Effects of melt flow index and equivalent weight on the dimensional stability and mechanical behavior of perfluorosulfonic acid ionomer membranes

The effect of melt flow index (MFI) and equivalent weight (EW) on three aspects relating to perfluorosulfonic acid membrane mechanical durability were studied: dimensional stability, hygral stress during humidity cycles and resistance to rupture under biaxial stresses. Membranes used in this study have nominal EWs of 800 and 700 g/mol -SO3H and MFI ranging from >> 10 to 0.005 g/10 min. The total water-uptake, water-uptake per sulfonic acid group, and coefficient of hygral expansion all increase with decreasing EW. For the same EW, the ionomer with lower MFI absorbs less water, swells less and experiences less hygral stress during humidity cycles. Although a 700 EW membrane with an MFI of 0.005 g/10 min swells more than the 800EW ionomers, it is projected to be the most durable because it experiences the least hygral stress during humidity cycles and is the most resistant to rupture under biaxial stress. These findings are supported by in-situ RH-cycling tests, which show a strong inverse correlation between log[MFI] and cycles to failure, and a weaker inverse dependence on EW. The results suggest that high-performing, low-EW ionomers can be mechanically robust enough for fuel cell applications provided they have a sufficiently low MFI.