Phenyl-Substituted Cage Silsesquioxane-Based Star-Shaped Giant Molecular Clusters: Synthesis, Properties, and Surface Segregation Behavior.

The crystallinity, solubility, and physical properties of polyhedral oligomeric silsesquioxane (POSS) compounds are highly dependent on their organic substituents. We previously synthesized a series of isobutyl-substituted star-shaped POSS derivatives with aliphatic chain linkers of different length. In this study, we prepared C3- and C6-linked phenyl-substituted star-shaped POSS derivatives ( 3 _Ph-C3 and 3 _Ph-C6 ) by the hydrosilylation of heptaphenylallyl- and hexenyl-POSS ( 1a and 1b ) and octadimethylsiloxy-Q ₈ -silsesquioxane (Q ₈ M ₈ ^H ) ( 2 ), respectively, and their properties were compared with those of the corresponding isobutyl-substituted derivatives ( 5 _iBu-C3 and 5 _iBu-C6 ). Although 3 _Ph-C6 was only soluble in chloroform and insoluble in tetrahydrofuran (THF) and toluene, 3 _Ph-C3 was soluble even in THF and toluene, suggesting that the shorter linkers of the derivative afford a wider range of solvents for dissolution. Differential scanning calorimetry analysis showed that 3 _Ph-C3 exhibited a baseline shift at 190 °C and an endothermic peak at 316 °C. However, no clear baseline shift was observed for 3 _Ph-C6 . Thermogravimetric analysis showed that the shorter linker in the phenyl-substituted star-shaped POSS derivative significantly increased the decomposition temperature compared with the longer linker. The annealed cast film of 3 _Ph-C3 at 340 °C above its melting temperature formed a transparent film even after cooling to room temperature. However, an opaque whitish film was formed in the case of 3 _Ph-C6 . Poly(methyl methacrylate) (PMMA) films containing 2 wt % 3 _Ph-C3 and 3 _Ph-C6 were prepared by casting their chloroform solutions onto glass substrates overnight at room temperature. The static water contact angle measurements and XPS analysis for the castings film containing 3 _Ph-C3 and 3 _Ph-C6 suggested that degree of the segregation amount of 3 _Ph-C3 was larger than that of 3 _Ph-C6 . The shorter linker length in the phenyl-substituted star-shaped POSS derivative, 3 _Ph-C3 , with its greater predicted solubility in PMMA, exhibited entropy-driven surface segregation.