Infrared spectroscopy study of microstructures of poly(silsesquioxane)s

Results are presented from theoretical and experimental infrared (IR) spectroscopy studies of the microstructures of poly(silsesquioxane)s (PSSQs) of varying chemical composition. The calculated IR spectra show two distinct asymmetric Si-O-Si stretch vibration bands for models of complete polyhedral cages, incomplete open cages, and short ladder structures. Close analyses of the calculated results indicate that the higher frequency IR band at about 1150 cm(-1) is derived from the parallel asymmetric Si-O-Si stretch vibration mode in the (Si-O)(n) ring subunit while the lower frequency band at about 1050 cm(-1) is due to the asymmetric Si-O-Si stretch symmetric with respect to the inversion point at the center of the (Si-O)(n) ring and is absent in highly symmetric cage structures. Experimentally, poly(methylsilses-quioxane) (PMSQ), poly(isobutylsilsesquioxane) (PiBSQ), and poly(phenylsilsesquioxane) (PPhSQ) exhibit a varying tendency of cage-like structures, rather than ladder structures, in as-polymerized samples. When the thermal conversion (curing) temperature is increased to 400 degrees C, the microstructure of PMSQ in thin solid films transforms from open cage-like structure toward a random network with lower symmetry. This change in microstructure is caused by the secondary condensation reaction and the evaporation of cage structures, and the effect of cage evaporation becomes most pronounced for PiBSQ films, which are mostly comprised of cage-like structures that evaporate around 280 degrees C. In comparison, PPhSQ films retain cage-like structure upon curing to 400 degrees C as a result of the high evaporation temperature (ca. 500 degrees C) of the cages.