(357f) Molecular Simulations of Neat, Hydrated, and Acid-Doped Polybenzimidazoles

Polybenzimidazoles are a set of linear aromatic polymers that exhibit excellent mechanical strength, thermal stability, and high resistance to acids, bases, and oxidative attack. These properties have led to their use in a variety of applications from fire-resistant materials to membrane applications. In particular, acid-doped polybenzimidazoles have shown promise as proton-exchange membranes (PEMs) for fuel cell use up to 200 ^oC. The most frequently used dopant is phosphoric acid (PA), introduced in 1995 by Wainright. As generally observed, acid doping level and water uptake are two critical factors that influence not only the conductivity but also the mechanical strength of the membranes. Ma et al. also proposed a comprehensive conductivity mechanism for proton transfer (PT) through the polymer-acid-water system based on their experimental results, and noted that hydrogen bonding played an important role in the organization of solvent molecules and the polymer chains to promote conductivity.

            While there have been many experimental studies of membrane conductivity and structural and mechanical properties available in the literature, simulation studies of the acid-doped polybenzimidazole system have not been reported. In this study, molecular dynamics simulations have been used to investigate the density, X-ray diffraction patterns, radial distribution functions (RDFs), diffusion properties, and, most importantly, the hydrogen bonding behavior of the neat, hydrated, and PA-doped benzimidazole polymer systems under different levels of water uptake, acid doping, and temperature. Results are compared experimental data available in the literature. Poly(2,2'-m-phenylene-5,5'-bibenzimidazole) (PBI, Fig. 1A), poly(2,5-benzimidazole) (ABPBI, Fig. 1B) and poly(p-phenylene benzobisimidazole) (PBDI, Fig. 1C) are three important examples of benzimidazole polymers.  PBI, with a reported glass transition temperature, T_g, of 420 ^oC, is the only commercially used polybenzimidazoles and the majority of research has been focused on the acid-doped PBI membranes. ABPBI, which has the simplest repeat-unit structure in the family, has also been widely investigated for applications as PEMs. Although PBDI has not been widely studied as potential PEMs, its hydrated structure has been characterized through X-ray crystallography of a similar diimidazole model compound. The polymer-acid-water interactions illustrated in that study provided valuable insight into the behavior of solvent molecules for both the hydrated and acid-doped polymer systems.

Figure 1 Structures of poly(2,2'-m-phenylene-5,5'-bibenzimidazole) (PBI), poly(2,5-benzimidazole) (ABPBI), and poly(p-phenylene benzobisimidazole) (PBDI)

            As a first step in this study, density, X-ray diffraction patterns, RDFs, and hydrogen bonding behavior were analyzed for all there neat benzimidazole polymers and their corresponding hydrated and PA-doped systems at the end of 800-ps NPT dynamics. Single chains of each of the three benzimidazole polymers were built head-to-tail at random torsion angles to represent the neat polymers. The chain length was 75 repeat units (RUs), 200 RUs, and 100 RUs for PBI, ABPBI, and PBDI, respectively. The hydrated systems were constructed by mixing the polymer chains with water molecules to make the ratio of imidazole units to water molecules to be 1:2. Similarly, the acid-doped system was constructed by mixing polymer chains with both PA and water molecules to make the ratio of imidazole units to PA molecules to water molecules to be 1:2:2.

            Second, the effect of the number of polybenzimidazole chains used in the simulation, the water uptake, acid doping level, and temperatures on structural and diffusion properties, and evidence of hydrogen bonding behavior in the neat, hydrated, and PA-doped ABPBI systems were studied. Three 80-RU ABPBI chains were built at 25 ^oC and 180 ^oC, respectively, to represent the neat ABPBI. Three 80-RU ABPBI chains and different numbers of water molecules (the ratio of imidazole units to water molecules was 1:0.5, 1:1, 1:1.5, 1:2, 1:2.5, and 1:3, respectively) were constructed at 25 ^oC to represent the hydrated systems of interest. For the acid-doped systems, two 80-RU ABPBI chains were selected to represent ABPBI.  At 25 ^oC, two acid-doped systems were constructed by mixing ABPBI chains with both PA and water molecules to make the ratio of imidazole units to PA molecules to water molecules to be 1:1.5:1.25 and 1:3:2.5, respectively. At 180 ^oC, another two acid-doped systems were built. One of them was constructed by mixing ABPBI chains with both PA and water molecules to make the ratio of imidazole units to PA molecules to water molecules to be 1:3:2.5. The other was constructed by mixing ABPBI chains with PA molecules only to make the ratio of imidazole units to PA molecules to be 1:3. Density, X-ray diffraction patterns, RDFs, and hydrogen bonding behavior were analyzed and compared between the constructed systems at the end of the 800-ps NPT dynamics. For the four acid-doped systems, another 3-ns NVT dynamics was run at 298 K or 453 K, following the NPT dynamics, to calculate the diffusion coefficients.

Good agreement between available experimental data and the simulated density, X-ray diffraction patterns, RDFs, and diffusion coefficients of the neat, hydrated, and acid-doped polybenzimidazoles provides validation for the simulation results. The results, especially the hydrogen bonding behavior, are consistent with the expected PT mechanism in polybenzimidazole-based PEMs.