Thermal Properties of Open-Cage Silsesquioxanes: The Effect of Substituents at the Corners and Opening Moieties

Abstract

Abstract Incompletely condensed polyhedral oligomeric silsesquioxane (IC-POSS), which has an open-cage structure, exhibits high optical transparency even incorporating 30 wt% of the IC-POSS fillers in a poly(methyl methacrylate) (PMMA) matrix dependent on their substituents. However, real understanding of the substituent-dependent dispersibility of the IC-POSS fillers is unclear. In this work, we studied structure-dependent bulk thermal properties of the IC-POSS derivatives substituting isobutyl, phenyl and cyclohexyl groups at the Si corners and dimethylsilyl, trimethylsilyl, dimethylethylsilyl, dimethylvinylsilyl, dimethylphenylsilyl, and dimethylethylcyclohexyl groups at the opening moieties. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) measurement revealed that both the substituents at the corners and opening moieties significantly affected the thermal properties of the IC-POSS derivatives. We found volcano-type dependence of the phase transition temperature on the molecular weights of the substituents at the opening moieties in all the tested IC-POSS derivatives. The trimethylsilyl groups and the dimethylphenylsilyl groups at the open moieties showed highest and lowest phase transition temperatures, respectively. Especially, the phenyl groups at the opening moieties lead to low melting point and high thermal stability.