Correlation of the Nitrogen 1s and Oxygen 1s XPS Binding Energies with Compositional Changes During Oxidation of Ethylene Diamine Plasma Polymers

In the course of plasma deposition of organic-polymeric thin films, radicals are incorporated into the growing film. These radicals initiate spontaneous oxidation reactions that continue over many weeks when the plasma polymers are stored in air. The resultant changes to the composition with time were monitored by XPS for ethylene diamine plasma polymer samples in order to improve understanding of the products of the oxidative reactions. The broadness of all the peaks, and the multitude of functional groups expected to be present, precluded the obtainment of detailed compositional information by curve fitting for components, but shifts in the binding energies (BEs) of the N 1s and O 1s peaks with time provided useful evidence of compositional changes. As the oxygen content of the plasma polymer increased upon oxidation, the binding energy of the N 1s photoelectrons increased from 399.1 to 399.8 eV. Concurrently, the amount of nitrogen in relation to carbon (N/C) decreased from 0.42 to 0.34. As the nitrogen content of the plasma polymer decreased, the binding energy of the O 1s photoelectrons increased from 531.1 to 531.8 eV. The BE values and their shifts with time/compositional changes suggested that the oxidation process predominantly caused oxidation of the carbon atoms that had amine groups attached to them, leading to the formation of amide groups and perhaps also some imides.