Separation and Purification Technology 73 (2010) 421–428 Contents lists available at ScienceDirect Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur Preparation and characterization of cross-linked Matrimid ® membranes using para-phenylenediamine for O 2 /N 2 separation F. Aziz, A.F. Ismail ∗ Advanced Membrane Technology Research Centre, (AMTEC), Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia article info Article history: Received 28 December 2009 Received in revised form 3 May 2010 Accepted 5 May 2010 Keywords: Gas separation Cross-linking para-Phenylenediamine Matrimid abstract The cross-linking modification of asymmetric Matrimid membranes with para-phenylenediamine (pPD) has been investigated. The modification was performed by immersing Matrimid membranes in 10% (w/v) pPD/methanol solution at ambient temperature for certain period of time. The effects of immersion time on degree of cross-linking, physicochemical properties and gas transport properties of the membrane have been studied. The cross-linked Matrimid membranes were characterized using gel test, FTIR, TGA, XRD and SEM. The FTIR spectra confirmed that the cross-linking process had taken place, which indicates that imide groups were turned to amide groups during modification process. The degree of cross-linking increased with increasing the immersion time. This study found that, there were small amounts of insol- uble materials found from the gel test. The possible reasons were that the proposed cross-linking method using pPD solution occurs only on the surface of the membranes or only low degree of cross-linking takes place. The XRD results revealed that the d-space of the polymeric chains change after the modification due to the methanol swelling and structure-tightening effects induced by cross-linking. The TGA results indicated that the degradation temperature is sufficient for gas separation applications. The gas transport properties of the pure and cross-linked Matrimid membranes were investigated using gas permeation tests. The results showed that the cross-linked Matrimid membranes decrease the O 2 ,N 2 and CH 4 per- meance compared to the unmodified membranes. Interestingly, 10% pPD-120 min samples showed an impressive enhancement of selectivity for O 2 /N 2 compared to the unmodified membrane from 6.19 to 10.01 due to the affinity of nitrogen-containing molecules (pPD) towards oxygen. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Membrane separation process has become one of the most exciting and significant new unit processes to have appeared in many years. Developing membrane technology for gas separa- tion is challenging, but economically rewarding from industrial prospect [1–3]. Among many polymeric materials for gas separa- tion, polyimides have been found to possess high gas permeability as well as high intrinsic selectivity in comparison to polycarbonate, polysulfone and other materials [4]. Meanwhile, cross-linking mod- ification of polyimides has received much attention since it appears to provide a promising approach toward improving selectivity and chemical and plasticization resistances [5]. The cross-linking modification tends to increase chain packing and inhibits the intra- segmental and inter-segmental mobilities, resulting in higher gas selectivity [6]. Cross-linking can be achieved by thermal, ion beam and UV irradiation treatment, or by reactions with added chemi- cals [7,8]. Currently, the chemical cross-linking approach seems to ∗ Corresponding author. Tel.: +60 7 5535592; fax: +60 7 5535925. E-mail addresses: afauzi@utm.my, fauzi.ismail@gmail.com (A.F. Ismail). be an important method and widely studied by many researchers [8–11]. Wind et al. [8] chemically cross-linked polyimide 6FDA- DAM:DABA 2:1 with ethylene glycol, 1,4-butylene glycol, 1,4- cyclohexanedimethanol, and 1,4-benzenedimethanol to study the effects of the cross-linking agent structure on membrane perfor- mance. The results showed that the physical properties of the cross-linking agent play a major role in determining the final cross-linking degree which in turn increases membrane produc- tivity. Shao et al. [9] also studied the chemical cross-linking of different polyimides on membrane performance. Interestingly, the cross-linking of 6FDA-durene membranes by the same diamine resulted in a greater decline in gas permeability compared to that of Matrimid membranes due to the different electrophilici- ties of polyimides and the different diffusion rates of diamines into membranes. Tin et al. [10] applied p-xylenediamine to cross-link Matrimid ® 5218 hollow fiber membrane. They observed that when the degree of cross-linking is higher, lower will be the membrane permeability probably due to the reduction in the free volume, the chain mobility and the interstitial space among chains after cross-linking modification. It was also found that the He/N 2 selectivity increased with the degree of cross-linking, but the 1383-5866/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2010.05.002