and compared to the predictions of the Non-Equilibrium Lattice Fluid (NELF) model. The polymer characteristic Lattice Fluid (LF) parameters were determined by best fitting the infinite dilution solubility coefficients, S0, of a wide series of n-alkane penetrants, from ethane to n-dodecane. The solubility of alkanes, which is comparable quantitatively and qualitatively to that measured in PTMSP, is well represented by the model. The solubility isotherms of alcohols (methanol, ethanol and 1-propanol), which exhibit a peculiar sigmoidal behavior that finds no explanation within the dual mode model, are also in line with the NELF model prediction without any ad hoc correction. The model is also used to represent successfully the dependence of the solubility on the alkane size, in PTMSN as well as in PTMSP and Teflon AF2400. The three polymers exhibit different trends: in PTMSP the vapor-pressure normalized solubility coefficient (S0pvap) increases with the penetrant size, while in Teflon AF2400 the behavior is opposite and in PTMSN the size plays a very little role on the activity-based solubility. Such a behavior, that is useful to determine the selective performance of the polymer, was analyzed and discussed taking advantage of the support offered by the NELF model: it was seen that the three main factors affecting the extent of solubility dependence on the alkane size are the polymer fractional free volume and cohesive energy density, as well as its energetic interaction with the penetrant; an approximate formula is used to estimate a priori the behavior of the solubility based on the knowledge of the polymer properties.

► PTMSN is glassy with high free volume, high solubility for alkanes and alcohols.
► High solubility is accompanied by a rather high volume dilation.
► Isotherms for alkanes are concave, while for alcohols are initially convex to p axis.
► Isotherms are correctly modeled by the NELF model both for alkanes and for alcohols.
► The role of solute size on solubility is explained through enthalpic and entropic effects.