Significance of interfacial redox reactions and formation of metal-organic complexes for the adhesion of metals on pristine and plasma-treated polymers

Abstract

The oxygen low-pressure plasma treatment of polymer surfaces promotes their adhesion to metals. It is well known that such treatment results in the formation of a wide variety of O functional groups, polymer degradation and crosslinking. Only a few watts and short exposure times (0.1 to 2 s) provide the optimum in functionalization and in preserving the original polymer structure. Therefore, these are the best conditions for maximum adhesion in many metal-polymer systems. Carbonate, ester, and aromatic groups are rapidly split by an oxygen plasma treatment leading to scissions of polymer backbones and loss in molecular weight. The formation of macrocycles and a large number of C=C bonds in a region of around 4 nm in depth is of interest. Crosslinking of polymers due to oxygen plasma exposure is a very important process. It can be verified by Thermal Field Flow Fractionation (ThFFF) by which gel particle concentration and particle diameters can be measured. The investigated polymers could be divided into different types of degradation behaviour on exposure to an oxygen plasma. Chromium or potassium strongly react with functional groups at polymer surfaces and may cause complete disordering of macromolecules in a ≈ 2 nm thick near-surface layer. Cr complexes formed by dπ-pπ interactions and the formation of carbide-like species are identified. Generally, a redox reaction at metal-polymer interfaces is accompanied by a modification of the interface free energies. The redox potential of the metal component (K>Al>Cr; Cu=electropositive) is the key parameter here. The oxygen plasma pretreatment was applied to a C18 Self-Assembled Monolayer followed by Cr deposition. A redox reaction between Cr and plasma introduced C=O groups was also observed. In order to analyse treated polymer surfaces and interfaces the well-established X-ray Photoelectron Spectroscopy (XPS) and also X-ray Absorption Spectroscopy (XAS) were employed. Furthermore, molecular weight distributions of thin polymer layers were measured by the chromatographic methods: Size Exclusion Chromatography (SEC) and Thermal Field Flow Fractionation (ThFFF) and also by mass spectrometry (Matrix-Assisted Laser Desorption Ionization Mass Spectrometry, MALDI-MS).