Study on the aggregation behavior of polyhedral oligomeric silsesquioxane compounds in solution

Polyhedral oligomeric silsesquioxane (POSS) has been extensively studied as an essential building block into polymer matrices to improve their thermal and mechanical properties. Although aggregation and dispersion state of POSS segments influence these properties, a deep understanding of POSS aggregation behavior is lacking. In this study, the aggregation behavior of heptaisobutyl-mono-substituted-POSS (7B1R-POSS) was investigated in chloroform (CHCl3), using a simplified model that eliminates various factors within polymer matrices, to understand the mechanism of POSS molecules aggregation. The UV-VIS spectra of 7B1R-POSS solutions with varying concentrations revealed a critical aggregation concentration (CAC), indicating that 7B1R-POSS formed aggregates in CHCl3. Dynamic light scattering (DLS) measurements of 7B1R-POSS solutions above the CAC suggested that the most of aggregation particles were on the order of nanometers, and very few aggregates of a few hundred nanometers were formed. It was found that all 7B1R-POSS exhibited temperature-responsive reversible aggregation behavior, aggregating upon heating, and dispersing upon cooling. This behavior was similar to the entropy-driven behavior observed in polymer micelles, which is known as hydrophobic behavior. In this work, the entropy-driven dominant aggregation behavior in the aggregates of 7B1R-POSS was clarified.