Hydrophilic Surface Modification of Microporous Polymer Membranes Using A Variety of Low-Temperature Plasma Treatments
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Hydrophilic Surface Modification of Microporous Polymer Membranes Using A Variety of Low-Temperature Plasma Treatments Dattatray S. Wavhal, Kristen R. Kull, Michelle L. Steen, and Ellen R. Fisher1 1 Department of Chemistry, Colorado State University, Fort Collins, CO 80523-1872 ABSTRACT A variety of plasma treatments have been employed to achieve permanent hydrophilic surfaces throughout the membrane structure. Specifically, we have modified microporous polyethersulfone (PES) membranes using H2O, CO2, and N2 plasma treatments to implant polar functional groups; alternatively, Ar-plasma treatment followed by grafting of hydrophilic monomers (acrylic acid and acrylamide) in the vapor phase has also been successful at modifying PES membranes. PES membranes treated with H2O and CO2 plasmas as well as the grafted membranes are found to be permanently hydrophilic (for a minimum of six months), and the entire membrane cross-section is modified. Chemical changes to the modified PES membranes were determined with FTIR and XPS measurements. Furthermore, water bubble point measurements and electron microscopy results reveal that pore sizes of the modified membranes are only slightly affected, depending on the treatment. Incorporation of polar functionalities results in an increase in the glass transition temperature (Tg) and a moderate change in tensile strength of the modified membranes. Most importantly, the surfaces of the modified membrane are less susceptible to absorption by bovine serum albumin (BSA) proteins. INTRODUCTION Surface modification of membranes is thought to be equally as important to the membrane industry as membrane material and process development. Surface modification of the membrane can enhance the membrane’s performance properties such as flux and selectivity. Moreover, it can reduce fouling and endow the membrane with various chemical functionalities [1]. Plasma treatment is a useful tool in the modification of surface properties and more attention has recently been given to its applications in membrane separation science. Plasma can alter membranes in various ways, from changes in the pore diameter through plasma etching or deposition [2] to the formation of functional groups on the membrane surfaces [1,3]. Plasma treatment can also result in a significant change in the surface energy, which plays crucial role in membrane fouling and cleaning phenomenon. In the present study we explored the use of various plasmas for permanent hydrophilic modification of microporous polyethersulfone (PES) membranes. EXPERIMENTAL DETAILS PES membranes obtained from Millipore Corporation were cleaned in methanol prior to use. The reactor used in this study is described elsewhere [1]. The membranes were ∼9 cm downstream from the most intense region of the plasma glow and oriented perpendicular to plasma flow. The large pore side of the asymmetric membrane was facing the plasma (upstream side). This arrangement facilitates the plasma to pass through the membrane and modify its entire cross section. For acrylic a
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