Stabilization of immiscible polymer blends using structure directing metal organic frameworks (MOFs) Nimanka P. Panapitiya a , Sumudu N. Wijenayake a , Yu Huang a , David Bushdiecker a , Do Nguyen a , Chalita Ratanawanate a , Grace J. Kalaw a , Christopher J. Gilpin b , Inga H. Musselman a , Kenneth J. Balkus Jr. a , John P. Ferraris a, * a Department of Chemistry, The University of Texas at Dallas, 800 West Campbell Road, Richardson, TX 75080, USA b Purdue Life Science Microscopy Facility, Purdue University, West Lafayette, IN 47907, USA article info Article history: Received 13 January 2014 Received in revised form 24 February 2014 Accepted 5 March 2014 Available online 13 March 2014 Keywords: Immiscible polymer blends Metal organic frameworks Compatibilizers abstract We have developed a novel approach for compatibilizing immiscible polymer blends using metal organic frameworks (MOFs). For the first time we demonstrated that the droplet diameter of the dispersed phase in a 1:1 immiscible polymer blend composed of 6FDA-DAM:DABA [copolymer of 4,4-hexafluoroisopropylidene diphthalic anhydride (6FDA), 2,4,6-trimethyl-1,3-phenylenediamine and 3,5-diaminobenzoic acid (DABA)], and polybenzimidazole (PBI), is dramatically reduced obtaining a uniform microstructure with the incor- poration of as low as 5% (w/w) of the zeolitic imidazolate framework-8 (ZIF-8). This indicates a large improvement in the compatibility of the immiscible polymers with the inclusion of ZIF-8. As the ZIF-8 loading was further increased to 10% (w/w), the droplet diameter further decreased resulting in even higher compatibility. The compatibilizing effect can be attributed to a reduction in the interfacial energy of the immiscible polymers due to the interfacial localization of ZIF-8. This MOF based compatibilization of immiscible polymer blends can open up opportunities for the combination of different properties of poly- mers in membrane-based separations and in more applications. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction The technology of polymer blends is one of the major areas of research and development in polymer science [1]. Blending of polymers leads to new materials that can synergistically combine the properties of polymers that are not achievable with individual components [2]. Moreover, it overcomes the need to synthesize new macromolecules for particular applications. However, due to the unfavorable entropy of mixing, most polymer blends tend to macroscopically phase separate [2]. In order to obtain complete miscibility, a favorable enthalpy of mixing is required, which can be achieved through specific interactions between polymers such as hydrogen bonding, dipoleedipole interactions or ionedipole in- teractions [1,2]. In contrast, immiscible polymer blends are more common and have been used for a variety of applications including tires [3], organic light-emitting diodes [4], and sensors [5]. How- ever, immiscible polymer blends are often characterized by uncontrolled phase separation leading to inconsistent properties and poor mechanical stability [1]. Therefore, polymers in phase- separated blends have been commonly compatibilized with co- polymers [6e8] and nanoparticles [9e13] to obtain uniform and stable morphologies [14e16]. The use of nanoparticles to stabilize immiscible polymer blends is more attractive since copolymers are difficult to synthesize and are specific to one polymer blend family [17]. Nanoparticle-compatibilized (NPC) immiscible polymer blends have afforded uniform and stable morphologies comprising both matrix-droplet and co-continuous morphologies [9]. Polyimides are an important class of polymers used in a variety of applications in fields such as fuel cells [18,19] gas separation membranes [20e23] and energy storage [24]. Furthermore, blends of polyimides are used for many applications due to their favorable properties such as toughness, high thermal stability, high temper- ature rigidity and good solvent resistance [1]. The co-polyimide 6FDA-DAM:DABA (3:2) (6FDD) (Fig. 1A) and PBI (Fig. 1B) are high performance polymers that have been used for membrane based separations [25e29]. Use of miscible polymer blends of polyimides/ PBI has shown enhanced performances as compared to the indi- vidual polymers in the above applications [30,31]. However this approach is limited by the unavailability of many miscible polymer * Corresponding author. Tel.: þ9728832905. E-mail addresses: gilpin@purdue.edu (C.J. Gilpin), ferraris@utdallas.edu (J. P. Ferraris). Contents lists available at ScienceDirect Polymer journal homepage: www.elsevier.com/locate/polymer http://dx.doi.org/10.1016/j.polymer.2014.03.008 0032-3861/Ó 2014 Elsevier Ltd. All rights reserved. Polymer 55 (2014) 2028e2034