Hydration of Polymer Membranes
The behavior of water in polymer membranes and films is important for many applications. In hydrogen fuel cells, the polymer membranes that conduct protons from the anode to the cathode require that the polymer be well hydrated. In polymer films used for packaging the ability to prevent the transport of water is important to ensure that the contents inside the packaging remain dry.
On this page, we provide a visualization of the distribution of water in two polymers that contain ionic species. Nafion is a perfluorosulfonic acid polymer that has a hydrophobic teflon backbone and a hydrophilic side chain terminated with a sulfonic acid group. Nafion is used in fuel cells. Chitosan is a naturally occurring polymer that has amine groups that can be hydrated. However, the backbone of Chitosan is hydrophilic relative to Nafion. Therefore the distribution of water in these systems is very different.
On this page, the visualizations show only the water. All of the polymer has been rendered invisible, with the exception of the sulfur atoms in Nafion. There is also a page in which all atoms in the system are shown, although in this case it is more difficult to see the distribution of water within the polymer. There is also a a page with a single isolated Nafion chain and Chitosan chain, which clearly shows their structures.
Interactive Structures
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The aqueous phase in hydrated Nafion (degree of polymerization = 15, equivalent weight = 1144, l = 22 H2O/SO3-) at 300 K. This structure shows the nanoscale phase segregation between the aqueous domain and the Nafion backbone. |
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The water distribution in a chitosan polymer film at T = 300 K and a relative humidity of 95%. This structure shows that the water molecules are distributed fairly homogeneously through-out the polymer membrane. Note that some bonds are split by periodic boundary conditions. |
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References
Nafion References:
Cui, S.T., Liu, J., Esai Selvan, M., Keffer, D.J., Edwards, B.J., Steele, W.V., "A Molecular Dynamics Study of a Nafion Polyelectrolyte Membrane and the Aqueous Phase Structure for Proton Transport", J. Phys. Chem. B 111(9) 2007 p. 2208-2218. doi: 10.1021/jp066388n.
Cui, S., Liu, J., Esai Selvan, M., Paddison, S.J., Keffer, D.J., Edwards, B.J., "Comparison of the Hydration and Diffusion of Protons in Perfluorosulfonic Acid Membranes with Molecular Dynamics Simulations", J. Phys. Chem. B 112(42) 2008 pp. 13273-13284. doi: 10.1021/jp8039803.
Liu, J., Suraweera, N., Keffer, D.J., Cui, S., Paddison, S.J., "On the Relationship Between Polymer Electrolyte Structure and Hydrated Morphology of Perfluorosulfonic Acid Membranes", J. Phys. Chem. C 114(25) 2010 pp 11279-11292. doi: 10.1021/jp911972e.
Esai Selvan, M., Calvo-Muñoz, E.M., Keffer, D.J., "Toward a Predictive Understanding of Water and Charge Transport in Proton Exchange Membranes", J. Phys. Chem. B 115(12) 2011 pp 3052-3061. doi: 10.1021/jp1115004.
Esai Selvan, M., Keffer, D.J., Cui, S., "A Reactive Molecular Dynamics Study of Proton Transport in Polymer Electrolyte Membrane", J. Phys. Chem. C 115(38) 2011 pp. 18835-18846. doi: 10.1021/jp203443c.
Chitosan References:
McDonnell, M.T., Greeley, D., Kit, K., Keffer, D.J., "Modeling Oxygen Permeability in Biodegradable Polymer Films", in preparation, 2014.
Similar structures are available for other polymer membranes including, xs-PCHD and xs-PCHD/PEG membranes:
Wang, Q., Suraweera, N.S., Keffer, D.J., Deng, S., Mays, J.W., "Atomistic and Coarse-Grained Molecular Dynamics Simulation of a Cross-Linked Sulfonated Poly (1,3-cyclohexadiene)-based Proton Exchange Membrane", Macromolecules, 45(16) 2012 pp 6669-6685. doi: 10.1021/ma300383z
Wang, Q., Keffer, D.J., Deng, S., Mays, J.W., "Structure and Diffusion in Cross-linked and Sulfonated Poly (1, 3-cyclohexadiene)/Polyethylene Glycol-based Proton Exchange Membranes", J. Phys. Chem. C 117(10) 2013 pp. 4901-4912. doi: 10.1021/jp309793z.
posted: July 2014.
updated: July 2014.