Anion exchange membranes composed of a poly(2,6-dimethyl-1,4-phenylene oxide) random copolymer functionalized with a bulky phosphonium cation
Graphical abstract
Introduction
Anion exchange membranes (AEMs) exhibit potential for a large variety of processes, such as mass separation (electrodialysis, filtration, water purification) [1], [2], chemical synthesis (electrolysis) [3], and energy conversion and storage (fuel cells and advanced batteries) [4], [5], [6]. In all of these processes, the cationic groups attached to the polymer backbone control the transport of the anionic species [7]. However, many AEMs are not chemically stable to hydroxide, which is essential to some applications, especially under drier and hotter conditions. High hydroxide concentrations decompose the cationic groups via SN2, Hofmann elimination, or other degradation processes [8], [9]. Therefore, durable AEMs with efficient, selective transport are desired, and so new stable cations and polymer backbones must be developed.
Recently, bulky cations have shown improved chemical stability. Zhang et al. designed a new diphenyl (3-methyl-4-methoxyphenyl) tertiary sulfonium cationic group [10]. This sulfonium cation remained stable in 1M KOD solution at 60 °C for 10 days. A 1,4,5-trimethyl-2-(2,4,6-trimethoxyphenyl) imidazolium cation [11], [12] demonstrated that attaching the 2,4,6-trimethoxyphenyl group to the cation improves stability. In Gu et al.'s work [13], a tris(2,4,6-trimethoxyphenyl) phosphonium functionalized polysulfone membrane survived for 48 h when immersed either in 10 M KOH at room temperature or in 1 M KOH at 60 °C. Jiang et al. [14] investigated a tris(2,4,6-trimethoxyphenyl) phosphonium functionalized bromomethylated poly(2,6-dimethyl-1,4-phenylene oxide) that maintains stability in 1 M NaOH at 60 °C for 75 h. Zha et al. [15] first reported AEMs with a ruthenium bis(terpyridine) complex cation. This membrane exhibited excellent stability in 1 M NaOH solution at room temperature over 6 months. The promising chemical stability of these bulky cations results from the contributions of the electron donating function as well as increased steric effects from the attached side groups. It has also been recently realized that the polymer backbone stability is also a crucial factor towards durable polymers for AEM applications. In particular, residual bromide from incomplete quaternization is a site for hydroxyl attack and so should be avoided [16].
The polymer morphology, ionic nanostructure, and degree of ionic dissociation all affect transport properties in AEMs [17], [18]. Solvents can alter the polymeric configuration and properties due to distinct interactions between the polymer chain and the solvent. Effects of the casting solvent on the phase behavior of the diblock [19], [20] or triblock [21] copolymers have been well studied. The conductivities of AEMs can also be influenced by changing solvents during the casting process. Elabd et al. [22] investigated the conductivity of sulfonated poly(styrene-b-isobutylene-b-styrene) triblock copolymers applied in proton exchange membrane fuel cells. The proton conductivities of these triblock copolymers varied by 3 orders of magnitude, ranging from 1.07×10−2 mS/cm cast from a toluene/ethanol mixture (85/15 w/w) to 5.95 mS/cm cast from toluene. Ong et al. [23] developed a random poly(2,6-dimethyl-1,4-phenylene oxide) AEM with a hydroxide conductivity of 17 mS/cm at 60 °C in water when cast from NMP, while the conductivity of a membrane cast from a chlorobenzene/DMF mixture solvents was 8 mS/cm.
Little work has investigated the casting solvents’ influence on water and ionic transport in AEMs. In this study, we report the synthesis of a new polymer with a bulky phosphonium cation attached to a polyphenylene oxide polymer via a selective bromination method used to avoid the large amount of excess bromide present in a previously synthesized similar AEM [16]. The polymer is fully characterized by NMR, FTIR, SAXS and microscopy. By studying the random copolymer, instead of a block copolymer, the intrinsic transport properties can be examined in a system with more random phase separation and structuring. By varying the solvent used for casting, based on the solubility of each of the polymer components, the impact of polymer morphology with transport and conductivity is established. Insights into the anionic conductivity are gained from a study of the water self-diffusion coefficients from NMR measurements.
Section snippets
Materials
Poly(2,6-dimethyl-1,4-phenylene oxide), N-bromosuccinimide, chlorobenzene, 2,2-azobis(2-methylpropionitrile), ethanol, 1-methyl-2-pyrrolidone, tetrahydrofuran and tris(2,4,6-trimethoxyphenyl)phosphine were purchased from Sigma-Aldrich. The molecular weight of poly(2,6-dimethyl-1,4-phenyloxide) was reported as Mw=35,000, Mn=16,000 (Sigma-Aldrich).
Synthesis of tris(2,4,6-trimethoxyphenyl) phosphonium functionalized poly(2,6-dimethyl-1,4-phenylene oxide) (PPO–TPQP)
A solution of poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) (6 g, 50 mmol in 60 mL chlorobenzene) was heated to 140 °C, followed by addition of N
PPO–TPQP synthesis and structure
Tris(2,4,6-trimethoxyphenyl) phosphonium functionalized poly(2,6-dimethyl-1,4-phenylene oxide) (PPO–TPQP) was successfully synthesized by bromination of poly(2,6-dimethyl-1,4-phenylene oxide), followed by quaternization with tris(2,4,6-trimethoxyphenyl) phosphine (Fig. 1).
The synthesis of the intermediate BPPO was validated by 1H NMR spectroscopy (Fig. S1(a)); the methyl and methylene protons of the brominated polymer were seen at 2.1 and 4.3 ppm, respectively. Integration of these two peaks
Conclusions
In this study, DMSO, Ethyl lactate and DMSO & Ethyl lactate mix solvents were selected based on Hansen solubility parameter to drop cast PPO–TPQP containing the large phosphonium cation. All three membranes have modest water uptake in the range of 10–28 wt% after being soaked in DI water at 60 °C. From SAXS measurements, the membranes' nanoscale morphology was shown to be invariant with humidity. The peak at about d-spacing of 7–15 was consistent with the optimized cation size calculated by DFT
Acknowledgments
The authors thank the Army Research Office for support of this research under the MURI Grant number #W911NF-10-1-0520. The Advanced Photon Source operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, was support by the U. S. DOE under Contract No. DE-AC02-06CH11357. We also thank Dr. Steven Abbott for the HSP calculation on PPO-TPQP by using the HSPiP package.
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