Effects of solvents on post-irradiation grafting of styrene onto fluoropolymer films

Abstract

Due to the interest in the development of low cost membranes with similar capabilities to Nafion®, grafting of styrene on fluorinated polymers by simultaneous irradiation and pre-irradiation methods at high temperature (higher than 50 °C) followed by sulfonation has been a process studied broadly. However, there is very little information about grafting yield at room temperature after simultaneous irradiation. Previous works reported that the surface morphology of the styrene grafts has showed an uneven profile under the studied conditions. This work aims to evaluate the grafting of styrene onto these films after simultaneous irradiation (in post-irradiation condition), using different solvent in periods until 28 days at room temperature. Films of poly(tetrafluoroethylene) (PTFE) and poly(vinylidene fluoride) (PVDF) were immersed in styrene/toluene 1:4 or styrene/methanol 1:4 and then submitted to gamma radiation at 40 and 80 kGy doses. The degree of grafting (DOG) was determined gravimetrically and physical and chemical changes were evaluated by infrared spectroscopic analysis (FTIR), differential scanning calorimeter analysis (DSC), thermogravimetric analysis (TGA), and also by scanning electron microscopy (SEM). For PTFE films, the highest DOG was achieved using toluene as solvent while for PVDF films, the highest DOG was achieved with methanol. Surface images of the grafted films by SEM technique have presented a strong effect of the solvents on the films morphology. Finally, PVDF-g-PS in styrene/methanol film revealed higher DOG, although the surface showed small cavities and discontinuities. On the contrary, the surface of PTFE-g-PS in styrene/toluene presented homogeneous surface.