362 Research Article Received: 22 February 2010 Revised: 16 May 2010 Accepted: 17 June 2010 Published online in Wiley Online Library: 28 October 2010 (wileyonlinelibrary.com) DOI 10.1002/pi.2956 Morphological dependency of polymer electrolyte membranes on transient salt type: effects of anion species Majid Esmaeili, a Sayed S Madaeni a* and Jalal Barzin b Abstract Two types of transition metal salts, i.e. Cu(NO 3 ) 2 and CuCl 2 , with different anion species were used to prepare various polyethersulfone-based poly(N-vinyl pyrrolidone) (PVP) composite membranes. The polymer crystallinity, strength of some of the bonds in the membrane structure and effects of anion species on the membrane morphology were investigated through X-ray diffraction (XRD), scanning electron microscopy with energy-dispersive X-ray spectrometry (SEM-EDX), Fourier transform infrared (FTIR) spectroscopy and atomic force microscopy. The XRD results indicated an enhancement of the PVP crystallinity after addition of Cu(NO 3 ) 2 . Moreover, addition of copper salt was accompanied by an increase of effective membrane distances. The FTIR analysis revealed that the nitrate ions might be better distributed than the other salt ions. The complexes created between them and carbonyl oxygens on the PVP chains were thus stronger. More powerful interactions caused a crystallinity enhancement following the addition of Cu(NO 3 ) 2 . The SEM-EDX experiment gave insight into the copper ion and carbonyl oxygen distributions in the membrane surface and active layer. The uniform distribution of copper ions resulted in a clear distribution of interchain interactions and complexes in the active layer structure and also caused structural order improvement. c  2010 Society of Chemical Industry Keywords: anion; copper salt; facilitated transport; membrane structure; morphology INTRODUCTION In the last decade, increasing attention has been devoted to facilitated transport membranes with improved permeability and selectivity to olefin-based unsaturated hydrocarbons. The use of various transient salts as carrier agents for the separation and recovery of various olefins such as ethylene and propylene from paraffins is well known. 1–3 Compared to paraffins, olefin molecules are shorter due to their carbon double bonds and have a better capability for specific interactions with a membrane matrix. Consequently, the preferred selective separation of olefins over paraffins is evident in membrane separation. 4–6 Polymers containing aliphatic or aromatic C O bonds can dissolve transient metal salts because of complex formation between the C O bonds and metal ions. Silver and copper salts such as AgCF 3 SO 3 , AgNO 3 , AgBF 4 , Cu(NO 3 ) 2 and CuAlCl 4 dissociate in a membrane structure and form carrier ions. An interaction takes place between salt cations and electrons on heteroatoms in the polymer backbone (e.g. the ether oxygens in poly(ethylene oxide) (PEO)). Therefore, the metal salt can be stabilized in the polymer matrix. Metal salts dissolve in polymers such as PEO, poly(N-vinyl pyrrolidone) (PVP), poly(vinyl alcohol) or poly(2-ethyl-2-oxazoline) and become active in olefin complexation. 7,8 The carrier stabilizes in the polymer matrix and coordinates a specific solute reversibly, such as propylene in the case of olefin/paraffin mixtures. 9 In all applications of facilitated transport membranes, transition metal anion and cation species play significant roles in determining the strength and the rate of interactions between carrier and olefin. 10 Also, the structure and complexation dynamics of solute – carrier complexes play vital roles in facilitating the desired separation. 11 Studies have revealed that different carrier anions have significant effects on the sorption capacity of the carrier or on the separation performance of polymer–carrier complexes. 12–15 For example, a silver ion which interacts weakly with anions 16 such as BF 4 − , CF 3 SO 3 − or ClO 4 − readily interacts with olefins making reversible olefin – silver complexes which in turn will become good olefin carriers in facilitated olefin transport. Despite many studies of the effect of the anion or cation of a transient salt on the separation performance of solid electrolyte membranes, the dependency of membrane structure on cation or anion species has to date been poorly investigated. The applied salt dissociation energy or lattice energy has some influence on the distribution of ions released or complexes created in the membrane structure. Therefore, the structure of a polymer matrix is affected easily by changes in the anion species of any transient metal salt. In the study reported here, two different copper salts were used in the preparation of solid PVP electrolyte membranes to investigate the effect of anion species on the strength of complexes formed between carbonyl oxygen bonds present on the PVP chains, on the crystallinity of the polymer structure and on the effective distribution of carrier agents among polymer chains. The experimental results were obtained through XRD, ∗ Correspondence to: Sayed S Madaeni, Membrane Research Center, Chemical Engineering Department, Razi University, Tagh Bostan, Kermanshah 67149, Iran. E-mail: smadaeni@yahoo.com a MembraneResearchCenter,ChemicalEngineeringDepartment,RaziUniversity, Tagh Bostan, Kermanshah 67149, Iran b Iran Polymer and Petrochemical Institute, Tehran 14965/115, Iran Polym Int 2011; 60: 362–370 www.soci.org c  2010 Society of Chemical Industry