The lateral motion of charge on thin films of polyethylene terephthalate

Measurements are reported on the lateral motion of charges along thin films of polyethylene terephthalate (PET) in air at 40% humidity. A sensitive, high-resolution induction probe coupled to a computer is used to scan the sample in a raster mode. Thus it is possible to produce not only line scans of the charge distribution across the film at any time, but also to display the charge as contour or relief maps as well. The behaviour of charge can thus be studied in more detail than previously. It is established that charges of either sign deposited from a corona source move laterally according to a diffusion law although there is some evidence that this motion may also be controlled by the electric fields induced. The diffusion coefficient is of order 10^-9 m² s^-1 with an activation energy of 1.5 eV and is much greater than the coefficient for the diffusion of neutral gases in the polymer. The diffusion is probably within the bulk of the polymer but washing in deionised water induces a much higher rate or diffusion which is probably a true surface phenomenon.