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Several approaches were investigated to produce monosort functionalized polymer surfaces with a high density and homogeneity of functional groups: (i) Plasma oxidation followed by wet-chemical reduction, (ii) formation of radicals and grafting on of functional group carrying molecules, (iii) plasma bromination followed by (iv) Williamson or Gabriel-like synthesis of spacer molecules, and (v) a pulsed plasma polymerization of functional groups bearing monomers or (vi) their copolymerisation with other comonomers. The formation of hydroxyl (OH), primary amino (NH2), and carboxyl (COOH) groups was studied in detail. The oxygen plasma treatment (i) in a low-pressure non-isothermal glow discharge results in the formation of a wide variety of O functional groups, polymer degradation and crosslinking. Low power densities and short exposure times (0.1 to 2 s) are required to functionalize a surface while preserving the original polymer structure. Carbonate, ester, and aromatic groups are rapidly degraded by an oxygen plasma treatment leading to scissions of polymer backbones and loss in molecular weight. Also the formation of macrocycles and C=C bonds was observed in a region of around 4 nm in depth. The investigated polymers could be classified by their degradation behaviour on exposure to the oxygen plasma.___TAGSTART___BR___TAGEND___ In order to maximize the process selectivity for OH groups, the variety of oxygen functionalities formed by the oxygen plasma was wet-chemically reduced by diborane, vitridetrade (Na complex), and LiAlH4. Typical yields were 9 to 14 OH groups per 100 carbon atoms.___TAGSTART___BR___TAGEND___ Plasma bromination (iii) (40 Br per 100 C atoms) of polymers, followed by grafting of spacer molecules (iv), has been proved to be a highly selective reaction.___TAGSTART___BR___TAGEND___ Another way to produce high densities of monosort functionalities was the pulsed plasma polymerization of functional group bearing monomers such as allylamine, allylalcohol or acrylic acid (v). The retention of chemical structure and functional groups during plasma polymerization was achieved by using low power densities and the pulsed plasma technique. The maximum yields were 30 OH, 18 NH2, and 24 COOH groups per 100 C atoms. To vary the density of functional groups a chemical copolymerization with ''chain-extending'' comonomers such as butadiene and ethylene was initiated in the pulsed plasma (vi). Additionally, the often-observed post-plasma oxidations of such layers initiated by reaction of trapped radicals with oxygen from the air were successfully suppressed by using NO gas as radical quencher.
Overlayers of SiO2 (nominally 4, 6 and 8 nm thick) on silicon, prepared by thermal oxidation, were investigated using x-ray photoelectron spectroscopy (XPS). The thickness of these overlayers was obtained from a measurement of the photoelectron intensities originating from the substrate and the oxide overlayer by applying an appropriate quantitative model. The uncertainty budget of that thickness measurement method is given. The relative combined standard uncertainty of the method was found to be 15%. The effective attenuation lengths or the corresponding electron inelastic mean free paths are of considerable importance for both the estimated values of overlayer thickness and combined standard uncertainties. The XPS results were compared with ellipsometry data.
The dismutation of CCl2F2 was used to probe the effect of halogenation of chromia by Cl/F exchange reactions to find out the difference between the halogenated inactive and active catalysts. The heterogeneous reactions were performed in a continuous flow Ni reactor and also under simulated reaction conditions in a reactor where after the reaction X-ray photoelectron spectroscopy (XPS) and X-ray excited Auger electron spectroscopy (XAES) analyses are possible without air exposure of the catalyst, i.e., under so-called "in situ" conditions. The Cr(III) 2p XP spectra, which revealed multiplet splitting features and satellite emission, were used for chemical analysis by using a simple evaluation procedure which neglects this inherent complexity. Chemical analysis was also applied by using chemical state plots for Cr 3s in order to cross-check Cr 2p related results. Both ex and in situ XPS show that as soon as Cr2O3 is exposed to CCl2F2 at 390°C fluorination as well as chlorination takes place at the catalyst surface. When the XPS surface composition reaches approximately 4 at. % fluorination and 6 at. % chlorination, maximum catalytic activity was obtained. Application of longer reaction times did not change significantly the obtained surface composition of the activated chromia. The fluorination and chlorination of chromia was further investigated by various HF and HCl treatments. The activated chromia samples and the Cr2O3, Cr(OH)3, CrF2OH, CrF3·H2O, α-CrF3, β-CrF3, and CrCl3 reference samples with well-known chemical structures were also characterized by X-ray absorption near edge structure (XANES), time-of-flight secondary ion mass spectroscopy (TOF-SIMS), pyridine-FTIR, wet chemical (F and Cl) analysis, X-ray powder diffraction (XRD), and surface area (BET) analysis. The results suggest that the formation of chromium oxide chloride fluoride species, e.g., chromium oxide halides, at the surface is sufficient to provide catalytic activity. The presence of any CrF3 and/or CrCl3 phases on the activated chromia samples was not found.
Plasma polymerised films of acrylic acid, allyl alcohol and allyl amine were prepared and studied by XPS (X-ray photoelectron spectroscopy) and XAS (X-ray absorption spectroscopy). The influence of external plasma parameters on certain chemical aspects of the films was investigated. The use of XPS and NEXAFS (near edge X-ray absorption fine structure) as complimentary tools in the surface characterisation of thin films has been highlighted. Certain chemical aspects of the plasma polymerised films such as presence of ROCdouble bond; length as m-dashO in the case of plasma polymerised acrylic acid films, presence of RCdouble bond; length as m-dashO and RCOR bonds in case of plasma polymerised allyl alcohol films and presence of RCdouble bond; length as m-dashN and C?N in case of plasma polymerised allyl amine films were studied. The concentration of ROCdouble bond; length as m-dashO species was found to decrease with increase in r.f. duty cycle in the case of plasma polymerised acrylic acid films. In case of plasma polymerised allyl alcohol films there is a decrease in the concentration of RCdouble bond; length as m-dashO species and increase in the concentration of RCOR species at higher pressures, while in the case of plasma polymerised allyl amine films the concentration of RCdouble bond; length as m-dashN and C?N increases at higher powers, with an increase in C?N more pronounced at higher powers.
The formation of plasma-deposited materials made from organic molecules is a technologically attractive way to obtain films with unique properties, e.g. adhesion, wettability, conductivity or selected chemical functionalization. A detailed chemical characterization of these films is a great challenge for the analyst because of the co-existence of a large number of chemical species in plasma-deposited films. We investigated these types of organic films using photoelectron spectroscopy for chemical analysis (ESCA, XPS), near edge X-ray absorption fine structure (NEXAFS) spectroscopy at synchrotron radiation source BESSY II and time-of-flight secondary ion mass spectroscopy (ToF-SIMS). A dedicated plasma preparation chamber was designed and added to the main analysis chamber of the respective spectrometers. This approach offers the possibility of studying plasma-deposited films without exposure to air before analysis (often called in-situ analysis) and the influence of post-plasma reactions, i.e. the ageing process. The present investigations are aimed at finding correlations between basic chemical properties of the deposited films and the plasma deposition parameters, e.g. duty cycle in pulsed plasmas and the power of discharge.
Plasma deposited allylamine films were studied by in situ ToF-SSIMS, XPS and NEXAFS Spectroscopy before exposure to ambient air. The influence of external plasma parameters such as duty cycle, plasma power, and monomer flow rate on (i) unsaturation, (ii) branching and cross-linking, (iii) nitrogen surface concentration and (iv) retention of amino groups was investigated. Harder plasma conditions, which can be obtained when high duty cycles, high plasma power, and low monomer flow rates are employed, increase the unsaturated, branched and cross-linked character of the plasma deposited films, while the surface concentration of N as well as the retention of the monomer's amino group decrease. As proven by NEXAFS findings and cross-checked by ToF-SSIMS results, the allylamine monomer's primary amino groups are partially transformed into other nitrogen functionalities during its plasma polymerization. Amongst them imines and nitriles are the conversion products with the highest probabilities of formation. Another conversion channel is the formation of nitrogen species not participating in the film growth. This is the reason for an increased N loss in plasma deposited allylamine films as observed independently by XPS and ToF-SSIMS when the plasma parameters are changed from mild to hard.
Chemical properties of plasma-polymerized styrene films can be effectively controlled by the appropriate choice of external plasma parameters, such as duty cycle, power and monomer pressure. Chemical properties here are the regularity of the primary structure, the amount of long living radicals and branching or cross-linking or both in plasma polymerized styrene films, which can be indirectly monitored by X-ray photoelectron spectroscopy (XPS) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. Principally, XPS and NEXAFS spectra of plasma-polymerized styrene show all the spectroscopic fingerprints characteristic of atactic polystyrene indicating an inherent similarity of the respective primary chemical structures. There is an increase in irregularity and radical concentration for the plasma deposited films with increasing duty cycle or power. With an increase in monomer pressure, a decreased level of monomer fragmentation and, consequently, an increase in regularity must be concluded from the experimental data. The applied power regulates the fragmentation of the monomers as monitored by spectroscopic signals correlated to the aromatic rings. A trend in the extent of the post-plasma oxygen incorporation according to the variation of the various external plasma parameters was deduced, which agrees with earlier ToF-SIMS findings
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.