E. Papirer

Fluorination of carbon blacks: An X-ray photoelectron spectroscopy study: I. A literature review of XPS studies of fluorinated carbons. XPS investigation of some reference compounds

Abstract A critical review of the literature relative to the chemical characterisation of fluorinated carbon compounds by X-ray photoelectron spectroscopy is proposed. Guidelines for identification and assignation of the different components which are individualized by fitting of the C 1s and F 1s peaks are given. It is suggested that in fluorocarbons, the chemical shifts of the C and F core levels depend not only on the nature of the CF bond (ionic, semi-ionic or covalent), the number of F atoms linked to the C atom and the chemical environment of the CF group (β effect), but also on the structure and conformation of the C atom skeleton in the fluorinated material. The types of fluorinated structural unit that are formed vary as a function of the conditions of fluorination (temperature, eventual presence of traces of HF or fluoride acting as catalysts) and of the structure and graphitic character on the starting material. Since the experimental values of the binding energy of the different types of CF bonds proposed by the authors differ to a slight extent, some fluorinated polymers and fluorographites of known composition have been analysed by XPS. The binding energies of the components of the C 1s and F 1s peaks, the energy separation ΔCiCr of the former relative to the component (Cr) corresponding to hydrocarbon contamination and the energy separation ΔCiFi between the corresponding components of the C 1s and F 1s peak have been determined and correlated to the nature of the chemical and structural environment of the involved atom.

Comparison of the surface properties of graphite, carbon black and fullerene samples, measured by inverse gas chromatography

Three types of carbon were examined by inverse gas chromatography (IGC), at infinite and finite concentration conditions: graphite, carbon black and fullerene (C60) samples. Those samples differ by their structure and also by their surface properties (existence of defects, of polar surface groups...): surface free energy characteristics and surface energetic heterogeneity. IGC allows to determine in part these properties and evidences strong differences amongst the 3 carbons.

Inverse gas chromatography: A valuable method for the surface characterization of fillers for polymers (glass fibres and silicas)

Abstract Inverse gas chromatography (IGC) is most useful for the study of the surface characteristics of powders, such as silicas and glass fibres, which can hardly be examined otherwise. Examples are provided to illustrate the potential of IGC measurements either at finite concentration or at infinite dilution chromatographic conditions. It is shown that the interpretation of the results is facilitated when studying solid surfaces modified in a controlled way.

Characterization of silicas by inverse gas chromatography at finite concentration: Determination of the adsorption energy distribution function

Abstract Amorphous and crystalline silicas, eventually modified by heat treatment or by surface grafting of alkyl chains, were investigated by inverse gas chromatography, a method which easily provides adsorption isotherms of various linear alkanes. The isotherms were analyzed in order to obtain the adsorption energy distribution of alkane probes. The distributions were calculated using a local solution of the adsorption integral equation which is evaluated and discussed. Silicas show typical energy distribution patterns. Furthermore, the grafting of hexadecanol chains on the silica surface brings the greatest changes in the energy distributions.

London component of the surface energy of heat-treated silicas

Abstract The surface properties of silicas of various origins were measured by inverse gas chromatography. Thus, after thermal treatment of silicas, the variation in the dispersive components (γSD) of their surface energies was determined. Different but typical behaviors either for amorphous or for crystalline silicas were observed. In comparison with other studies, a correlation is suggested between the variation in γSD and the modification of chemical and physical properties of silica surfaces after heat treatment.

Inverse gas chromatography investigation of the surface properties of cellulose

Cellulose samples differing in origin and crystallinity were examined by inverse gas chromatography (IGC). Surface energy characteristics such as the apparent dispersive component of the surface energy and acid-base properties, and also nanomorphology indices of the different cellulose samples, were assessed. The influence of humidity on these properties was demonstrated. A decrease of 10-20% of the interaction potential was detected. Crystalline cellulose is the most sensitive to the presence of adsorbed water, which possibly disorders its surface structure. IGC appears to be an appropriate method for the evaluation of the surface characteristics of cellulose. However, care has to be taken when trying to understand differences in surface properties between various cellulose samples, since IGC is very sensitive to minor variations in surface characteristics.

Fluorination of carbon blacks: an X-ray photoelectron spectroscopy study: IV. Reactivity of different carbon blacks in CF4 radiofrequency plasma

Abstract The effect of CF 4 -plasma enhanced fluorination on the surface modification of carbon blacks has been examined using XPS. Three different types of carbon blacks have been studied: a thermal black, a furnace black and a high electrical conducting black. The analysis of the XPS spectra of fluorinated carbon black samples indicates that all fluorine atoms, fixed at the surface and in a subsuperficial zone of the particles, are covalently linked to carbon atoms. The influence of the physicochemical properties and morphology of these three types of carbon blacks on the fluorination reaction has also been investigated. The proportion of different types of fluorinated carbon atoms, i.e. on one hand CF x surface and border groups of graphitic bulk domains for which the planar configuration of the graphene layers is preserved together with the sp 2 character of C, i.e. structures of type I, on the other hand polyalicyclic perfluorinated structures in which sp 3 C form puckered layers similar to those of covalent fluorographites, i.e. structures of type II, and also the F/C ratio of the fluorinated groups are related to the surface morphology and depend on the microstructural organization of particles. When the microstructure ordering and graphitic character of the carbon increase, the size of the ordered graphitic domain also increases. At the same time the density, the size of defects and proportion of protonated sp 3 C entities bridging the graphene layers decrease. As a consequence, the proportion of carbon atoms, potentially able to form perfluorinated CF 2 and CF 3 groups, decreases. The relative contribution of those groups is appreciably higher in fluorinated compounds which are derived from carbon blacks with a lower structural order.

Fluorination of carbon blacks: An X-ray photoelectron spectroscopy study: III. Fluorination of different carbon blacks with gaseous fluorine at temperatures below 100 °C influence of the morphology, structure and physico-chemical characteristics of the carbon black on the fluorine fixation

Abstract Furnace, thermal and a high electrical conductive blacks were submitted to fluorination at room temperature using a gaseous mix of fluorine and nitrogen. The weight uptake of the samples and their elemental composition were determined and compared to the composition of the outer surface and subsuperficial zone of the particles as determined by XPS. Analysis of the XPS spectra indicates that F is merrily fixed at the surface and in a subsuperficial layer of limited thickness and covalently bonded to C. In the furnace blacks, a minor amount of F2 is trapped in micropores. When the blacks are activated (air oxidation at 450 °C), the amount of fluorine fixed by the furnace and thermal blacks increases due to a slight increase of the density of reactive sites consecutively to the development of porosity. The high conductive black, made of empty carbon shells shows a peculiar behavior as the activation does not lead to an augmentation of the F fixation.

Chemical modifications and surface properties of carbon blacks

Five carbon blacks, differing in origin or specific surface area, were submitted to a series of chemical treatments: moderate oxidation with potassium persulfate, severe air oxidation, halogenation with chlorine or bromine, and grafting of short (ethyl, butyl, pentyl) or long (lauroic) alkyl chains in order to modify their surface properties and to compare their reactivity in relation to their atomic structure. Various techniques such as elemental analysis, active surface-area determination and inverse gas chromatography were used to assess the consequences of the modifications. In particular, inverse gas chromatography at infinite dilution is shown to detect essentially the most active sites, and allows monitoring of the variation of surface properties upon chemical modification of the blacks.

Organization of the layers of polyethylene oxide grafted with different densities on silica

Abstract EPR spectroscopy of labeled grafted polyethylene oxide chains has been used to estimate the ratio of the population of free end segments in solution to that of adsorbed segments on a silica surface as a function of temperature. The configurations of the chains have been studied in the case of certain extreme coverages. Grafted molecules of mol wt 2000 with grafting ratios of 0.045, 0.057, 0.126, and 0.42 molecules/nm 2 were in contact with benzene. DSC thermograms are also shown. At low grafting ratios the chains lie very flat, whereas at higher grafting ratios the layer is more organized.