Synthesis and Characterization of Supported PDMS Membranes

I.O. CAO; P.M. HOCH; M.A. VILLAR

Buenos Aires, Argentina

Congreso; XXII Congreso Interamericano de Ingeniería Química y V Congreso Argentino de Ingenier¨ªa Qu¨ªmica; 2006

Asociaci¨®n Argentina de Ingenieros Qu¨ªmicos (AAIQ) y Confederaci¨®n Interamericana de Ingenier¨ªa Qu¨ªmica (CIIQ)

Polymeric materials are widely used in membrane separation processes such as dialysis, reverse osmosis, gas permeation and pervaporation. Polydimethylsiloxane (PDMS) is among the most interesting and promising materials for membrane preparation and can be used in alcohol dehydration, organic/organic separations and volatile organic compounds  (VOCs) removal from water in a highly efficient fashion (1). Nevertheless, PDMS networks exhibit poor mechanical properties because of weak intermolecular interactions between polymer chains. In order to reinforce the material and obtain a better performance, a composite structure consisting of a porous support and a thin dense PDMS top layer is prepared. The support is generally chosen according to its microstructure in a way that the composite material does not present a considerable increase in mass transfer resistance in comparison to the PDMS film (2). For an optimal performance, the PDMS film should be cast on the support taking into account that polymer intrusion in the pores can lead to a high mass transfer resistance. On the other hand, scarce intrusion results in weak polymer-support interaction and therefore poor membrane final properties. Accordingly, an important parameter to evaluate is the extent of reaction of the mixture before casting. In this work, model PDMS networks with a well defined structure are synthesized by reacting a commercial a,w-divinyl poly(dimethylsiloxane) (B2) with a trifunctional cross-linker bearing silane groups (A3) and known amounts of w-vinyl poly(dimethylsiloxane) (B1) synthesized by anionic polymerization. Rheological characterization of the obtained networks is carried out in a rotational rheometer by dynamic tests. Viscoelastic properties of the networks depend on both concentration and molecular weight of pendant chains (3) and these can be related to the separation properties of the membranes prepared afterwards.  Extraction of soluble material using toluene as solvent is performed in order to remove non-cross-linked polymer. After extraction, weight fraction of solubles (Ws) and equilibrium degree of swelling (¦Ô2m) are calculated. Membranes with different thickness are obtained by spin coating varying the amount of polymer involved and the spinning speed. This technique leads to uniform coating and easy preparation of the samples. In order to gain insight into the final membrane structure, SEM micrographs of the cross-section are analyzed. This tool enables us to determine an optimal time for casting the polymer on the support as well as the thickness of the PDMS coating. A good knowledge of these parameters is essential for conducting permeation tests with these tailor-made membranes in a reproducible way. (1)   Ghoreyshi, S.A.A., Farhadpour, F.A., Soltanieh, M. Multicomponent transport across nonporous polymeric membranes. Desalination 2002; 144: 93-101. (2)   Kim, H.J., Nah, S.S., Min, B.R. A new technique for preparation of PDMS pervaporation membrane for VOC removal. Advances in Environmental Research 2002; 6:255-264. (3) Roth, L.E., Vega, D.A., Vall¨¦s, E.M., Villar, M.A. Viscoelastic properties of networks with low concentration of pendant chains. Polymer 2004; 45:5923-5931.