(180af) Solubility and Diffusivity of Water and of Organic Liquids in Glassy and Rubbery Polymers

Separation processes play a major role in chemical, food and pharmaceutical industry, where they are required for the fractionation of mixtures, the recovery of high valued molecules from complex feedstock and the removal of unwanted substances that  could give rise to off specific products, outflavours in food and beverages and shelf-life reduction.  The industry of aromas and nutriceuticals  requires to deal with feedstock with an initially low concentration of  the molecules of interest. Moreover, these molecules should be treated with care, since they are frequently prone to denaturation and degradation, that can be triggered by thermal and chemical stimuli.

In several of the above mentioned processes, therefore, membrane-based techniques, such as, for instance, pervaporation and organic solvent nanofiltration (OSN), can offer a viable separation method in alternative to chemical and thermal techniques. In both such processes, the solution-diffusion mechanism of transport inside the membrane plays a relevant role [1].

Therefore, in the design of such processes, the knowledge of the fundamental transport parameters, e.g. diffusivity and solubility, of the interesting liquid components into membrane materials, is required. Indeed, the knowledge of such basic parameters can be the basis for a fundamental understanding and knowledge of the thermodynamic and of the kinetic aspects which control the separation.

Unfortunately, a considerable lack of basic transport parameters of liquids in the most interesting polymers is observed in the literature. Such data are useful for the design of membrane modules, but also for the validation of models which can be used to reduce the number of experimental data required.

In the present work, the solubility of a series of liquid penetrants (water, n-alkanes, alkyl-alcohols, ethyl acetate, acetone, edible oil components) has been inspected at room temperature, into dense films of both glassy (Matrimid 5218) and rubbery (crosslinked PDMS) polymers.

Matrimid 5218 is a commercial polyimide that is wholly amorphous and exhibits a high glass transition temperature with respect to the common operating conditions of  membrane process [2].

Dense films of Matrimid 5218 were prepared via solution casting, starting from a 5% solution of  Matrimid 5218 in dichloromethane. After drying, the film was kept under vacuum in a oven for 24 hours, at 200°C. Since glassy polymers exhibit history-dependent physical properties, some tests were run also on films that were treated at 100°C and 50°C, in order to understand the effect of the membrane thermal pre-treatment. Crosslinked PDMS preparation was performed using a proprietary crosslinker-catalyst system supplied by Wacker Silicones Corp, followed by hexane extraction in order to remove unreacted low molecular weight species. Since PDMS is rubbery at the operating conditions of our test, it is assumed that thermal history does not exert any relevant influence on the final physical properties. The measurements of pure component sorption and of total sorption, for mixtures, has been performed gravimetrically.

Samples were kept inside flasks filled with the liquids, stored in a oven for the temperature control. The samples were removed from the liquid on a periodic basis, quickly dried with a sheet of paper and weighted on a laboratory precision balance. Then the samples were placed back in liquid

In several cases it was also possible to obtain relevant data on sorption kinetics and the respective diffusion coefficients. Both parameters can affect the performance of those membrane separations processes in which the solution-diffusion mechanism contributes to the transport. The experimental solubility data collected for the rubbery crosslinked PDMS were compared to the predictions of  Equation of State (EOS) model, such as Sanchez Lacombe EOS and PC-SAFT EOS, while the solubility of data collected for Matrimid 5218 were compared to the predictions of the Non Equilibrium Thermodynamics for Glassy Polymers (NET-GP) approach, applied to the same EOSs [3].

References:

[1]  S.P. Nunes, K.V. Peinemann, Membrane Technology in the Chemical Industry, 2^nd Edition, 2006, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany.

[2] A.C. Comer, D. S. Kalika, B. W. Rowe, B. D. Freeman, D. R. Paul, Dynamic Relaxation Characteristics of Matrimid® Polyimide, Polymer, 50, 2009, 891-897.

[3] M.G. De Angelis, G.C. Sarti, Solubility of Gases and Liquids in Glassy Polymers,  Annual Review of Chemical and Biomolecular Engineering, 2011, 2, 97-120.