Methylsilsesquioxane-Based Aerogel Systems-Insights into the Role of the Formation of Molecular Clusters

Condensed clusters of hydrolyzed methyltrimethoxysilane (MTMS) were studied using two complementary approaches: (i) Fourier transform infrared (FTIR) spectroscopy was applied along with the hydrolysis and condensation stages of a sol-gel process from the condensation of colloidal suspension of nanoparticles to the solid phase of bulk material; (ii) density functional theory calculations of energies, structural and vibrational data of pertinent MTMS hydrolysis products, specifically, methylsilanetriol-based species with different number of silicon atoms (from two to eight atoms) and different structures/conformations (linear, cyclic, and cage, in a total of 13 structures), were performed at B3LYP/6-311+G(d,p) level of theory. The calculated infrared spectra show two distinct Si-O-Si stretch vibration bands for models of caged structures. The higher-frequency IR band at ca. 1120 cm(-1) is derived from the antisymmetric Si-O-Si stretch vibration mode, while the lower-frequency band at 1035 cm(-1) is due to the symmetric Si-O-Si stretch and is characteristic of the cyclic clusters, being absent in highly symmetric cage structures. The calculated versus the experimental FTIR spectra of poly(methylsilsesquioxane) (PMSQ) dried aerogel in KBr pellet show that cage/cyclic-like structures prevail over ladder structures (linear) in actual PMSQ.