Abstract
The solubility and diffusivity of several liquid species in crosslinked polydimethylsiloxane (PDMS) films were determined with a gravimetric method at 35°C. The series of liquids considered includes alkanes (from n-C5 to n-C18; cyclo-C6), water, acetone, alcohols (ethanol, 1-propanol, 1-butanol, iso-butanol, tert-butanol, 1-pentanol, 1-hexanol), terpenes (squalene, limonene, linalool, geraniol) and edible oils with different oleic acid contents. The effect of size, structure and solubility parameter of the different molecules on the sorption and transport properties in PDMS was discussed. The diffusivity values of the different penetrants inspected span over one decade while the molar solubility spans over four decades, indicating that the membrane selective behavior is a strong function of liquid solubility. In particular, the solubility of hydrocarbons (alkanes, hydrocarbon terpenes) that have favorable energetic interactions with the polymer is entropy-driven, i.e. decreases with molecule size. The solubility of substances bearing one alcoholic group is also strongly affected by the energetic interactions with the polymer, especially if their alkyl chain length is small. The trends are due to the dual entropic/enthalpic nature of solubility, which obeys a nice exponential decreasing trend when reported versus the product MW0.75 (δ−δPDMS)2 which accounts for both entropic and energetic nonideality of the polymer–penetrant mixture. The diffusivity decreases roughly monotonically with the molecular size of the penetrant within a series of homologous penetrants, unless the penetrants have very high solubility differences and induce different swelling in the polymer matrix. Solubility data are well represented by the Flory−Rehner model, that also allows to explain differences between experimental sources based on the crosslinking degree.
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