Enhancement of Salt Rejection and Water Flux by Crosslinking-Induced Microstructure Change of N- substituted Polybenzimi
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Enhancement of Salt Rejection and Water Flux by Crosslinking-Induced Microstructure Change of N-substituted Polybenzimidazole Membranes Motohiro Aiba1, Takahiro Tokuyama2, Hidetoshi Matsumoto1, Hiroki Tomioka2, Tomoya Higashihara3, and Mitsuru Ueda3 1 Department of Organic and Polymeric Materials, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1, O-okayama, Meguro-ku, Tokyo 152-8552, Japan. 2 Toray Industries Inc., Sonoyama, Otsu, Shiga 520-0842, Japan. 3 Department of Polymer Science and Engineering, Faculty of Engineering, Yamagata University, 4-3-16 Jonan, Yonezawa City, Yamagata 992-8510, Japan. ABSTRACT Crosslinked or non-crosslinked ultrathin semipermeable membranes based on the Nbutylated and N-butylsulfonated polybenzimidazole (BPBI and BSPBI) were successfully prepared by spin-coating method. Structural characterization by FTIR and WAXD revealed that the N-substitution and the crosslinked structure of PBI suppressed the hydrogen bonding and increased the d-spacing. Furthermore, positron annihilation lifetime spectroscopy (PALS) clearly showed the pore radius change from 0.27-0.29 nm to 0.33 nm by crosslinking. As a result, the enhancement of water flux and NaCl rejection was achieved by the crosslinking of the BPBI and BSPBI. Especially, the crosslinked N-butylsulfonated PBI (CL-BSPBI) membrane significantly improved not only salt rejection but also water flux (NaCl rejection : 46 %, water flux : 22.1 L m-2 h-1) compared to those of non-crosslinked BSPBI one (NaCl rejection : 11 %, water flux : 1.88 L m-2 h-1) due to both the Donnan effect and the formation of larger pores in the membrane, respectively. INTRODUCTION Semipermeable membranes have become important materials from the viewpoint of energy and environment.[1] Up to present, crosslinked aromatic polyamides have been used as a membrane material of the active layer due to their high water flux and excellent NaCl rejection. However, PA membrane suffered from critical low chlorine resistance of the amide linkage, which leads to irreversible performance loss over time and requires the expensive dechlorination and rechlorination treatment steps.[2] Thus, the novel synthetic polymers designed to be high chlorine resistance and transport properties should be used as semipermeable membranes to address this problem. Polybenzimidazole (PBI) without the amide linkage was expected to be one of the alternative materials as the active layer due to its high mechanical properties, thermal stability, and chemical stability over a wide range of pH.[3,4] In spite of its excellent properties, the hydrogen bonding between the N-H unit and the nitrogen with lone pair in the imidazole ring is so strong that the PBI membrane has become dense and shows low water transport properties. Therefore, it is necessary to suppress the polymer/polymer interaction and increase the free volume of PBI membrane. In this study, a half of the N-H groups in the imidazole rings of PBI were substituted to butyl or butylsulfonate group in order to suppress the
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