A 29Si solid-state magic angle spinning nuclear magnetic resonance study of TEOS-based hybrid materials

Abstract

Tetraethylorthosilicate (TEOS)-based materials were prepared via the sol—gel method by acid catalysed hydrolysis and condensation of mixtures of trifunctional alkoxysilanes (T) and tetraethylorthosilicate (TEOS, Q). The size of the organic tail and the mole percentage of T-substitution were varied. The effect of the length of the organic tail of the trifunctional silicon-alkoxide on the degree of condensation was studied with solid-state magic angle spinning (MAS) nuclear magnetic resonance (NMR). Cross-polarisation (CP) and single-pulse excitation (SPE) MAS-NMR measurements were compared. CP can not be used to obtain quantitatively reliable data for these systems due to the large distance between protons and part of the Q4 atoms. For quantitatively reliable data SPE MAS-NMR measurements with a repetition delay of 300 s have to be performed (at 7.05 T). The actual functionality (the realised number of network bonds) of the T-atoms decreases with increasing size of the organic tail due to steric hindrance. The functionality of the Q-atoms (AFQ), on the other hand, increases in the presence of T-atoms. An impressive increase in the AFQ parameter is observed for the samples containing 3-glycidoxy-propyltrimethoxysilane (Glymo). In all cases, except for low levels of Glymo substitution, the total number of network bonds decreases with increasing level of substitution. The evolution of the system starting from the sol phase indicates differences in the size of the primary particles formed from a pure TEOS sample on the one hand, and Glymo/TEOS or Phenyltrimethoxysilane (PhTMS)/TEOS samples on the other. In the latter two cases smaller primary particles are obtained. The different time dependences observed for the formation of the T-and Q-distributions for the sample containing Glymo implies phase segregation, in agreement with the results of CP experiments. The formations of the T- and Q-distributions for the PhTMS/TEOS sample appear to be synchronous. This effect, together with the results of the CP and 1H/29Si 2D heteronuclear correlation experiments, indicates the formation of a homogeneous system for PhTMS/TEOS systems.