Gas sorption, diffusion, and permeation in thermally rearranged poly(benzoxazole-co-imide) membranes Seungju Kim a,b , Kyung Taek Woo a , Jong Myeong Lee a , Jeffrey R. Quay c , M. Keith Murphy d , Young Moo Lee a,b,n a WCU Department of Energy Engineering, Hanyang University, Seoul 133-791, Republic of Korea b School of Chemical Engineering, College of Engineering, Hanyang University, Seoul 133-791, Republic of Korea c Air Products and Chemicals, Inc., Allentown, PA, USA d Air Products and Chemicals, Inc., St. Louis, MO, USA article info Article history: Received 18 September 2013 Received in revised form 16 November 2013 Accepted 19 November 2013 Available online 28 November 2013 Keywords: Thermally rearranged polymers Gas sorption Solubility coefficient Solution diffusion model Polybenzoxazole-co-imide abstract Thermally rearranged polymer membranes exhibit extraordinary gas permeability based on a rigid polymer structure with a high free volume element. In this study, TR poly(benzoxazole-co-imide) membranes from 4,4′-hexafluoroisopropylidene diphthalic anhydride (6FDA), 3,3′-dihydroxyl-4-4′- diamino-biphenyl (HAB), 2,2′-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane (bisAPAF), and 2,4,6- trimethyl-m-phenylenediamine (DAM) were prepared to improve their gas separation properties. Copolymer membranes of polyimides and TR polybenzoxazoles may be desirable to generate efficient gas transport properties as well as to process polymers into fiber or film forms. Gas permeability, diffusivity, and solubility of the precursor polyimide and TR poly(benzoxazole-co-imide) membranes were investigated to characterize gas transport properties for small gas molecules including H 2 ,O 2 ,N 2 , CH 4 , and CO 2 . Thermal rearrangement process resulted in an increase in polymer free volume, which improved the diffusion and sorption coefficients. & 2013 Elsevier B.V. All rights reserved. 1. Introduction Membrane gas separations have played an important role in the gas production industry as an alternative technology of traditional gas separation processes such as cryogenic distillation and absorption [1]. Membrane processes allow for a small foot- print and low energy consumption that are advantageous com- pared to other separation processes [2,3]. Membrane materials for gas separation applications have been investigated to overcome the current material performance limits, the “upper-bound” estab- lished by Robeson in 1991 and revisited in 2008 [4,5]. Among various membrane materials, including metallic and inorganic materials, polymeric membranes have been commercialized with high gas flux based on the asymmetric structure. However, only a few commercial polymers, such as cellulose acetate (CA), poly- imides (PI), and poly(phenylene oxide) (PPO), have been used [1,6]. Various studies have been conducted to improve the mem- brane performance in aspects of gas permeability and selectivity. As a result, a number of microporous polymer membranes have exhibited improved gas transport performance [7–11]. Fundamental studies on the gas transport mechanism of polymer membranes are required to improve membrane performance and to overcome the upper bound. For polymeric membranes in gas separation, the solution–diffusion model is considered as a well- defined gas transport mechanism [10]. Gas permeability through polymeric membranes is usually determined as a product of gas diffusivity and solubility in the solution–diffusion model [10]. Gas solubility is a thermodynamic parameter that provides the amount of penetrants sorbed in the polymer matrix in equilibrium states. Gas diffusivity is a kinetic parameter related to the rate at which penetrants move in the polymer matrix [1]. Polymer membranes have been investigated to improve gas permeability in a way of enhancing the gas diffusivity and/or gas solubility of materials. Rubbery polymer membranes typically exhibit high gas solubility for condensable gases and sorption-selective gas separation properties [12]. The membranes demonstrate a high CO 2 solubility coefficient, result- ing in high CO 2 permeability. Rubbery polymer membranes, such as poly(ethylene oxide) (PEO) and polyvinylamine (PVAm), have demonstrated high CO 2 separation performance with sorption- based CO 2 permeation [13–16]. Glassy polymer membranes demonstrate diffusivity-based gas permeability that is governed by chain rigidity and free volume elements. Moreover, free volume in the polymer acts as a gas transport pathway through the membrane matrix [17–19]. Microporous organic polymers, Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/memsci Journal of Membrane Science 0376-7388/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2013.11.031 n Corresponding author at: School of Chemical Engineering, College of Engineering, Hanyang University, Seoul 133-791, Republic of Korea. Tel.: þ82 2220 0525. E-mail addresses: ymlee@hanyang.ac.kr, jms@hanyang.ac.kr (Y.M. Lee). Journal of Membrane Science 453 (2014) 556–565