J. Friedrich

Chemical analysis of plasma-polymerized films: The application of X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (NEXAFS) and fourier transform infrared spectroscopy (FTIR)

Abstract Selected FTIR, XPS and NEXAFS spectroscopy results obtained with films deposited with different plasma polymerization processes and different monomers (styrene, acetylene, ethylene and butadiene) are presented. In detail FTIR spectra, XPS surveys, XPS valence bands and core level signals including shake-up features as well as carbon K-edge absorption spectra are qualitatively and, in some cases, semi-quantitatively considered. Information on the film formation, the chemistry of the films and the film stability against air exposure are derived from spectroscopic features. With styrene chemically rather well defined plasma polymer layers can be formed with rather high deposition rates applying a pulse plasma process. Using ethylene, acetylene or butadiene as a monomer in the plasma deposition process the deposition rates are smaller. Using these monomers plasma polymer films can be obtained with a primary chemical structure which is similar to each other but more or less different from those of the respective conventional polymers. The main difference between these samples is their individual concentration of unsaturated carbon species. Finally, a technologically relevant example, i.e. a plasma deposited polymer barrier layer deposited on the inner wall of a poly(ethylene) vessel is briefly considered.

Selective surface functionalization of polyolefins by plasma treatment followed by chemical reduction

Abstract Polymer surfaces can be finished with functional groups upon exposure to a plasma. Species of the plasma gas are attached at surface carbon atoms, forming functional groups of different composition. To produce a modified polymer surface with a high density and homogeneity of hydroxyl groups only, the oxygen-plasma-formed oxygen functional groups were chemically reduced by diborane and LiAlH4 with yields of 10 to 11 OH groups per 100 carbon atoms in the 3 to 5 nm near-surface layer as detected by X-ray photoelectron spectroscopy (XPS). The identification of hydroxyl groups was performed by means of attenuated total reflectance–Fourier transform infrared spectroscopy and XPS.

Homofunctionalized polymer surfaces formed by selective plasma processes

Abstract Several possibilities exist to produce a modified polymer surface with a high density of only one sort of functional group such as: (i) the plasma grafting of unfragmented monomer molecules and their polymerization forms OH, NH2, COOH groups, etc. in concentrations of approximately 25 groups per 100 C atoms; (ii) selective plasma bromination provides 10–25 CBr groups; (iii) the plasma oxidation of polymer surfaces in an O2 plasma followed by the chemical reduction of all O-containing groups to OH groups by diborane, vitride™ (Na complex) or LiAlH4 yields 9–14 OH groups per 100 carbon atoms; and (iv) the grafting of spacers with different endgroups onto OH or CBr groups produces 7–10 spacer molecules/100 C. This work was focused on the formation of thin plasma deposited polymer layers with a maximum of (homo)functional groups and with a minimum of chemical irregularities using the pulsed plasma technique. The monomers were allylalcohol, allylamine, acrylonitrile and acrylic acid. The further intent was to study the interactions of functional groups (OH, COOH, NH2) and deposited metals (Cr, Al, Ti). It was expected that more basic (NH2), weakly basic or neutral (OH) or more acidic (COOH) groups would show different interactions and chemical reactions with metal atoms.

Chemical reactions at polymer surfaces interacting with a gas plasma or with metal atoms — their relevance to adhesion

Abstract The chemical and morphological stabilities of polymer segments in the near-surface layer were investigated by spectroscopic methods such as X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure spectroscopy. Model studies were undertaken with Langmuir–Blodgett films, self-assembled monolayers and oligomer films. For thin polymer layers (30 to 500 nm), the changes in molecular-weight distributions of some polymers were investigated systematically by size exclusion chromatography, matrix-assisted laser desorption/ionization–time-of-flight mass spectrometry and thermal-field flow fractionation for oxygen- and helium-plasma exposures. The polymer surfaces were found to be relatively stable at exposure to an oxygen low-pressure plasma up to ca. 2 s. This is important information to get maximum adhesion to metals in composites. In correlation to their redox potentials, potassium, aluminium and chromium react with oxygen functional groups at the polymer/metal interface. In a dedicated study, chromium was found to attack aromatic rings and form different reaction products.

Corona, spark and combined UV and ozone modification of polymer films WeBP23

Abstract Different types of plasma, irradiative and chemical activation were compared in terms of surface functionalization. Corona and spark jet plasmas are characterized by low gas temperatures and high rates in surface modification. UV irradiation in the presence of ozone does not involve any particle bombardment and acts only by enhanced photooxidative processes. Although ion implantation can be avoided, this method is not free of radiative damage in both the surface-near region and the bulk of polymers. Furthermore, its functionalization rate is low. In relation to low-pressure O 2 plasma modification, all treatments mentioned here have a low efficiency in adhesion promotion due to oxidative degradation of macromolecules and formation of molecular debris known as the “weak boundary layer”.

Plasmaoxidative and Chemical Degradation of Poly(ethylene terephthalate) Studied by Matrix-assisted Laser Desorption/Ionization Mass Spectrometry

A model system of linear ethylene glycol terminated poly(ethylene terephthalate) oligomers of the general formula H–[GT]n–G (where G is an ethylene glycol unit and T represents a terephthalic acid unit) was synthesized and exposed to an oxygen plasma. The degradation of the oligomers was investigated by matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS). The results were compared to a chemical degradation of commercial poly(ethylene terephthalate) which had been exposed to an aqueous solution of natural chalk for many years. In both cases an ester scission process was found which generates terephthalic acid terminated oligomers (H–[GT]m–OH) and decomposes linear ethylene glycol terminated oligomers (H–[GT]m–G). The scision of the ester bonds during the chemical treatment additionally leads to the formation of T–[GT]m–OH like oligomers and to a strong decrease of the number of cyclic oligomers ([GT]m). Furthermore during the plasma treatment an additional formation of [GT]m–G like cyclic oligomers was observed.

Fluorescence spectroscopic studies on plasma-chemically modified polymer surfaces with fluorophore-labeled functionalities.

Molecular engineering of polymer surfaces that includes the attachment of functional molecules to existing or previously generated reactive chemical groups like e.g., − OH, − NH2, or − CHO requires simple strategies and tools for the controlled generation of surface functionalities and their derivatization as well as for their identification and eventually quantification. Here, we systematically investigate the plasma-chemical surface modification of polypropylene films in combination with dansyl labeling chemistry and conventional, yet costly, XPS and highly sensitive fluorescence spectroscopy for the detection of surface groups. Based on these results, the potential of and requirements on the fluorometric characterization and quantification of surfaces functionalities are discussed.

Improvement of bonding properties of polypropylene by low-pressure plasma treatment

Abstract Bonding of rapidly expanded polypropylene (PP) remains poor unless a special pretreatment of the PP substrate is performed. In the past mainly chemical primers were used. A replacement of this type of pretreatment by plasma pretreatment offers advantages from an environmental viewpoint. From the present investigations, where the plasma parameters were systematically varied, it can be concluded that a very short exposure to the plasma and new curing conditions led to the best results. The lap shear (bond) strength after plasma pretreatment became even higher than after a primer pretreatment. ESCA analyses and ultrasonic C-scan analyses of the fracture areas were performed. It was found that a long exposure to the plasma weakened the boundary layer. The C-scan analyses showed that the failure of the plasma pretreated bonds appears different from that at the primer pretreated specimens. Another important finding — especially from a practical viewpoint — was that the beneficial effect of the plasma pretreatment before bonding lasted in a laboratory environment much longer than after a primer pretreatment.

Reactions and Intermediates at the Metal-Polymer Interface as Observed by XPS and NEXAFS Spectroscopy

Abstract Potassium or chromium were evaporated by means of a Knudsen effusion cell under ultra-high vacuum conditions onto a number of common polymers, prepared as stretched foils and spin-coated films. The metal-polymer interface was studied by X-ray Absorption and X-ray Photoelectron Spectroscopy. Evaporated samples were analyzed without exposure to the atmosphere. Different general types of reactions of the metal atoms with the polymers were observed. With deposited K and Cr a redox process including the transfer of substrate oxygen atoms across the interface was found. The formation of π-electron complexes and covalent metal-carbon bonds were obtained exclusively with chromium. Aromatic rings, carbonyl groups and, to a lesser extent, ether linkages are scissioned by metal-polymer interactions. The fourfold substitution of aromatic rings and the exclusive existence of C ─ O ─ C structures within polyphenylene ether (PPE) make this polymer most stable toward reactions with chromium. In contrast, bisphen...

Plasma functionalization and reorientation of macromolecules at polymer surfaces

The functionalization of polypropylene surfaces and octadecyltrichlorosilane layers with oxygen-containing groups, and changes in molecular and supermolecular orientations resulting from low pressure oxygen glow discharge plasma treatment were investigated. The main results are the fast reorientation under plasma exposure, and the discrepancy between the slow increase in the functional group concentration and the fast improvement in the maximum adhesion to polyurethane. The reoriented polymer surface should also be a precondition for improved adhesion properties. Labelling reactions allow us to detect low concentrations of OH groups at polymer surfaces by Fourier transform IR spectroscopy combined with ATR spectroscopy. The concentration of OH groups was twofold increased with diborane reduction to convert C=C, C=O, COOH and COOR groups into OH groups.