The solubility and transport properties of small molecules in polymers — A modeling approach

Abstract

AbstractThe molecular mechanisms acting in the transport of “guest” molecules through polymeric solids still pose considerable difficulty: despite a manifest technological demand there has been a considerable lack of fundamental understanding of the mechanisms underlying the solubility and diffusion of even the smallest molecules in macromolecular substances. It is well known that the transport coefficients of low‐molar‐mass substances in polymers are strongly dependent on a number of factors, prominent among them the chemical structure of macromolecules. Nevertheless, the models applied for interpretation of transport of small molecules through macromolecular systems have been usually of a rather unspecific nature.Recently, modeling of the transport of gases through polymers in atomistic detail has been rather successful in elucidating the mechanisms as well as in predicting, without adjustable parameters, the transport coefficients. For the solubility, the quantitative estimate is rather rough since the estimation of the free energy of residence of guest molecules in a host matrix is plagued by the usual inaccuracies, the topological information is accurate, however; on the other hand, it is possible to estimate the diffusion coefficient to within a factor or ca. 2–5 over the entire range investigated (105 < D/(cm2/s) 10< −10).Mechanistically, one can state that the interstitial unoccupied column in the polymer can accommodate gas molecules with only small effects on the matrix. The solubility isotherm follows Fermi‐type statistics as long as the solute concentration is not too high. The gas transport through polymers occurs by a hopping mechanism in a matrix of macromolecular material that executes rapid thermal motion not coupled to large‐scale changes in the polymer. Consequently, the mechanism for gas transport in glasses and melts (or rubbers) is the same. On a short time scale, typically for times between 1013s and ns (or more), the diffusion is anomalous, due to the interstitial structure of the polymer. Einstein diffusion occurs only on time scales that are significantly longer (sometimes up to ms).