Dissipative Particle Dynamics Simulations of Anion Exchange Membranes
Authors: Luo Xubo , Sepehr Fatemeh , Paddison Stephen J. 
Published: February 2020

Anion exchange membranes (AEM) are currently under investigation for their application in energy and conversion devices.1,2 The main challenges of the AEMs are the low stability and ionic conductivity. Rather than increasing the ion exchange capacity, a realistic strategy to enhance the conductivity is to use phase segregated AEMs.3 Hence, the morphology of the AEM polymer is very important to achieve high performance. However, there is still a lack of fundamental understanding of stability and transport in AEMs.4 Recently, researchers have reported elastomeric AEMs exhibiting satisfactory chemical stability.5 These AEMs are based on the triblock copolymer, polystyrene-b-poly(ethylene-co-butylene)-b-polystyrene (SEBS), and functionalized with various cationic groups.

In this research, dissipative particle dynamics (DPD) simulations were chosen to model the morphology of SEBS systems functionalized with tetra alkyl ammonium groups. DPD, a meso-scale simulation technique, enables relatively large time and length scales to be examined,6,7 and therefore permits the morphology of the polymer system in acceptable time and computational expenses. The SEBS polymer was coarse-grained in to DPD beads, balancing both chemical distinction and fine structural variants. The optimized structure and the interaction parameters of these beads were calculated by using a recently developed methodology employing DFT based electronic structure calculations.8
,9 The morphology of the aforementioned AEMs were simulated at different hydration levels. The result shows that the morphology changes with the hydration levels. The effects of the alkyl chain tethered to the cation group, the ion exchange capacity, and the copolymer composition were also investigated.

References:

(1) Couture, G.; Alaaeddine, A.; Boschet, F.; Ameduri, B. Polymeric Materials as Anion-Exchange Membranes for Alkaline Fuel Cells. Prog. Polym. Sci.
2011, 36 (11), 1521–1557.

(2) Varcoe, J. R.; Atanassov, P.; Dekel, D. R.; Herring, A. M.; Hickner, M. A.; Kohl, P. A.; Kucernak, A. R.; Mustain, W. E.; Nijmeijer, K.; Scott, K.; et al. Anion-Exchange Membranes in Electrochemical Energy Systems. Energy Environ. Sci.
2014, 7, 3135–3191.

(3) Li, N.; Guiver, M. D. Ion Transport by Nanochannels in Ion-Containing Aromatic Copolymers. Macromolecules
2014, 47 (7), 2175–2198.

(4) Marino, M. G.; Melchior, J. P.; Wohlfarth, A.; Kreuer, K. D. Hydroxide, Halide and Water Transport in a Model Anion Exchange Membrane. J. Memb. Sci.
2014, 464, 61–71.

(5) Mohanty, A. D.; Ryu, C. Y.; Kim, Y. S.; Bae, C. Stable Elastomeric Anion Exchange Membranes Based on Quaternary Ammonium-Tethered Polystyrene-B-Poly(ethylene-Co-Butylene)-B-Polystyrene Triblock Copolymers. Macromolecules
2015, 48 (19), 7085–7095.

(6) Groot, R. D.; Warren, P. B. Dissipative Particle Dynamics: Bridging the Gap between Atomistic and Mesoscopic Simulation. J. Chem. Phys.
1997, 107 (11), 4423.

(7) Espanol, P. Hydrodynamics from Dissipative Particle Dynamics. Phys. Rev. E
1995, 52 (2), 1734–1742.

(8) Sepehr, F. Morphology of Elastomeric Anion Exchange Membranes: A Dissipative Particle Dynamics Study. In PRiME 2016/230th ECS Meeting (October 2-7, 2016); 2016.

(9) Sepehr, F.; Paddison, S. J. Dissipative Particle Dynamics Interaction Parameters from Ab Initio Calculations. Chem. Phys. Lett.
2016, 645, 20–26.