Investigation of the water states of poly(styrene sulfonic acid) grafted poly(ethylene-co-tetrafluoroethylene) copolymer using FT-IR analysis

Abstract

The water states of poly(styrene sulfonic acid) (PSSA) grafted poly(ethylene-co-tetrafluoroethylene) copolymers (ETFE-PEMs) with grafting degree (GD)=8.8-30.5% are investigated under ambient conditions by using FTIR analysis. Detailed analysis of the FT-IR spectra of ETFE-PEMs allows the observation of each component of water in the two broad bands of 1500-2000 cm^-1 and 2400-3800 cm^-1. These components include protonated water (H₃O⁺…(H₂O)n), H-bonded water to sulfonic acid groups ((SO₃)…(H₃O⁺)(H₂O)n), H-bonded water to other water molecules ((H₂O)…(H₂O)), and non-H-bonded water (HOH…F). Of these water forms, ((SO₃)…(H₃O⁺)(H₂O)n) is predominantly represented. Free water (non-H-bonded water) is assumed to be trapped in the holes of the ETFE backbone and is thus difficult to remove at 50^oC over a 24 h period. The total peak areas of the water molecules significantly increase with GD indicating that the ETFE-PEM with higher GD possesses higher water content. Moreover, the increase of the total peak area with GD via two steps reveals an interesting relationship between the hierarchical structures and the capacity of water content under ambient conditions. The observed water content at ambient conditions can significantly affect the conductance of ETFE-PEMs under conditions of low relative humidity such as during fuel cell operation or measurement procedures of specific characterisations or testing methods.