Ferruccio Doghieri

Predictions of the solubility of gases in glassy polymers based on the NELF model

Abstract A reliable, predictive model for the solubility of gases and vapours in glassy polymers has been recently presented: it is based on the free energy expression for the polymer penetrant mixture as obtained from the lattice fluid theory and on the idea that the partial polymer density is an internal state variable for the system. When the polymer density for the sorption condition is known, the model can be used in a pure predictive way as the solubility is estimated on the basis of pure penetrant and pure polymer PVT properties. In this work the basic assumptions of the model and its development are discussed in detail. The pseudo-equilibrium condition and the calculation of the penetrant solubility under pseudo-equilibrium conditions are clearly stated. In closure, several examples comparing model predictions and experimental results for the solubility in glassy polymers are presented. In all the cases considered, the comparison confirms the model capability to satisfactorily predict solubility data in nonequilibrium glassy polymers and supports the validity of the underlying physical model.

Hydrocarbon and fluorocarbon solubility and dilation in poly(dimethylsiloxane): Comparison of experimental data with predictions of the Sanchez–Lacombe equation of state

Sorption and dilation isotherms are reported for a series of gases (N2, O2, CO2), hydrocarbon vapors (CH4, C2H6, C3H8), and their fluorocarbon analogs (CF4, C2F6, C3F8) in poly(dimethylsiloxane) (PDMS) at 35°C and pressures up to 27 atmospheres. The hydrocarbons are significantly more soluble in hydrocarbon-based PDMS than their fluorocarbon analogs. Infinite dilution partial molar volumes of both hydrocarbons and fluorocarbons in PDMS were similar to their partial molar volumes in other hydrocarbon polymers and in organic liquids. Except for C2H6 and C3H8, partial molar volume was independent of penetrant concentration. For these penetrants, partial molar volume increased with increasing concentration. The Sanchez–Lacombe equation of state is used to predict gas solubility and polymer dilation. If the Sanchez–Lacombe model is used with no adjustable parameters, solubility is always overpredicted. The extent of overprediction is more substantial for fluorocarbon penetrants than for hydrocarbons. Very good fits of the model to the experimental sorption and dilation data are obtained when the mixture interaction parameter is treated as an adjustable parameter. For the hydrocarbons, the interaction parameter is approximately 0.96, and for the fluorocarbons, it is approximately 0.87. These values suggest less favorable interactions between the hydrocarbon-based PDMS matrix and the fluorocarbon penetrants than between PDMS and hydrocarbons.

Pervaporation of methanol-MTBE mixtures through modified poly(phenylene oxide) membranes

Abstract Modified poly(phenylene oxide) membranes were used to separate methanol from methyl tert -butyl ether solutions through a pervaporation process. The process characterization was performed considering the influence of both the permeate side pressure and the methanol concentration in the feed mixture. The membrane performance has been expressed in terms of transmembrane flux and overall separation factor. Experimental results are discussed in terms of solubility, diffusivity and driving force for the diffusion process of each component through the polymeric membrane.

Solubility, diffusivity, and mobility of n‐pentane and ethanol in poly(1‐trimethylsilyl‐1‐propyne)

The diffusion coefficient of ethanol and of n-pentane in PTMSP, at 27°C, was measured as a function of concentration up to a penetrant content of about 12% by weight, for polymer samples obtained through different processes; differential sorptions and desorptions with vapor phases were considered. In the case of ethanol a nonmonotonous behavior was observed for the diffusivity, while in the case of n-pentane the same property was found to monotonously decrease with increasing the penetrant content. The sorption isotherms were also reported, indicating that n-pentane exhibits a typical dual mode behavior, while ethanol follows an unusual s-shape curve. The chemical potential of the dissolved penetrants, calculated directly from the isotherms, shows the very different importance of the energetic interactions of the two penetrants with the polymer units. In spite of the remarkably different concentration dependencies observed for both solubility and diffusivity of the two penetrants, the mobility factors are in both cases monotonously decreasing with the penetrant concentration, and follow very similar trends. The significant differences observed for the concentration dependence of the diffusion coefficients are, thus, associated to the thermodynamic contributions, which are very different for n-pentane and ethanol. Different polymeric films, obtained through different solvent evaporation processes, show quite different solubility, diffusivity and mobility for both ethanol and n-pentane. On the other hand, the ratio between the mobility of the two penetrants as well as the slope of mobility as function of the concentration remains the same for all the different samples inspected.

Sorption and diffusion of vapors in PTMSP and PTMSP/PTMSE copolymers☆

The mass transport properties of n-alkanes in pure poly(1-trimethylsilyl-1-propyne) (PTMSP) and in its copolymers with poly(1-trimethylsilyl-1-hexyne) (TMSE) have been investigated using gravimetric techniques. The results show in all cases a decrease in solubility and diffusivity as the fraction of TMSE increases; that indicates a significant effect of TMSE on the fractional free volume of the polymeric matrix. The evaluation of the mobility parameter from the diffusion data further confirms this effect. The behavior at different temperatures shows that the sorption process is exothermic; the absolute value of the heat of mixing decreases with increasing the TMSE content, while the activation energy for diffusion increases. The effect of the penetrant molecular weight on solubility is examined in some detail and interpreted based on the NELF model.

Gas solubility in glassy polymers: predictions from non-equilibrium EoS

Abstract Extension of well established EoS for fluid mixtures to the description of thermodynamic properties of non-equilibrium glassy polymeric phases is considered and corresponding calculations for pseudo-equilibrium gas content are performed. Results obtained using three different “tangent hard spheres chain” EoS are discussed and compared with experimetnal data for low-pressure gas solubility in glassy polycarbonate. While the overall result allows to conclude that tools developed herein may be confidently used for pure predictions of gas and vapor solubility in polymeric material below the glass transition temperature, interesting differences among the performance of different models were observed which can be attibuted to the different form of interaction potential considered.

Non-fickian transport of alkyl alcohols through prestretched poly-methylmethacrylate

Abstract The effect of the deformation of the solid matrix on the sorption kinetics of low molecular weight alcohols into PMMA was studied. Sorbed mass and cross-sectional area were measured as a function of time for sorption experiments into polymeric films which had been previously stretched. The effect of different draw ratios and different molecular weights of the solvent on the sorption process is discussed in terms of transport kinetics character, swelling front velocity and relaxation time. An interpretation of the experimental data is also presented based on the relevant dimensionless numbers comparing the characteristic times of the elementary processes involved.

Structure and sieving mechanism of high selective graphene-based membranes

Graphene oxide was used as charge able to confer high selectivity to the final product. A self-assembling technique, namely layer-by-layer has been developed to stratify graphene-based coating on polymeric films; this coating is composed by nanolayers of graphene oxide alternated with polymers, bonded each other by electrostatic forces. Permeability measurement on layered Matrimid®, a commercial polyimide, showed incredibly high selectivity values to small particle mixtures, as O2, CO2, He and H2. Through simple post-treatments the selective performance was also improved, as demonstration of potentiality of the well-ordered bi-dimensional system: improvement on the coating would make this material one of the viable solution for industrial separations, e.g. hydrogen purification in sustainable energy production. A further investigation on similar structures obtained by other strategies shall demonstrate the peculiar mechanism occurring in this material for high selective performance.

Probing effect of solvent concentration on glass transition and sub-Tg structural relaxation in polymer solvent mixtures: The case of polystyrene-toluene system

A novel experimental method for the analysis of volume relaxation induced by solvents in glassy polymers is presented. A gravimetric technique is used to evaluate the isothermal solvent mass uptake at controlled increasing/decreasing solvent pressure at constant rate. Fundamental properties of the solvent/polymer system can be obtained directly, and models can be applied, combining both nonequilibrium thermodynamics and mechanics of volume relaxation contribution. The fundamental case of polystyrene and toluene mixtures are thus accounted for, and various experimental conditions have been explored, varying the temperature, and spanning over different pressure increase/decrease rates. The results obtained allowed to evaluate the isothermal second order transition induced by solvent sorption, as well as the determination of the effect of the pressure rate. Therefore, this work proposes a new standard for the characterization and the understanding of the relaxational behavior of glassy polymers.

Solute induced relaxation in glassy polymers: Experimental measurements and nonequilibrium thermodynamic model

Data for kinetics of mass uptake from vapor sorption experiments in thin glassy polymer samples are here interpreted in terms of relaxation times for volume dilation. To this result, both models from non-equilibrium thermodynamics and from mechanics of volume relaxation contribute. Different kind of sorption experiments have been considered in order to facilitate the direct comparison between kinetics of solute induced volume dilation and corresponding data from process driven by pressure or temperature jumps.