J.Lopes da Silva

An XPS study of the fibre-matrix interface using sized carbon fibres as a model

This work is part of a larger project whose main objective is a better understanding of the mechanism of adhesion between the surface of carbon fibres and resins in composites. The effect over the surface of commercial PAN-based carbon fibres (Courtaulds IM CG43-750) induced by several degrees of a commercial wet oxidative surface treatment (STL) as well as the nature of the fibre-size interface have been studied by X-ray photoelectron spectroscopy. As far as functional groups are concerned, a good qualitative and quantitative characterization of the fibre surface has been achieved, using a criterium based on the requirement of internal consistency between results obtained with photoelectrons from C 1s, O 1s and N 1s peaks. Oxidative treatment induces an enrichment in oxygen and nitrogen at the surface; the presence of the former on the surface is due to the treatment but strong evidence for a residual origin of nitrogen is obtained. Coating of the commercial fibres by a prepolymer similar to the resin used as the matrix, has been used to test the type of bonding between fibre and resin before curing. Evidence is given supporting the idea that the coating (size) does not entirely cover the surface of the fibre and that only a part of it is covalently bound, alcohol groups seeming to play an important role in the adhesion mechanism.

Study of polymer film surfaces by EELS using selectively deuterated polystyrene

HREELS spectra of selectively deuterated polystyrene films show loss energies corresponding to the excitation of vibrational modes of the surface region of the organic medium. The thickness of the analyzed surface region can be varied by changing the incident electron energy and/or the scattering geometry. Selective labelling allows to better separate the contributions of main chain and of pendant-groups, and to eliminate the influence of contaminants. It is shown for instance that isotactic and atactic polymer surfaces differ, the former being mainly composed of phenyl rings. End group segregation at the surface may also be important. Multiple losses are much more intense than the corresponding combination modes in IR: the theory of interaction of low energy electrons with polymer films should be further investigated.

The chemical adsorption of 1-hexene on Ru(0001) studied by reflection-absorption infrared spectroscopy

The chemisorption of 1-hexene on a Ru(0001) surface has been studied using reflection-absorption infrared spectroscopy (RAIRS). At 92 K, a very low dose of 1-hexene produced a RAIR spectrum consistent with the formation of a di-σ type complex. On annealing to 100 K, this species started dehydrogenating on the surface, forming a mixture of rotational conformers of hexylidyne, [C(CH2)4CH3]. A similar result was obtained when adsorbing at 100 K, with predominance of the all-trans conformer of hexylidyne for exposure at a high rate. Hexylidyne proved to be stable up to at least 120 K, although the conformational equilibrium changed, gradually favouring species with gauche rotations. Between 130 and 290 K, the decomposition products did not produce detectable RAIR spectra. Further thermal decomposition into methylidyne was observed at 300 K, in contrast with shorter chain alkylidynes previously studied on Ru(0001), which decompose via ethylidyne.

Electronic excitation and secondary electron emission studies by low-energy electrons backscattered from thin polystyrene film surfaces

Thin atactic polystyrene films were studied by backscattering of low-energy electrons. Energy losses corresponding to the excitation in the film of triplet and singlet electronic excitation of the pendant groups are observed. The variation of the corresponding cross sections with incident electron energy are in good agreement with theoretical calculations for benzene. The variation of the backscattered intensity integrated over electron energy in a given direction, yields a ionization threshold of about 7 eV and shows evidence for structure of the valence band.

High-resolution electron energy loss spectroscopy studies on the electronic structure of quinquethiophene (5T) and sexithiophene (6T) films

High-resolution electron energy loss spectroscopy (HREELS) and optical absorption spectra were recorded for 5- and 6-thiophene films. These compounds are characterized by a large excitonic band of u symmetry. Optical transitions are only allowed to the high-energy edge (top) of this band. Due to different selection rules, HREELS spectra reveal transitions to the low-energy edge (bottom) of the exciton band, and allow a precise determination of the excitonic gap. Furthermore, a second threshold above 3 eV is tentatively assigned to the electronic gap.

Characterization of oligothiophene films by high resolution electron energy loss spectroscopy

Electronic structures of quinquethiophene (5T) and sexithiophene (6T) polycrystalline films deposited by evaporation on gold substrates were studied by high resolution electron energy loss spectroscopy (HREELS). Vibrational spectra were compared to infrared spectroscopy and Raman scattering results. They reveal that surfaces are not rough and are free from contaminants. Energy gap values of 2.38 ∠ 0.02 and 2.27 ∠ 0.02 eV for 5T and 6T films, respectively, were evaluated from electronic spectra. Comparison with optical spectroscopy enabled the assignment of losses corresponding to optically allowed and forbidden transitions. Some of the latter ones located at 1.2, 1.7 and 2.0 eV were observed in 5T films and were assigned to S 0 φι Tn transitions. Differential cross section analysis reveals the presence of resonance processes in the excitation mechanism. Ionization potentials were estimated to be 4.7 ∠ 0.5 eV for both films.© 1998 Elsevier Science S.A. All rights reserved

Surface characterization of poly(acrylonitrile) based intermediate modulus carbon fibres

The interface between the fibre and the matrix is a very important factor influencing the mechanical behaviour of composite materials. For superior composite performance, one must not only select optimal fibres and matrices, but also optimize the interface between them. However, the control of the interface properties is not an easy task. This work is an interdisciplinary and integrated approach to the problem. The effect of different degrees of a wet oxidative surface treatment on the surface of poly(acrylonitrile) based intermediate modulus carbon fibres (Courtaulds IM CG43–750) has been studied using classical thermodynamic as well as spectroscopic techniques, aimed at obtaining a complete physical and chemical characterization of the fibre surface. The results show that all aspects of the fibre surface are influenced by the surface treatments, which are specially designed to improve the adhesion between fibre and matrix. The study outlines the most important surface features improving this adhesion. The results concerning the characterization of the fibre surface contribute, when combined with micromechanical tests, to clarifying the adhesion mechanisms, revealing, at the same time, a mechanical interlocking and a chemical interaction.

Interaction of slow electrons with Langmuir-Blodgett films studied by high resolution electron energy loss spectroscopy

Abstract Vibrational and complete high resolution spectra (HREELS) of Langmuir-Blodgett (LB) monolayers of mixtures of pure hydrogenated and deuterated stearic acids on aluminum substrates were recorded. The results show that electrons penetrate into the film and relax in the conduction band of the insulating film. Complete spectra are mainly composed of different regions corresponding to vibrational losses, and to exponential decay of the intensity of backscattered electrons where superimposed structures, fixed to the vacuum level, are observed for primary energies higher than 4 eV. An emission of thermalized electrons is also observed near the vacuum level. Conclusions about the different aspects of this interaction are inferred combining information obtained from the different regions in the spectra.

Diblock copolymer ultrathin films studied by high resolution electron energy loss spectroscopy

Abstract Surface segregation of polymers forming thin films is relevant in several industrial applications. This paper studies the influence of preparation parameters on the surface composition of poly(ethylene oxide)–polystyrene diblock copolymer ultrathin films. Silicon wafers were used as substrates. The film preparation method (spin coating vs. casting followed by solvent evaporation), solvent (THF vs. CCl 4 ), annealing vs. non-annealing and the substrate nature (native silicon oxide vs. silanized substrate) were analyzed. High resolution electron energy spectroscopy in vibrational energy loss range was used to estimate the extreme surface composition. Generally, THF favors the surface segregation of polystyrene. After annealing, whatever the preparation method, the polystyrene preferentially segregates at the surface without forming a pure overlayer. These results were compared with the surface compositions calculated from X-ray photoelectron spectroscopy spectra of the films.

Delayed specific luminescence of aromatic scintillators

Abstract A modified expression for the delayed specific luminescence of organic scintillators as a function of the incident-particle linear energy transfer is presented. Theoretical curves thus produced show a good fit to the experimental results obtained with doped polystyrene films irradiated with α particles of energy ranging from 1 to 5 MeV. The adjusted value for the bimolecular interaction parameter B T for the different concentrations of solute in the films gives consistency to the assumptions made.