Andreas Lippitz

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.

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.

Oxygen plasma induced degradation of the surface of poly(styrene), poly(bisphenol-A-carbonate) and poly(ethylene terephthalate) as observed by soft X-ray absorption spectroscopy (NEXAFS)

Biaxially stretched poly(ethylene terephthalate) foils, poly(ethylene terephthalate) and poly(styrene) spin coated films as well as self-supporting poly(bisphenol-A-carbonate) films were analysed by surface sensitive near edge X-ray absorption fine structure spectroscopy (NEXAFS) before and after low pressure oxygen DC plasma treatments. The degradation effects of the treatment are discussed in a common way in terms of probable reaction pathways. The results are, when possible, cross-checked by XPS. NEXAFS analysis provides data on the phenyl ring, ester and carbonate group degradation in a straightforward manner. It can be shown that the phenyl rings in partially crystalline poly(ethylene terephthalate) are, in comparison, most effectively protected against plasma attack. In the case of poly(styrene), where the rings are located as side chains, they are most efficiently degraded.

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”.

Surface analysis of partially crystalline and amorphous poly(ethylene terephthalate) samples by X-ray absorption spectroscopy (NEXAFS)

Biaxially stretched poly(ethylene terephthalate) (PET) foils and spin-coated PET films were analysed by surface-sensitive X-ray absorption spectroscopy (NEXAFS). The surfaces of partially crystalline foils and amorphous films can be easily distinguished when the angle of incidence of the linearly polarized synchrotron light beam is varied relative to the surface normal of the respective sample. In contrast to the amorphous state, we found a strong angle dependence in the NEXAFS spectra of the surface of crystalline samples.

The improvement in adhesion of polyurethane-polypropylene composites by short-time exposure of polypropylene to low and atmospheric pressure plasmas

The surfaces of polymers, namely polypropylene, copolymers and blends, were exposed to low pressure oxygen and atmospheric pressure air plasmas to improve their adhesion to polyurethane adhesives. A correlation is attempted between lap shear strengths of polypropylene-polyurethane composites and the relevant XPS, AFM and NEXAFS data. It was found that plasma functionalization improved the adhesion to maximum values even when the time of exposure was low: 1 to 10 seconds for low pressure plasmas, and 0.1 to 1 seconds in case of atmospheric plasma jet treatments. Thus, high lap shear strengths were obtained at relatively small oxygen contents. The improvement in shear strength at short time plasma exposures seems to be correlated to the complete smoothening of the supermolecular structure of stretched polypropylene foils as shown by AFM. Valence band XPS and derivatization techniques revealed more details of the oxygen functionalization on polypropylene. NEXAFS experiments confirmed a re-orientation of bonds and segments of the macromolecules by plasma exposure which are assumed to be responsible for adhesion improvement

Adlayers of dimannoside thiols on gold: surface chemical analysis.

Carbohydrate films on gold based on dimannoside thiols (DMT) were prepared, and a complementary surface chemical analysis was performed in detail by X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (ToF-SIMS), near-edge X-ray absorption fine structure (NEXAFS), FT-IR, and contact angle measurements in order to verify formation of ω-carbohydrate-functionalized alkylthiol films. XPS (C 1s, O 1s, and S 2p) reveals information on carbohydrate specific alkoxy (C-O) and acetal moieties (O-C-O) as well as thiolate species attached to gold. Angle-resolved synchrotron XPS was used for chemical speciation at ultimate surface sensitivity. Angle-resolved XPS analysis suggests the presence of an excess top layer composed of unbound sulfur components combined with alkyl moieties. Further support for DMT attachment on Au is given by ToF-SIMS and FT-IR analysis. Carbon and oxygen K-edge NEXAFS spectra were interpreted by applying the building block model supported by comparison to data of 1-undecanethiol, poly(vinyl alcohol), and polyoxymethylene. No linear dichroism effect was observed in the angle-resolved C K-edge NEXAFS.

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.