Thin-film membranes derived from co-continuous polymer blends: preparation and performance Danut Riscanu a , Basil D. Favis a, * , Chaoyang Feng b , Takeshi Matsuura b a De ´partement de Ge ´nie Chimique, Centre de Recherche Applique ´ Sur les Polyme `res (CRASP), E ´ cole Polytechnique de Montre ´al, P.O. Box 6079 Station Centre-Ville Montre ´al, Montreal, Que., Canada H3C 3A7 b Department of Chemical Engineering, Industrial Membranes Research Institute, University of Ottawa, Ottawa, Ont., Canada Received 2 December 2003; received in revised form 21 May 2004; accepted 27 May 2004 Abstract Two approaches for preparing thin-film membranes from immiscible co-continuous polymer blends are presented. Approach 1 involves the melt blending of co-continuous polymer blends followed by the selective extraction of one of the phases and results in a microporous membrane material of high void volume. In that case, the pore size is defined by the phase size of one of the phases in the blend and hence composition, interfacial tension, viscosity ratio and other parameters influencing phase morphology can be used to control porosity. For that first approach, the blend system studied is high density polyethylene/polystyrene, compatibilized with SEBS (styrene – ethylene – buthylene – styrene) triblock copolymer. Both symmetric and asymmetric type membranes can be obtained. The symmetric membrane demonstrates porosity ranging from 80 to 230 nm. It is shown that extraction time can be used to develop asymmetry in the membrane and the effects of extraction time on the morphology, pore size distribution and performance are presented. High flux values and high apparent rejection factors estimated from permeability testing indicate that these materials could have potential in a variety of membrane applications. Approach 2 is a solventless approach that results in a membrane of very low void volume. A high interfacial tension immiscible co- continuous blend compatibilized at different levels by a weak interfacial modifier is prepared by melt mixing and extrusion through a sheet die. Microporosity in the bulk of the material is generated in situ during cooling by this approach. The thin sheet is then subjected to uniaxial or biaxial cold stretching to develop surface porosity. This technique exploits interfacial debonding and the weak interface of the co- continuous morphology acts as a template to guide the direction of porosity development. Highly percolated membranes of polycarbonate and high-density polyethylene with SEBS were prepared. These membranes possess pore sizes in the range of 100 nm and are of very low void volume. Oxygen permeation tests, carried out under atmospheric pressure, demonstrate a dramatic increase in oxygen flux from 1378 cm 3 /m 2 /day (non-stretched 50PE/50PC/15SEBS sample) to 106,270 cm 3 /m 2 /day (biaxially stretched sample). The results indicate that they could have potential as breathable barrier type materials. The effects of draw ratio on the permeation values are presented. q 2004 Elsevier Ltd. All rights reserved. Keywords: Co-continuous polymer blends; Melt processing; Nanoporous membranes 1. Introduction Polymers are the most often used materials to produce porous membranes. These kinds of membranes find wide application in various separation processes, such as membrane gas separation, pervaporation, reverse osmosis, ultrafiltration, and microfiltration depending on the pore size [1]. A wide variety of methods exist to prepare polymer membranes [1]. The most widely used approach is the preparation of membranes using three main casting techniques: air, immersion and melt casting [2]. The casting technique takes the polymer through complex polymer- solvent phase separation processes. Using the phase separation technique, cellulose acetate membranes, devel- oped by Loeb and Sourirajan for the purpose of seawater desalination, were the first successful reverse osmosis polymer membranes [1]. Although this approach is widely used, it requires the control of highly complex binodal and spinodal type morphologies is highly complex due to the non-equilibrium nature of the phase separation process. A frozen-in, phase separated material is obtained. Such 0032-3861/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2004.05.067 Polymer 45 (2004) 5597–5609 www.elsevier.com/locate/polymer * Corresponding author. Tel: þ1-514-340-4711/4527; fax: þ 1-514-340- 4159. E-mail address: favis@chimie.polymtl.ca (B.D. Favis).