Abstract
Solubility and permeability of gases in glassy polymers have been considered with the aim of illustrating the applicability of thermodynamically-based models for their description and prediction. The solubility isotherms are described by using the nonequilibrium lattice fluid (NELF) (model, already known to be appropriate for nonequilibrium glassy polymers, while the permeability isotherms are described through a general transport model in which diffusivity is the product of a purely kinetic factor, the mobility coefficient, and a thermodynamic factor. The latter is calculated from the NELF model and mobility is considered concentration-dependent through an exponential relationship containing two parameters only. The models are tested explicitly considering solubility and permeability data of various penetrants in three glassy polymers, PSf, PPh and 6FDA-6FpDA, selected as the reference for different behaviors. It is shown that the models are able to calculate the different behaviors observed, and in particular the permeability dependence on upstream pressure, both when it is decreasing as well as when it is increasing, with no need to invoke the onset of additional plasticization phenomena. The correlations found between polymer and penetrant properties with the two parameters of the mobility coefficient also lead to the predictive ability of the transport model.
Highlights
The analysis of the solubility and permeability of gases, vapors and liquids in polymeric phases is of remarkable relevance for various applications [1,2,3,4,5], and among the others for membrane-based gas separation [6]
Such noequilibrium approach has been widely employed as NELF model by using the lattice fluid equation of state (EoS) model by Sanchez and Lacombe [27,28], or as nonequilibrium perturbed hard sphere chain theory (NE-PHSC) [29] and nonequilibrium statistical associating fluid theory (NE-SAFT) [30] by using tangent spheres-based model perturbed hard sphere chain theory (PHSCT) [31] and statistical associating fluid theory (SAFT) [32,33], to predict the solubility behavior of gases [34], vapors [35], liquids [36] and gas mixtures [37,38] in all kinds of glassy polymers [39,40,41]
CO2 is able to induce a significant dilation of all polymer matrices, as indicated by the rather large swelling coefficients calculated by the NELF model from the experimental solubility data (Table 4), while much lower values are obtained for CH4, and almost no swelling is associated with N2 sorption
Summary
The analysis of the solubility and permeability of gases, vapors and liquids in polymeric phases is of remarkable relevance for various applications [1,2,3,4,5], and among the others for membrane-based gas separation [6]. The solid theoretical basis of the model allowed the derivation of a predictive tool for the a priori evaluation of the gas permeability in glassy membranes, based only on the correlations found between model parameters and the properties of pure penetrant and pure polymer [26] Such a transport model represents a simple but effective tool for the description and prediction of gas permeability, and for the evaluation of its dependence on relevant process conditions as feed pressure, composition and temperature, required for the design and development of novel membrane materials, as well as for the optimization of gas separation processes. The model has been described and applied in detail to the case of gas sorption and transport in three relevant glassy polymeric systems: (i) polysulfone (PSf), selected as a traditional and commercial membrane material; (ii) polyimide 2,2-bis(3,4-carboxyphenyl) hexafluoropropane dianhydride, 4,4-hexafluoro diamine (6FDA-6FpDA), and poly (phenolphthalein terephthalate) (PPh), selected as representative of innovative materials
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