On the mechanisms and the characterization of the pulsed electron grafting of styrene onto poly(tetrafluoroethylene-co-hexafluoropropylene) to synthesize a polymer electrolyte membrane
Byung N. Kim, Alia Weaver, Marina Chumakov, Ileana M. Pazos, Dianne L. Poster, Karen Gaskell, Do H. Han, Gunther Scherer, Michael A. Yandrasits, Byung C. Lee, Mohamad Al-Sheikhly
During the pulsed-electron beam direct grafting of neat styrene onto poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP) substrate, the radiolytically-produced styryl and carbon-centered FEP radicals undergo various desired and undesired competing reactions. In this study, a high dose rate is used to impede the undesired free radical homopolymerization of styrene and ensure uniform covalent grafting through 125 µm FEP films. This outweighs the enhancement of the undesired crosslinking reactions of carbon-centered FEP radicals and the dimerization of the styryl radicals. The degree of uniform grafting through 125 µm FEP films increases from ~8%, immediately after pulsed electron irradiation to 33 % with the subsequent thermal treatment exceeding the glass transition temperature of FEP of 39 oC. On the contrary, steady-state radiolysis using 60Co γ radiolysis, shows that the undesired homopolymerization of the styrene has become the predominant reaction with a negligible degree of grafting. Time-resolved fast kinetic measurements on pulsed neat styrene show that the styryl radicals undergo fast decays via propagation homopolymerization and termination reactions at an observed reaction rate constant of 5 x 108 L·mol-1·s-1. The proton conductivity of 25µm film at 80 oC is 0.29 ± 0.01 S cm-1, and 0.007 S cm-1 at relative humidity of 92 % and 28 %, respectively.