Hydrogen plasma treatment of poly(ethylene terephthalate) surfaces

Amorphous hydrogenated carbon (a-C:H) layers on poly(ethylene terephthalate) (PET) act as permeation barriers reducing the permeation coefficient by up to 70%. It has been found recently that high density a-C:H layers show a lower reduction of the permeability coefficient than low-density layers. This was attributed to the occurrence of cracks in the high density layers, most probably originating from the relief of the high internal stress throughout the a-C:H layers on the flexible polymer substrate. A conceivable reason for this effect could be a change of the polymer surface or bulk properties due to interaction with the plasma in: the initial phase of the deposition process. To investigate this, we studied the interaction of hydrogen plasmas with PET surfaces. Hydrogen plasmas contain many of the active ingredients of hydrocarbon plasmas, such as hydrogen ions, hydrogen atoms, and intense UV radiation, so that they can be applied to simulate to a certain extent the plasma surface interaction in the initial phase of deposition without the protection by the growing him. Chemical and morphological modifications of the PET surfaces were analyzed by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy, after treating the polymer in hydrogen plasmas at high and low ion energy conditions. The influence of such treatments on the permeability coefficients of several gases was also investigated. A magnesium fluoride window was applied to protect the PET surface from ion bombardment during the hydrogen plasma treatment and to investigate the effect of the UV radiation on the barrier properties. Distinct surface modifications and changes of the barrier properties were observed for PET treated for 1 h under high ion energy conditions. The oxygen content of the PET surface decreases by approximately 30%, if PET is exposed to the hydrogen plasmas used in this work. The morphology and the XPS spectra of the hydrogen plasma treated PET indicate a severe degradation of the polymer matrix. (C) 2001 Elsevier Science B.V. All rights reserved.