Free volume study of sulfonated FEP proton exchange membrane

Changes in the free volume of poly(tetrafluoroethylene-co-hexafluoropropylene) proton exchange membrane caused by the grafting of styrene and incorporation of a sulfonic acid group (-SO3H) was investigated by using a positron annihilation lifetime measurement. The free volume size of the film decreases because of not only the incorporation of-SO3H but also structural changes in hydrogen bonds during proton transportation. The decrease of free volume size leads to the close distribution of-SO3H; therefore, the performance of the proton exchange membrane increases with the increase of the degree of grafting.