470 EXTENDED ABSTRACTS

of distinguishing relationships, which help to identify the surface components.

In the case of elements other than carbon and oxygen being present in the surface components, these impart further dimensions to the carbon-oxygen relationships, which increases the information. This is illustrated by means of a silicon-containing component in the investigated system.

An analysis of this kind requires spectra with good resolution and good statistical quality and is of particular interest within fields of applications, where spectra are record-

ed for a large number of objects with variability of the relative amount of surface components. In the early stages of the development of a product where the problem is to remove surface contaminants, samples with higher and variable levels of contamination may occur. The informational value of this can be used for the interpretation and analysis of ESCA spectra of the final product with much lower levels of con- tamination. We have also used ESCA in a similar way for the characterization and control of surface modified intraocular lenses.

Surface Effects on a Polysulphone Induced by X-Ray-, Electron-, or Ion-beams

Andras Toth, Imre Bertoti, Tamas Szekely Research Laboratory for Inorganic Chemistry, Hungarian Academy of Sciences, H-1112 Budapest, Budaorsi ut 45, Hungary

Giovanni Marletta, Salvatore Pignataro Dipartimento di Scienze Chimiche, Universita di Catania, 1-95125 Catania, Wale A. Doria 6, Italy

Palazs Keszler Research Institute for the Plastics Industry, H-1950 Budapest, Hungaria krt. 114, Hungary

INTRODUCTION

The interaction of X-rays and energetic particles with poly- mers is of both theoretical and practical interest. The chemical changes brought about by these treatments are in the com- position, molecular weight distribution and electronic struc- ture. Simultaneously macroscopic properties like wettability, adhesion, conductivity, biocompatibility, solubility, mechan- ical properties, etc. are usually also largely modified. Although the radiation chemistry of polymers is a field of active research, the number of related surface studies is still limited.

In the radiolysis or during proton or electron bombardment of aromatic polyethersulphones, in particular, homolytic main-chain scission at the C-S bond is thought to be the principal degradation step, followed by a scission of the adjac- ent C-S bond liberating SO,. C-0 bond cleavage is con- sidered to be another important step of d e g r a d a t i ~ n . ~ . ~

Here we report some XPS data on the x-ray-, electron-, or ion-beam induced chemical changes of an aromatic poly- ethersulphone, demonstrating that, at least on the polymer surface and in the low keV range, other chemical processes may also be of importance in the degradation mechanism.

EXPERIMENTAL

Polyethersulphone (ICI 1OOP) samples were studied in the form of foils or films cast from N-methyl pyrrolidone. Expo- sure to beams of x-ray, or Ar+ ions, or N2+ ions, as well as the related XPS analysis were performed using a Kratos XSAM 800 electron spectrometer equipped with a Kratos MacroBeam ion gun system. The operation details have been described elsewhere.6 Exposure to electron beams as well as the related XPS studies were carried out using a Kratos ES 300 spectrometer equipped with an integral electron gun. The experimental details have been described earlier.’

For studying the x-ray beam effect, MgKa radiation

(E = 1253.6 eV) was used with an estimated flux density of - 10” photons cm-’ s-’. During the electron-beam studies electrons of 1.1 keV were applied with a flux density of - 10’’ electrons cm-’ s-’ . During the ion-beam studies 1 keV Ar+ or Nz+ beams were used with flux densities of -7 x 10” or - 1013 ions cm-, s-’ , respectively.

Data acquisition and data processing were performed partly by using the Kratos DS 300 data system running on a DEC LSI 11/23 computer, and partly by using routine programs of a HP-2627A computer.

RESULTS AND DISCUSSION

The Ols, Cls and S2p peaks of the polyethersulphone (PES) sample were asymmetric. Based on literature data8v9 they were separated into components whose assignments are shown in Table 1.

Table 1. Analysis of the starting sample.

Binding FWHM Area Core level Chemical feature energy eV eV Yo

-c-0-c- 533.9 1.80 33 01 s \ /

/ \

o=s=o 532.3 1.85 67

\

/

\ -c-s- 285.6 1.90 17 /

/ \

CIS -c-0- 286.6 1.90 17

\ / \ -C-C-, -C-H 285.0 1.75 66 / \ /

0 =s=o 169.4 1.78 33

S2P,,, 0 -s-0 168.1 1.78 67 / \

=PI,,

EXTENDED ABSTRACTS 471

On long-lasting x-ray exposure, however, there have been some alterations in the characteristic peaks. The relative intensity of the 0 Is peak decreased gradually. Simultaneously, a shoulder developed on the low binding energy side of the S2p peak with an energy separation of -4.3 eV, showing that reduction of the sulphur took place resulting in the formation of sulphide (or thioether) type sulphur atoms. This process, however, was rather slow: after 300 min, for instance, the rela- tive amount of sulphide reached only -23% of the total sulphur S2p signal.

Similarly, XPS spectra taken after the electron- bombardment of the samples demonstrated that, again, the sulphur in the sulphone groups was gradually reduced to sul- phide type ones.

The examination of the energy-loss region of the Cls peak reflected drastic changes in the electronic structure of the polymer : significant alterations took place in the intensity, shape and position of the Cls shape-up peaks on electron- bombardment. The component situated at - 10 eV, at the high binding energy side of the main peak, gradually decreased and almost disappeared after - 140 min of expo- sure. The separation of the second shape-up peak from the main Cls peak (having a value of -6.6 eV before bombardment) decreased on electron bombardment. The reactivity of the electron-bombarded sample was then quickly tested: after its exposure to the atmosphere for 48 h, the uptake of oxygen (as determined by XPS) was negligible.

Bombardment of the PES samples by 1 keV Ar' or N2+ ion beams again led to a gradual reduction of the sulphur. Simultaneously the relative content of oxygen decreased and that of carbon increased significantly, while that of sulphur showed a minor decrease in both cases. While incorporation of argon could not be observed by XPS, nitrogen was found to build into the polymer surface when bombarded with N2+ ions."

With regard to the degradation mechanism of PES on bom- bardment by either x-ray-, or electron-, or ion-beams of -1

(ii)

(iii)

(9 keV, it can be concluded that a gradual reduction of sulphur takes place; sulphone groups are reduced to sulphide type ones; and

sulphoxy S=O group-containing products are not

clearly detected by XPS, even if they are expected to be intermediate products in a consecutive degradation scheme.

\

/

REFERENCES

1. A. Dilks in Electron Spectroscopy: Theory, Techniques and Application, Vol. 4. pp. 277-359, ed. by C. R. Brundle and A. D. Baker, Academic Press (1 981 ).

2. W. L. Brown, Radiation Effects, 98, 281 (1 986). 3. D. Briggs and M. J. Hearn, Vacuum 36, 1005 (1986). 4. J. R. Brown and J. H. O'Donnell, J. Appl. Polym. Sci. 19, 405

(1975). 5. D. R. Coulter, M. V. Smith, F.-D. Tsay and A. Gupta, J. Appl.

Polym. Sci. 30, 1753 (1985). 6. A. Tbth, I. Bertbti, T. Szbkely and M. Mohai, Surf. Interface

Anal. 7, 282 (1 985). 7. G. Marletta and S. Pignataro, Chem. Phys. Letters 124, 414

(1986). 8. B. J. Lindberg, K. Harnrin, G. Johansson, U. Gelius, A.

Fahlman, C. Nordling and K. Siegbahn, Physica Scripta 1, 286 (1 970).

9. D. T. Clark and H. R. Thomas, J. Polym. Sci., Polym. Chem. Ed. 16, 791 (1978).

10. A. Licciardello, 0. Puglisi and S. Pignataro, J. Chem. SOC. Faraday Trans. 11. (1 985).

Analysis of the Yeast Cell Surface by X-ray Photoelectron Spectroscopy

D. E. Amory, M. J. Genet and P. G. Rouxhet Unite de Chimie des Interfaces. Universite Catholique de Louvain, Place Croix du Sud 1, B-1348 Louvain-la-Neuve, Belgium

The surface properties of the complex polymers constituting the surface of micro-organisms influence their behaviour in fermentation (flocculation, flotation) and determine the possi- bility to retain them in a bioreactor by adhesion to a support.

The surface of various strains of brewers' yeast has been analysed by x-ray photoelectron spectroscopy. Therefore the cells were washed with water and freeze-dried. The samples must be pressed in a trough in order to avoid variations due to surface roughness. The absolute intensity of XPS peaks could be compared between independent runs, owing to the use of silica as a separate reference in each run: this permitted also the estimation of apparent surface concentrations. The reproducibility of the results obtained over a period of 8 months was satisfactory. The main cause of variability is the freeze-drying operation ; for samples freeze-dried together, the

reproducibility of absolute surface concentrations is about 5% for C and about 15% for 0, N and P.

The N/P atomic concentration ratio of the various strains studied is inversely correlated with the electrophoretic mobil- ity at pH 4, i.e. with the zeta potential. A systematic difference is found between the P concentration of top fermentation and bottom fermentation yeasts: the average N/P ratio of the two sets of strains is respectively 17.8 k 4.0 and 6.5

Even when the cells have been equilibrated with CaCI, sol- utions, K + is the main cation found on the surface, owing to its diffusion from the cells (K-rich) into the solution. The K/P atomic concentration ratio is very close to 1, suggesting that K is retained by ion exchange on the phosphate groups present on the surface.

0.3.