Available online at www.sciencedirect.com Journal of Membrane Science 309 (2008) 209–221 Effect of molecular weight of PEG on membrane morphology and transport properties B. Chakrabarty, A.K. Ghoshal ∗ , M.K. Purkait ∗ Department of Chemical Engineering, Indian Institute of Technology Guwahati, Guwahati 781039, India Received 14 August 2007; received in revised form 16 October 2007; accepted 20 October 2007 Available online 26 October 2007 Abstract Flat sheet asymmetric polymeric membranes were prepared from homogeneous solution of polysulfone (PSf) by phase inversion method. N- methyl-2-pyrrolidone (NMP) and dimethyl acetamide (DMAc) were used as solvents separately. Polyethylene glycol (PEG) of three different molecular weights (400 Da, 6000 Da and 20000 Da, respectively) were used as the polymeric additives in the casting solution. The morphology and structure of the resulting membranes were observed by scanning electron microscope (SEM). The pore number, pore permeability and their distribution and average pore size of the membranes were determined by the liquid displacement method. The permeation performances of the membranes were evaluated in terms of pure water flux (PWF), equilibrium water content (EWC), hydraulic permeability, and solute rejection. Solution of bovine serum albumin (BSA) of molecular weight 68,000 Da was used to study the permeation performance of prepared membranes using a batch membrane cell of 100 mL capacity. Results showed that with increase in molecular weight of PEG, the pore number as well as pore area in membranes increases. Membranes with PEG of higher molecular weights have higher PWF and higher hydraulic permeability due to high porosity. With increase in molecular weight of PEG from 400 to 20000, the PWF increases from 15.3 to 2713.4 L m -2 h -1 with NMP as solvent while with DMAc as solvent, the PWF increases from 24.5 to 555.6 L m -2 h -1 . Similarly, EWC increases from 56.8% for PEG 400 to 78.8% for PEG 20000 for PSf/NMP/PEG membranes. Similar trend is observed for PSf/DMAc/PEG membranes. The BSA rejection data is maximum with PEG 6000 for both the solvents and the values are 56.4% for NMP (at pH 4.8) and 42.4% for DMAc (at pH 9.5). © 2007 Elsevier B.V. All rights reserved. Keywords: Polysulfone membrane; Phase inversion method; Polyethylene glycol; Scanning electron microscopy; Protein rejection 1. Introduction Phase inversion is one of the most important processes for preparing both symmetric and asymmetric polymeric mem- branes. The structure of phase inversion membranes results from a phase change of initially stable polymer solutions. These mem- branes are widely used today in various applications such as microfiltration, ultrafiltration, reverse osmosis and as supports for composite structures [1,2]. It is actually a diffusion-induced phase separation process, which involves conversion of a liquid polymer solution of two or more components into a two-phase system, like; solid polymer rich phase and the liquid polymer poor phase. The solid phase forms the membrane structure while ∗ Corresponding author. Tel.: +91 361 2582262; fax: +91 361 2582291. E-mail addresses: aloke@iitg.ernet.in (A.K. Ghoshal), mihir@iitg.ernet.in (M.K. Purkait). the liquid phase forms the membrane pores. The conversion is generally carried out by addition of a precipitating fluid which is usually miscible with the solvent but immiscible with the polymer. The mechanism of formation of these membranes has been the subject of investigation since many years. Reuvers et al. [3] developed a model which describes the mass transfer phenomena occurring during the immersion step. The model explained the two types of demixing taking place during the phase inversion process, i.e. instantaneous demixing and delayed demixing. Membranes formed by instantaneous demixing gen- erally show a highly porous substructure (with macrovoids) and a finely porous, thin skin layer. Membranes formed by a delayed demixing mechanism show a porous (often closed-cell, macrovoid-free) substructure with a dense, relatively thick skin layer. Structure and properties of membranes prepared by phase inversion method depend upon many factors. Addition of addi- tive into the casting solution is one of the major factors. An additive is used in the casting solution in order to have an opti- 0376-7388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2007.10.027