Preparation of porous polymer membranes using nano- or micro-pillar arrays as templates Xiaohu Yan a , Guojun Liu a, * , Michael Dickey b , C. Grant Willson b a Department of Chemistry, Queen’s University, 90 Queen’s Cresent, Kingston, Ont., Canada K7L 3N6 b Department of Chemical Engineering, University of Texas at Austin, Austin, TX 78712-1167, USA Received 12 August 2004; accepted 30 September 2004 Available online 18 October 2004 Abstract ZnO or polystyrene (PS) pillar arrays were formed on solid substrates and used as templates for the formation of porous polymer membranes. The membranes were formed by casting a polymer solution in the inter-pillar space of these templates and evaporating the solvent. Dissolution of the pillars in a selective solvent resulted in thin films containing monodisperse micrometer-sized channels. Membranes produced using the pillar template technique showed high water permeability and high size selectivity. q 2004 Elsevier Ltd. All rights reserved. Keywords: Membrane; Pillars; Templated synthesis 1. Introduction Microporous membranes can be formed in numerous ways. Few methods yield membranes with cylindrical pores of uniform size that span the membrane like those found in the ‘track-etched’ polymer membranes [1] or in the ‘electrochemically-etched’ alumina membranes [2]. The track-etched membranes suffer, however, from relatively low porosity and thus low effluence. Recently methods have emerged that are capable of forming large arrays of small, cylindrical pores in polymer films through the use of block copolymers [3,4] or with imprint lithography [5,6]. The ability to create ever-smaller, monodisperse pores is enabling more sophisticated applications of porous materials in areas as diverse as drug delivery [7], enantiomer separation [8], DNA and biomolecule separ- ation [9], lithography [10], chemical sensoring and catalysis [11] etc. The cylindrical pores of such membranes have been used recently also as ‘templates’ to grow various micro- and nano-structures including metal multiblocks [12]. In this paper, we report a simple method for creating mono-disperse cylindrical pores in polymer films based on ‘reverse templating’. A polymer film is cast about a template of a sacrificial pillar array. The pillars are removed after film formation, leaving behind a porous membrane with cylindrical pores that span the complete thickness of the membrane (w5 mm). The pore size may be tuned by adjusting the diameter of the templating pillars from 100 nm to 10 mm or by shrinking the pore diameter by plating the walls with metal [13]. Although the preparation of only polymer membranes will be discussed, the process may be adaptable also to the preparation of sol–gel inorganic or other films. The concept of replica molding against a template for the formation of porous materials is not without precedent [14]. The most notable example is the use of micelles of low- molar-mass [15] or block copolymer [16] surfactants as templates to prepare mesoporous bulk silica with three- dimensional porous structures. The technique is, however, not readily amenable to generate thin films with macro- scopically-aligned permeating channels [17]. Other popular templating techniques include the use of colloidal spheres [14,18] or water droplets [19] as templates to yield membranes with concave pores. There are numerous methods available for creating arrays of pillars for the template, such as photolithography or e-beam lithography [20]. We chose to form both organic 0032-3861/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2004.09.073 Polymer 45 (2004) 8469–8474 www.elsevier.com/locate/polymer * Corresponding author. Tel.: C1 613 533 6996; fax: C1 613 533 6669. E-mail address: gliu@chem.queensu.ca (G. Liu).