Charge transfer in metal/polymer contacts

Abstract

We have investigated the contact charging of polymers by metals, paying particular attention to the variation of charge transfer from place to place on the sample surface, and from one sample to another. For most polymers we find that charge transfer is unaffected by our attempts at purification and is much the same for samples prepared in quite different ways, and we conclude that charge transfer is an intrinsic property of polymers, not normally dominated by the presence of accidental impurities etc. There is one exception however: charge transfer to PTFE appears to be determined by physical damage. For some polymers the charge transfer tends to increase as the work function of the contacting metal increases but this tendency is not universal: charge transfer to some polymers is insensitive to the metal work function. (We show that this is true for multiple contacts and for sliding as well as for single non-sliding contacts.) The apparent charge density shows remarkably little variation from one polymer to another, and this implies that the density of the electron states responsible for contact electrification is much the same for a wide range of polymers. However, the density of states required to account for our observed charge densities is much smaller than the density of intrinsic states, such as the orbitals of 'pendant' chemical groups, and too small to allow significant back-tunnelling during separation. Our observations cannot be satisfactorily reconciled with current models of charge transfer which assume electron exchange between the metal and localised states in the polymer unless intrinsic states of sufficiently low density can be plausibly identified.

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