Short Communication New crosslinking method of polyamide–imide membranes for potential application in harsh polar aprotic solvents S.M. Dutczak a,1 , F.P. Cuperus b , M. Wessling a,2 , D.F. Stamatialis a,⇑ a Membrane Science and Technology Group, Institute for Nanotechnology MESA+, University of Twente, Faculty of Science and Technology, Membrane Technology Group, P.O. Box 217, NL-7500 AE Enschede, The Netherlands b SolSep B.V. Robust Membrane Technologies, St. Eustatius 65, NL-7333 NW Apeldoorn, The Netherlands article info Article history: Received 24 January 2012 Received in revised form 21 September 2012 Accepted 11 October 2012 Available online 23 October 2012 Keywords: Crosslinking Polyamide–imide Membranes Aprotic solvents abstract We report for the first time successful crosslinking of polyamide–imide (Torlon Ò ) based membranes using di-isocyanates. The crosslinked membranes are resistant to N-methyl pyrrolidone (which is solvent of the non-crosslinked membranes) and have very good mechanical properties. In contrast to the state-of-the-art polyimide crosslinked membranes, the created covalent bond is expected to be thermally stable enabling applications at elevated temperatures. The transport characteristics (permeance and molecular weight cut off – MWCO) of the crosslinked membranes are changed in comparison to non- crosslinked ones. This suggests that the crosslinking process could be used to tailor these characteristics. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction Solvent resistant nanofiltration (SRNF) attracts a lot of attention due to its proposed simplicity and great energy efficiency. Litera- ture already suggests numerous successful applications in many branches of industry ranging from petrochemistry [1], through pharmaceutical manufacturing [2–6] to catalytic processes [7–9]. The last years, lot of effort has been spent on development and improvement of membrane stability in various solvents [10–16]. Most of the membranes are either dense PDMS composites [17– 25] or porous integrally skinned made of polyimides (PIs) [10,26–29] or polyamide–imide (PAI) [15,30]. The PAI membranes are already robust and stable in many aprotic solvents like toluene, hexane, acetone, methyl ethyl ketone (MEK), methyl tert–butyl ether MTBE, pyridine, nitromethane, tetrahydrofuran (THF) etc. However, PI and PAI polymers are typically soluble in harsh polar aprotic solvents (e.g. N-methyl pyrrolidone (NMP), di-methyl sulf- oxide (DMSO), di-methyl formamide (DMF) or di-methyl acetam- ide (DMAc)) and therefore they cannot be used for separations in such solvents To overcome this limitation and improve their stabil- ity, PI membranes are crosslinked by thermal imidization [30] or diamines in an imide-ring opening reaction [31] either during phase separation [15,32] or in post casting process [10,33]. How- ever, long exposure to the diamine crosslinking solution may cause deterioration of the polymer and on long term cause membrane dissolution [34]. In this work we report a new method of crosslinking of PAI membranes using di-isocyanates. The resulting membranes are mainly evaluated concerning stability to NMP mixtures. 2. Experimental 2.1. Materials Toluene, methanol and THF all analytical grade, were purchased from Merck (The Netherlands). Styrene (Reagent Plus P99%), sec- butyllithium solution 1.4M in cyclohexane, 6-Hexamethylene Diis- ocyanate (HMDI) (purum, P98.0%), triethylene diamine (TEA) (P99%), Tin 2-ethylhexanoate (95%) were purchased from Sigma Aldrich (The Netherlands). NMP, extra pure, was purchased from Acros Organics. Acetone (technical grade) was purchased from As- sink (Hengelo, The Netherlands). All chemicals were used as sup- plied without additional purification. 2.2. Crosslinking PAI-based membranes, kindly provided by SolSep B.V. (Apel- doorn – The Netherlands), were used for crosslinking. These mem- 1383-5866/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.seppur.2012.10.018 ⇑ Corresponding author. Present address: Biomaterials Science and Technology, MIRA Institute for Biomedical Technology and Technical Medicine, University of Twente, Faculty of Science and Technology, P.O. Box 217, NL-7500 AE Enschede, The Netherlands. Tel.: +31 53 489 4675. E-mail address: d.stamatialis@utwente.nl (D.F. Stamatialis). 1 Present address: Gambro Dialysatoren GmbH, Membranforschung/Membrane & Device Research, Holger-Crafoord-Str. 26, 72379 Hechingen, Germany. 2 Present address: RWTH Aachen University, Chemische Verfahrenstechnik (CVT), 52064 Aachen, Germany. Separation and Purification Technology 102 (2013) 142–146 Contents lists available at SciVerse ScienceDirect Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur