Hydrophilization of Hydrophobic Membrane Surfaces By the Adsorption of Water-Soluble Polymers

The adsorption of biological compounds such as proteins on membrane surfaces which is driven by different protein-surface forces, including van der Waals, hydrophobic and electrostatic forces, is important in the fields of separation science. The physical adsorption of natural polymers such as proteins on hydrophobic surfaces is referred to as a part of ‘fouling phenomena’, which is a major drawback of the continuous flux decline due to the adsorption in question, pore blocking, precipitation and cake formation in membrane-based water purification processes. The low surface tension energies of hydrophobic membrane surfaces give rise to several problems of easy wetting, ease of initial flux, adsorption or spreading. Therefore, it is required that the surface energy should be increased through the introduction of polar functionality using chemical modification, gas-discharge treatment (plasma, corona, etc., g-radiation, UV irradiation, and coating. Since, however, these chemical approaches are limited to lab scale to date because of the complicated production equipments, strict operation requirements and high costs, the coating or impregnating with hydrophilization agents, such as alcohols, surfactants, or amphiphilic polymers is more favorable method to endow the hydrophilicity with hydrophobic membranes.

In this study, to improve the water flux of porous hydrophobic membranes, various water-soluble polymers including neutral, cationic and anionic polymers were adsorbed using ‘salting-out’ method. The adsorbed hydrophobic membrane surfaces were characterized mainly via the measurements of contact angles and scanning electron microscopy (SEM) images. To enhance the durability of the modified membranes, the water-soluble polymers such poly(vinyl alcohol) (PVA) were crosslinked with glutaraldehyde (GA) and found to be resistant for more than 2 months in vigorously stirred water. The water flux was much more increased when the ionic polymers used as the coating materials rather than the neutral polymer and in this case, about 70% of 0.31 L/m²∙h (LMH) to 0.50 LMH was increased when 300 mg/L of polyacrylamide (PAAm) was used as the coating agents. Among the cationic coating polymers such as poly(styrene sulfonic acid-co-maleic acid) (PSSA_MA), poly(acrylic acid-co-maleic acid) (PAM) and poly(acrylic acid) (PAA), PSSA_MA was found to be the best in terms of contact angle and water flux. In the case of PSSA_MA, the water flux was enhanced about 80%. The low concentration of the coating solution was better to hydrophilize while the high concentration inclined to block the pores on the membrane surfaces. The best coating condition was found: (1) coating concentration 150 to 300 mg/L, (2) ionic strength 0.15, (3) coating time 20 min.