Valence electronic structure of selected polyorganosiloxanes; x-ray photoelectron spectroscopy and quantum chemical studies

Abstract

The valence electronic structures of poly(dimethylsiloxane) (PDMeSO), poly(methylsilsesquioxane) (PMeSSO), and poly(phenylsilsesquioxane) (PPhSSO) were calculated by the ab initio Hartree-Fock LCAO crystal orbital method and were studied experimentally by x-ray photoelectron spectroscopy. The basic features of the experimental spectra could be reproduced by the calculations, and thus the peaks could be assigned to structural elements. The electrons of the methyl side-groups of these polymers are basically localized. The aromatic electrons of the phenyl side-groups interact with the σ -bonds of the silsesquioxane skeleton. This skeleton-sidegroup interaction is similar to what has been reported for polyorganosilanes, though in PPhSSO these states are delocalized to a lesser extent as their smaller k-dispersion shows. The methyl side-groups of PDMeSO and PMeSSO contribute to the spectra only in the inner valence regions (between 16 eV and 28 eV), while the contribution of the phenyl groups in PPhSSO is important also in the outer valence region. These observations may facilitate the interpretation of chemical changes occurring during the transformation of these polymers into ceramic-like materials.

Valence band structure of PDMS surface and a blend with MWCNTs: A UPS and MIES study of an insulating polymer

“…Assigning the fitted peaks to the functional groups of PDMS is complicated by the fact that in [36] the peak at the lowest binding energy is at a different binding energy than the peak at the lowest binding energy found in the UP and MIE spectra shown here. Measured and calculated binding energies usually differ thus need to be correlated.…”

“…In order to compare the measured spectra with theoretically calculated DOS of PDMS [36] and also to the previous experimental data using XPS for the valence band region, a Gaussian fit to the MIE and UP spectra were made to approximate the experimental peak positions for A c c e p t e d M a n u s c r i p t 19 PDMS and can be seen in Figure 3 and Figure 4. A table summary of peak positions can be seen in Table 2.…”

“…Correlating calculated binding energies to measured binding energies usually is achieved by offsetting the energy scales of the calculation such that the position of the peak with the lowest calculated binding energy aligns with the peak with the lowest binding energy in the experimentally measured spectrum [37]. Because the peak with the lowest binding energy in the XP spectrum in [36] is at about 1.5 eV higher binding energy than the peak with the lowest binding energy in the UP spectrum shown here, we take the energy difference of 1.3 eV into account in Table 2 for assigning the peaks in the UP spectra to specific functional groups. where Gsum is the sum of all Gaussians G1~G5, and Exp Bknd is the exponential background of the measurement.…”

“…G4 is spread across all components. Peak G5 in UPS can be attributed to the Si-O backbone (stated as broad peaks from 12~14 eV in [36]) and is not present in the MIE spectrum. The presence of the low binding energy peak (G1) in MIES shows that there is oxygen present at the outermost layer as the states around 6 eV are attributed to lone pair oxygen [36].…”

“…Peak G5 in UPS can be attributed to the Si-O backbone (stated as broad peaks from 12~14 eV in [36]) and is not present in the MIE spectrum. The presence of the low binding energy peak (G1) in MIES shows that there is oxygen present at the outermost layer as the states around 6 eV are attributed to lone pair oxygen [36]. This presence of G1 and absence of G5 in MIES indicates that any exposed areas of the polymeric backbone are primarily the O2p orbitals -which intuitively makes sense for two reasons.…”