Molecular Simulation of the Water Uptake and Glass Transition of Sulfonated Copolyimides as Polyelectrolytes for Fuel Ce
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Molecular Simulation of the Water Uptake and Glass Transition of Sulfonated Copolyimides as Polyelectrolytes for Fuel Cell Applications N. Hu and A.T. Hsu 1 1
Department of Mechanical Engineering Indiana University - Purdue University Indianapolis 723 W Michigan Street, Indianapolis, IN 46202, USA ABSTRACT Molecular dynamics and Grand Canonical Monte Carlo simulations have been used to obtain the glass transition temperatures and the water uptake for polyimides synthesized from Naphthalene-1,4,5,8-tetracaboxylic dianhydride (NTDA), 2,2’-benzidinedisulfonic acid (BDSA), 4,4’-diaminodiphenylether-2,2’-disulfonic acid (ODADS), and non-sulfonated diamine monomers. The glass transition temperature Tgs of these polyimide copolymers have been determined from plots of specific volumes versus temperatures above and below Tgs. The simulation results suggest that the ODADS-based polyimide membranes have lower Tgs and better mechanical properties than the BDSA-based polyimides, which can be attributed to high mobility of the backbones of ODADS as supported by the vectorial autocorrelation function (VACF) results of this study. In addition, comparisons of the simulated Tgs for ODADS-based polyimides of various degrees of hydration show that water content in polyimides may enhance their mechanical properties by lowering the Tgs. In the case of water uptake of these polyimide copolymers, the GCMC simulation results indicate better water solubility for more sulfonated polymers. INTRODUCTION Sulfonated block copolymers have shown potential as membranes for polymer electrolyte fuel cells (PEFCs) [1-3]. Examples of such copolymers are copolyimides synthesized from naphthalene-1,4,5,8-tetracaboxylic dianhydride (NTDA) and sulfonated diamine monomers [46]. Structures of the polyimide copolymers are shown as follows O
O
N A
N O
O
O
N O
O
N
B
O
where the naphthalene-1,4,5,8-tetracaboxylic dianhydride (NTDA) is linked to a sulfonic acid group A to form a sulfonated block and linked to B to form a non-sulfonated block. A slight modification of the functional groups A and B may result in significant change of the properties of these copolyimide membranes [4-6]. Thus a good understanding of the structure-property relationship is important for synthesizing new polymers as fuel cell membranes. This work seeks to explore the relationship between the water uptake and mechanical properties of model polyimide copolymers and their structures using molecular simulation techniques. The use of
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these molecular dynamics and Grand Carnonical Monte Carlo simulations may offer an opportunity to provide a guideline for the design of new polyimide copolymers. COMPUTATIONAL METHODOLOGIES Force Field The force field selected for this study is COMPASS (Condensed-phase Optimized Molecular Potentials for Atomistic Simulation Studies), a class II force field, which has been extensively parameterized for the simulations of a wide range of polymers [7-10]. To model the acidic polyimide copolymers, charge distributions in the SO3- group
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