New hyperbranched polymers for membranes of high-temperature polymer electrolyte membrane fuel cells: Determination of the crystal structure and free-volume size

The key requirements for a membrane in polymer electrolyte membrane fuel cells are a high ion conductivity, mechanical strength, and barrier properties. We reported earlier on two new promising hyperbranched polymers: poly(benzimidazole-co-aniline) (PBIANI), with a uniform rectangular net structure, and poly(benzimidazole-co-benzene) (PBIB), with a honeycomb structure. Both polymers exhibit a high ion conductivity and mechanical strength and have proven themselves suitable for the membranes of high-temperature polymer electrolyte membrane fuel cells. In this article, we deal with the determination of crystal structure and free-volume cell/microvoid size of these two polymers. Both PBIANI and PBIB had the same d-spacing (3.5 Å). However, the percentage of crystallinity was higher and the crystallite size was larger for PBIB. The kinetic diameters of hydrogen (2.89 Å), oxygen (3.46 Å), water (2.60 Å), and methanol (∼ 4.00 Å) were much larger than the free-volume cell/microvoid diameters of PBIANI (1.81 Å) and PBIB (1.96 Å) but much smaller than those of Nafion 115 (6.54 Å) and polybenzimidazole (PBI) (∼ 6.00 Å). The very small free-volume sizes of PBIANI and PBIB ensured good barrier properties against hydrogen, oxygen, water, and methanol, unlike those of Nafion- and PBI-type membranes.