Radiation-induced grafting of styrene onto ultra-high molecular weight polyethylene powder for polymer electrolyte fuel cell application : II. Sulfonation and characterization

Post-sulfonation of selective styrene-grafted ultra-high molecular weight polyethylene (UHMWPE-g-PS) films was carried out by chlorosulfonic acid and the properties of the membranes were investigated. The UHMWPE-g-PS films were previously prepared by initial γ-irradiation of polyethylene powder, grafting with styrene, followed by hot-pressing to process into films. The range of ion exchange capacities (IECs) obtained with different degrees of grafting (DG) of styrene was in the range of 0.97–2.77 mequiv./g. The water and methanol uptake of sulfonated films were studied on a weight and volume basis. FTIR spectroscopy and thermal gravimetric analysis (TGA) of control UHMWPE, UHMWPE-g-PS, and UHMWPE-g-PSSA were compared. The methanol permeability values at 30 °C for the series of UHMWPE-g-PSSA membranes were in the range 4.86 × 10−8 to 1.67 × 10−6 cm2/s, with corresponding proton conductivities of 0.025–0.29 S/cm at 90 °C. The methanol permeability values of UHMWPE-g-PSSA were generally lower than that of Nafion 117 for comparable or even higher proton conductivity. Transmission electron microscopy (TEM) analysis was used to observe microstructure for evidence of nanophase separation of hydrophilic and hydrophobic domains. Owing to its low cost and the results of the current study, UHMWPE-g-PSSA is suggested as a viable proton exchange membrane for low temperature hydrogen and direct methanol fuel cell applications.