Investigation of chemical degradation and water states in the graft-type polymer electrolyte membranes

Chemical degradation in the presence of H2O2 aqueous solution and water states of poly(styrene sulfonic acid) (PSSA) grafted poly(ethylene-co-tetrafluoroethylene) polymer electrolyte membranes (ETFE-PEMs) with grafting degree (GD) of 8.8%-21.1% are investigated using Fourier-transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), and electrochemical impedance spectroscopy (EIS). The ETFE-PEMs show the complete detachment at the alpha-hydrogen atoms of the whole graft PSSA chains from the ETFE backbones, while their precursor polystyrene-grafted ETFE films do not. This chemical degradation exhibits independently with GDs and takes place even at very low GD (<10%). Water molecules of the membranes dried at 40 degrees C are not completely de-absorbed. The remains are partial non-bonded water molecules, which are assumed to locate within the lamellar amorphous domains (constrained water molecules) of the crystalline phases. These constrained water molecules exist independently with the PSSA groups and do not detach together with them during the degradation. Note that the water molecules remained at 40 degrees C show the additional contribution of 18%-19% to the total water uptake but do not contribute clearly to the conductance.