Hybrid Sol-Gel Glasses with Glass-Transition Temperatures Below Room Temperature

Melting gels are hybrid gels that have the ability to soften and flow at around 100 degrees C for some combinations of mono-and di-substituted alkoxysiloxanes, where substitutions are either all aromatic or all aliphatic. In this study, melting gels were prepared using phenyltriethoxysilane (PhTES) and dimethyldiethoxysilane (DMDES), meaning both an aromatic and aliphatic substitution. Differential scanning calorimetry was performed to identify glass-transition temperatures, and thermal gravimetric analysis coupled with differential thermal analysis (TGA-DTA) was performed to measure weight loss. The glass-transition temperatures (Tg) ranged from -61 degrees C to +5.6 degrees C, which are between the values in the methyl only system, where all Tg values are less than 0 degrees C, and those values in the phenyl only system, where Tg values are greater than 0 degrees C. The Tg decreased with an increase in the DMDES fraction. Below 450 degrees C, the gels lost little weight, but around 600 degrees C there was a drop in weight. This temperature is lower than the temperature for gels prepared with only aromatic substitutions, but higher than that for gels prepared with only aliphatic substitutions. Final heat treatment was carried out at 150 degrees C for the gel with 80% PhTES-20% DMDES (in mol%), and the consolidation temperature increased with increasing DMDES content to 205 degrees C for the gel with 50% PhTES-50% DMDES. After this heat treatment, the melting gels no longer soften.