| 초록 |
We report that rheology of perfluorosulfonic acid ionomers (PFSI) monolayers are attributed to repulsive interaction of SO3- rather than percolation of backbones; electrostatic length of SO3- (Debye length, κ-1) is much longer than physical length of backbone (radius, r). To support our statement, we add salts to reduce κ-1 by screening the charge of SO3-. Until κ-1 is reduced to be similar to r, shear moduli does not change, probably because backbone does not contribute to rheology. Additionally, PFSI monolayers do not follow the Cox-Merz rule empirical for polymer solution/melt; when rheology arise from polymeric backbone, they are supposed to follow the rule. Despite rheology are attributed to SO3-, it is revealed that equivalent weight (EW) and side chains affect rheology indirectly: (1) As surface activity increases with EW, confirmed by the increase of surface pressure (Π), greater amounts of PFSI adsorb to the interface so that rheology increase; (2) PFSI of longer side chain exhibits larger rheology than PFSI of shorter side chain at the same Π, because the former packs better due to larger translational entropy. |
