Disorder of the sublattice of terminal groups and phase transition in octa (vinylsilsesquioxane)

Abstract

Octa(vinylsilsesquioxane) is one of the members of a series of materials which are important for microelectronics, viz., the organylsilsesquioxanes with the general formula (RSiO/sub 1.5/)/sub n/, where R is an organic substituent, and exists in the form of a molecular crystal consisting of (C2H3SiO/sub 1.5)9 molecules. The almost cubic silicon-oxygen skeleton is slightly extended along the C3 axis. According to the data from x-ray diffraction analysis, at room temperature there is disordering of the terminal C/sub US / carbon atoms of the substituent in the compound: among three positions for substituents lying on the third-order axis and among two positions for those in positions which are not symmetry centers. However, the question of the nature of the disordering (whether it is static or dynamic) remained open. In the present work the authors used the method of proton NMR (PMR), NMR relaxation, and precision calorimetry to establish that dynamic disordering appears in octa(vinylsilsesquioxane) at T = 230K in the sublattice of the terminal groups of atoms and that it is accompanied by a phase transition in the system.