Supercritical Gel Drying: A Powerful Tool for Tailoring Symmetric Porous PVDF-HFP Membranes S. Cardea,* ,† A. Gugliuzza, ‡ M. Sessa, † M. C. Aceto, ‡,§ E. Drioli, ‡,§ and E. Reverchon † Department of Chemical and Food Engineering, University of Salerno, Via Ponte Don Melillo 1, 84084 Fisciano, Italy, Research Institute on Membrane Technology, ITM-CNR, c/o University of Calabria, Via Pietro Bucci 17C, 87036 Rende (CS), Italy, and Department of Chemical Engineering and Materials, University of Calabria, Via Pietro Bucci 17C, 87030 Rende, Italy ABSTRACT In this work, poly(vinylidene fluoride) copolymer with hexafluoropropylene (PVDF-HFP) membrane-like aerogels have been generated for the first time. PVDF-HFP gels have been prepared from polymer-acetone solutions by adding various amounts of ethanol. A series of supercritical drying experiments have been performed at different pressures (from 100 to 200 bar) and temperatures (from 35 to 45 °C) and at various polymer concentrations (from 5 to 12 wt %). The effects of the process conditions on the membrane morphology have been evaluated, and structure-property relationships have been found. In all cases, the membranes exhibit interconnected structures with nanosized pores and high porosity, leading to reduced resistance to the gas mass transfer and high hydrophobic character of the surfaces. These membrane-like aerogels promise to form a new class of highly hydrophobic porous interfaces, potentially suitable to be used in membrane operations based, for example, on the contactor technology. KEYWORDS: membranes • aerogels • supercritical CO 2 , interfaces • contactors INTRODUCTION T he copolymer of poly(vinylidene fluoride) with hexaflu- oropropylene (PVDF-HFP) is an acid-resistant, inert, and semicristalline material. Traditionally, PVDF mem- branes find large application in advanced fields of contactor technology (1), catalysis (2-4), biomedicine (5-8), as well as transductors (9) for the polymorphism that characterize this material. Because of these important technological applications, many works have been developed with the aim of producing porous PVDF-HFP structures (1, 10-20), emphasizing often the events controlling the phase-separa- tion phenomena (1, 13, 19). However, porous PVDF mem- branes prepared according to traditional dry-wet processes rarely exhibit well-controlled morphology and chemistry, which are characteristics necessary to make them the ideal interfaces for membrane operations such as, for example, contactors (1). Non-well-defined pore size and pore distribu- tion, coalescence phenomena, low surface, and overall porosity influence, in turn, the final performance of the membranes, producing interfaces not suitable for promoting the uniformity and high productivity of the process (21). In addition, the lack of hydrophobicity due to the use of hydrophilic pore formers affects significantly the waterproof- ness of the films, reducing the period of operational time. Recently, PVDF-HFP membranes have also been generated by a new technique in which supercritical carbon dioxide (SC-CO 2 ) replaces the liquid nonsolvent (i.e., SC-IPS process) (22-24). Compared to the dry-wet process, the SC-IPS process can give several advantages: SC-CO 2 substitutes the nonsolvent, reducing the potential pollution; the membrane is obtained without additional post-treatments because SC- CO 2 completely extracts the solvent; it is possible to modu- late the membrane morphology, cells, and pore sizes simply by changing the operative conditions (22-34). The results obtained in the case of PVDF-HFP membranes confirmed the versatility of the process (through a change in the SC- CO 2 solvent power, either a leafy morphology or a cellular structure was obtained (22), but skinned surfaces were always obtained 22-24). Another interesting process that could be a valid alterna- tive for the preparation of porous membranes is the gel drying process (35-37). Unlike the phase-inversion process, the starting sample is a gel (not a solution) and the porous structure (i.e., aerogel) is formed during the gelation process; this process can assure a uniform and symmetric skinless nanostructured aerogel morphology, but some drawbacks can seriously affect gel formation. The surface tension of the solvent to be eliminated can cause the collapse of the gel polymeric structure (due to the cohesive forces between the liquid solvent and the polymeric nanosized network), leading to a partially nonporous structure, and long processing times are usually necessary. For these reasons, the removal of the solvent from the gel, without damaging the overall network of the polymer, represents an attractive strategy for the * Corresponding author. Tel.: +39(0)89964232. Fax: +39(0)89964057. E-mail: scardea@unisa.it. Received for review October 7, 2008 and accepted December 10, 2008 † University of Salerno. ‡ Research Institute on Membrane Technology, ITM-CNR, c/o University of Calabria. § Department of Chemical Engineering and Materials, University of Calabria. DOI: 10.1021/am800101a © 2009 American Chemical Society ARTICLE www.acsami.org VOL. 1 • NO. 1 • 171–180 • 2009 171 Published on Web 01/08/2009