5115
J. Phys. Chem. 1993,97, 5115-5123
Theoretical Study of Spectral Differences in the XPS Valence Bands of Polyethylene Lamellae and Films Micbagl Deleuze,’ Jean-Pierre Denis, and Joseph Delhalle Luboratoire de Chimie Thsorique Appliquke, Facultks Universitaires N.D. de la Paix 61, rue de Bruxelles B-5000 Namur, Belgium Barry T. Pickup Department of Chemistry, The University, Sheffield S3 7HF, United Kingdom Received: November 18, 1992; In Final Form: February 2, 1993
With the aim of searching for conformational signatures, the XPS valence spectra of selected forms of the n-nonane compound, modeling the extreme surface of polyethylene samples, are simulated a t the second-order level of many-body Green’s function theory, using the minimal STO-3Gbasis set. Variations observed in the shape of spectral bands are analyzed in terms of the combined influence of the pole strength factor together with the orbital relaxation, pair removal, and pair relaxation effects. Molecular orbital photoionization cross sections, line positions, and many-body effects exhibit evidence of the fundamental role played by directional orbitals: the origin of the striking conformational signature exhibited in the inner valence band is attributed to alterations of stabilizing interactions of hyperconjugation character and ultimately related to the large energetic dispersion of the Czsstates in an extended hydrocarbon.
Introduction The characterization of the molecular structure of polymer surfaces is critical for understanding phenomena such as polymer interfacial mixing, surface tension, interdiffusion, etc., which are of general importance in numerous applications.I.2 However, very few methods can be used effectively to probe the molecular structure at the surface (or regions near to it) of thin and ultrathin polymer films. Because of their large sampling depth, yielding information averaged over many layer^,^ infrared and Raman spectroscopies,powerful for solving molecularstructurequestions on systems in the bulk, are much less efficient when applied to surface problems. More recent techniques such as STM and AFM look very promising for providing information on the top layer,4*sbut their capacity to deal with samples as received, i.e., without specific preparations that could lead to alterations in the initial structure, must still be ascertained. Developing the potential of complementarysurfacesensitivetechniques to obtain information on the secondary structure of polymer surfaces is thus useful. In 1988, simulated XPS valence spectra based on Koopmans ionization energies of model oligomers, folded (fold A and fold B) and zig-zag planar (ZZ chain) as shown in Figure 1, revealed the possibility of detecting conformational signatures in the valence X-ray photoelectron spectra of ordered polyethylene surfaces.6 A recent X-ray photoelectron spectroscopy (XPS) study of polyethylene lamellae’ confirms that the observed modifications in the intensities of inner-valence lines, lines I and I1 in Figure 2, are in good qualitative agreement with the early theoretical predictions.6 Except for a very few mainly devoted to methodological tests, calculations of the XPS valence band features of oligomericand extended chains in connection with their primary and secondary structures are carried out at the Hartree-Fock level and interpreted in the framework of Koopman’s approximation. Reasons for this choice are (i) the Hartree-Fock theory is often reliable for questionspertaining to main peak assignation and relative positions upon molecular structure alterations and (ii) the minimal size of a finite chain required to mimic an actual systemI0.lI is such that it often precludesthe use of more rigorous but also costly approaches. Nevertheless, when dealing with 0022-3654/93/2097-5 115$04.00/0
I
region fold
Fold B
Fold A
straight segment region
(C)
22 chain
Figure 1. (a) Model of an ideal lamella, (b) details showing the zig-zag planar section and the fold region, and (c) molecular models of folds and zig-zag planar n-nonane as a model of polyethylene chains in the bulk.
intensity questions, special care must be exercised since electron relaxation contributionsin the inner-valenceregion, for instance, can account for a significant part of the net result. Because the early calculations6 were carried out at the Koopmans level, a more detailed analysis must be carried out to find the reasons for the observed differences in the line intensities of polyethylene lamellae and films ultimately to be in a position to transposesecurely the results to other situations and compounds. In thiscontribution,theXPSspectraof threeselectedstructures modeling the surface of polyethylene samples are simulated at the second-order level of many-body Green’s function theory. Although known to be quantitatively deficient, this approximation yields a qualitative description of the main relaxation and correlation effects on the ionization process. The first section deals with methodological considerations. Improvementsprovided by this method of simulationare analyzed in the following section. Finally, theoriginsof conformationalsignatures are tracked down.
0 1993 American Chemical Society
Deleuze et al.
5116 The Journal of Physical Chemistry, Vol. 97,No. 19, 1993
TABLE I: Successive Torsional Angles in the Model oligomers
torsional angles (deg)
fold A
fold B
ZZ chain
c,-c2-c3-c4
-176.088 55.615
174.783 68.421 91.614 -58.415 -64.390 174.425
180.000 180.000 180.000 180.000 180.000 180.000
C+23-C4
