Journal of Membrane Science 213 (2003) 159–180 Morphological study of fluorinated asymmetric polyetherimide ultrafiltration membranes by surface modifying macromolecules M. Khayet a,∗ , C.Y. Feng b , T. Matsuura b a Department of Applied Physics I, Faculty of Physics, University Complutense of Madrid, Av. Complutense s/n, 28040 Madrid, Spain b Industrial Membrane Research Institute, Department of Chemical Engineering, University of Ottawa, 161 Louis Pasteur, P.O. Box 450, Stn. A, Ottawa, Ont., Canada K1N 6N5 Received 20 June 2002; received in revised form 28 October 2002; accepted 28 October 2002 Abstract Phase inversion polyetherimide (PEI) flat sheet membranes were surface modified using fluorinated surface modifying macromolecules (SMMs) additives. Two SMM formulations were used. Each SMM was blended into polyetherimide casting solutions containing the solvent N,N-dimethyleacetamide and the non-solvent hydroxybutyric acid -lactone (GBL). The effects of the SMM and the PEI base polymer concentrations on the morphological properties of the prepared membranes have been investigated. Contact angle measurements indicate that PEI membrane surface becomes more hydrophobic after adding the SMMs to the PEI casting solutions, while X-ray photoelectron spectroscopy analysis shows enrichment of fluorine on the PEI membrane surfaces. The SMM modified and unmodified PEI membranes were also characterized by means of atomic force microscopy (AFM), gas permeation tests and ultrafiltration experiments using aqueous solutions of polyethylene glycol (PEG) and polyethylene oxide (PEO) of various molecular weights. Pore sizes and nodule sizes obtained from AFM images were remarkably fitted to the log-normal probability distribution curves. Mean pore sizes, pore size distributions, nodule sizes, nodule size distributions and roughness parameters of the membranes were determined. It was found that SMM actively migrated to the air surface and changed the surface properties of the PEI membranes. The mean pore size and the molecular weight cut-off (MWCO) of the SMM modified PEI membranes were lower than those corresponding to the unmodified membranes, while the nodule sizes were larger. The surface roughness parameters were reduced when SMMs were added. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Polyetherimide; Fluoro-polymer; Surface modification and surface properties; Membrane preparation and structure; Ultrafiltration 1. Introduction Surface fluorination has been shown to improve the transport properties of some polymeric membranes such as polyethylene [1], poly(4-methyl-1-pentene) [2], poly(trimethyl silyl propyne) [3] and polysulfone [4] used in gas separation processes; polyethersulfone ∗ Corresponding author. Tel.: +34-91-3945191; fax: +34-91-3945191. E-mail address: khayetm@fis.ucm.es (M. Khayet). used in pervaporation and in ultrafiltration [5,6]; and polyurethanes [7] applied in biomedical applications. In the membrane literature, various methods have been developed for direct fluorination of materials such as plasma treatment, grafting reaction, etc. [8]. A less common approach to modify the properties of a polymer is the introduction of additives that can migrate to the film surface and change its chemistry while maintaining the bulk properties unaltered. In this method, only a small weight percentage of active ad- ditive is required to modify the surface properties [7]. 0376-7388/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII:S0376-7388(02)00523-9