Design of low-crystalline and low-density isobutyl-substituted caged silsesquioxane derivatives by star-shaped architectures linked with short aliphatic chains

Abstract

The C2-, C3-, and C8-linked star-shaped isobutyl-substituted caged silsesquioxane derivatives (3a, 3b and 3c) were prepared by the hydrosilylation of mono-vinyl-, allyl-and octenylisobutyl-T 8-silsesquioxane (1a, 1b and 1c), respectively, along with the synthesis of octadimethylsiloxy-Q 8-silsesquioxane (2). 3a and 3b formed optically transparent films by casting the solutions onto glass pieces. However, 3c formed an opaque white film. Differential scanning calorimetry traces of 3a and 3b exhibited lower melting points than that of 3c and the appearance of a glass-transition point, suggesting their lower crystallinity, which promotes the formation of transparent films. Wide-angle X-ray scattering measurements suggested that 3a and 3b formed more tightly packed structures after melting. However, 3c formed a less dense structure after melting. The refractive index of the film of 3a (1.4529±0.0005) was lower than that of 3b (1.4556±0.0007), which is due to the density of 3a (1.1433 g cm-3) being less than that of 3b (1.1753 g cm-3).