The lateral migration of surface charges on poly(methyl methacrylate) graft-copolymerized onto polypropylene film, and its dependency on relative humidity

Abstract

The decay behavior of surface charges on poly(methyl methacrylate) graft-copolymerized onto polypropylene film has been investigated in air at 20°C by measuring a two-dimensional distribution of charges, and its dependency on relative humidity studied. At low humidity (0 or 20%), the charges were very stable and did not decay. At higher humidity (40 or 60%), the surface charges have been shown to decay and the lateral charge spreading along the film surface has been clearly observed. Both the decay and the lateral migration of surface charges are faster at 60% humidity than at 40%. The observed decay behavior and the lateral charge spreading have been successfully computer-simulated. From this computer simulation, it has been concluded that there are two types of surface charges trapped at two different sites; one exists just at the film surface and can migrate laterally along the surface but neither migrates into bulk nor decays, and the other exists at the more inner sites, migrates laterally more slowly and can migrate into bulk and decay. The ratio of amounts of these two types of charges was about 1 : 1. The diffusion constant of lateral migration obtained by the computer simulation for the charges at the film surface was about 10 −9 m 2 s −1 at 60% humidity and 10 −10 m 2 s −1 at 40%.