Molecular Dynamics Simulation of Swollen Membrane of Perfluorinated Ionomer

Shingo Urata, Jun Irisawa, Akira Takada, Wataru Shinoda, Seiji Tsuzuki, and Masuhiro Mikami

J. Phys. Chem. B 109 4269-4278 (2005).

Molecular dynamics simulations of the swollen membrane of perfluorinated ionomer, which is composed of polytetrafluoroethylene backbones and perfluosulfonic pendant side chains, have been undertaken to analyze the static and dynamical properties of the water and the side chain in the membrane. The calculations were carried out for four different water contents, such as 5, 10, 20 and 40wt%, at 358.15 K and 0.1MPa.The results are summarized as follows; (1) the sulfonic acid is the unique site to which water molecules can bind, and the other sites in the pendant side chain has no bound water even at high water concentration. (2) Sulfonic acids aggregate in the short range within 4.6 to 7.7A in spite of the electrostatic repulsion between them. In such aggregates, a water molecule bridges two sulfonic acids. (3) Pendant side chains prefer to orient perpendicular to the hydrophilic/hydrophobic interface, and long range correlation of side chain orientations is observed at 20 and 40wt% water uptake membranes. (4) In low water uptake membrane, dynamics of water is substantially restricted due to strong attractive interactions with acidic sites. In contrast, at high water content, even the water locating near the sulfonic acid is relatively mobile. The short residence time of the bound water reveals that such water can frequently exchange their position with relatively free water, which locates in center of water cluster, in highly swollen membranes.