UV Photoelectron Spectroscopic Study of the Electronic Structure of Poly(dimethylsiloxane) and the Comparison with Related Silicon Compounds

Abstract

The electronic structure of poly(dimethylsiloxane) was studied by UV photoelectron spectroscopy. The results using rare gas resonance lines (Fig.1) give an accurate ionization energy of 8.3 eV, while the spectrum by synchrotron radiation (Fig.2( a )) gives the whole valence electronic structure. From the comparison with the data of related compounds in Fig.2 and with the density of states of unsubstituted polysiloxane calculated by Takeda an d Shiraishi (Ref.7) (Fig.3), the features in Fig.2 a are assigned as follows: A is derived from 0 2p lone pairs, B from the Si-C bonds, C from the C-H bonds, D from the Si 3s leves, E from the C 2s levels, and F from the O2s levels. The comparison of the present results with those of poly(dimethylsilylene) shows that the σ conjugation in poly(dimethylsilylene), which leads to the low ionization threshold energy and high conductivity upon doping, is effectively broken in poly(dimethylsiloxane), resulting in good insulating property. The localized nature of the electronic states in the Si-O systems leads to the increase of the ionization threshold from poly(dimethylsiloxane) to SiO₂ by the removal of electron-donating methyl groups (Fig.4), in contrast to the decrease in the catenated silicon systems from poly(dimethylsilylene) to silicon due to more effective delocalization in the three-dimensional system than in the one-dimensional system.