From polymer to SiOC glass: structure, microstructure, mechanical, and thermal properties

The transformation of a methyl-siloxane into a SiOC glass in Ar and reactive CO2 atmosphere was studied through positron annihilation spectroscopy, infrared spectroscopy, density, and calorimetric measurements, whereas the mechanical properties were monitored by nanoindentation and the thermal diffusivity measured by the laser flash method. Two types of micropores were detected at the beginning of the ceramization (700 degrees C); one is related to the voids present in the parent polymer structure (approximate to 0.8 nm in diameter), and the other to the polymer decomposition (approximate to 1.2 nm in diameter at 700 degrees C). The interconnected microporosity collapses, forming isolated pores at 800 degrees C. These progressively sinter at higher temperatures with a final size between 0.3 and 0.7 nm at 1250 degrees C. The porosity evolution kinetics correlates with the SiOC compositions, the densification occurring at lower temperatures in CO2 where the matrix is close to SiO2. However, the amorphous network obtained in Ar, leading to a heavy anionic substitution C4- -> 2O2-, is denser than that obtained in CO2 at 1250 degrees C. In both atmospheres, the correlation length at 1250 degrees C is about 2.7-2.9 nm, well below the typical values of amorphous SiO2. The porosity and structure evolution cause a progressive increase in the nanomechanical properties and thermal conductivity as the pyrolysis temperature increases. The thermal conductivity of the SiOC obtained at 1250 degrees C in CO2 and Ar atmosphere is about 1 and 1.8 W m-1 K-1, respectively, such figures are symmetric to the values usually reported for amorphous silica (about 1.4 W m-1 K-1). It is argued that the low thermal conductivity of the SiOC pyrolyzed in CO2 is related to the presence of the free carbon phase rather than to the anionic substitution of the glass network.