Preparation of phenyl-substituted open-cage silsesquioxane-pendant polysiloxanes and their thermal and optical properties

Abstract

We prepared phenyl-substituted corner-opened type polyhedral oligomeric silsesquioxanes (CO-POSSs) bearing tris(dimethoxysilyl)-groups with variable linker lengths at the opening vertex; that is, tris(dimethoxysilyl-ethyl-dimethylsiloxy)- and tris(dimethoxysilyl-propylthioethyl-dimethylsiloxy)-heptaphenyl-substituted CO-POSSs (Ph-H and Ph-V). Optically transparent free-standing films of phenyl-substituted open-cage silsesquioxane-pendant polysiloxanes (PolyPh-H) were prepared by optimizing the sol–gel reaction conditions for Ph-H. Polycondensation of Ph-V afforded an optically transparent and flexible phenyl-substituted CO-POSS-pendant polysiloxane film (PolyPh-V). The polycondensations of Ph-H and Ph-V were fully completed even at 50 °C for 6 h under vacuum. ²⁹Si cross-polarization magic angle spinning (CP-MAS) NMR analysis suggests that the films included cyclotrisiloxane (D₃) and linear siloxane (D_linear) structures. The effects of the polysiloxane structures on the thermal and mechanical properties were studied. The highest temperature at which the sample lost 5 wt% of the original mass (T_d5) under N₂ (381 °C) was obtained for PolyPh-V, even though it contained a flexible linker unit. The predominant linear siloxane structures may provide increase higher thermal stability. The UV-vis spectra of the resulting transparent films were mostly unchanged even after six days of exposure to UV irradiation in air. The present study shows that phenyl-substituted CO-POSS-pendant polysiloxanes represent alternative UV-resistant, optically transparent materials with higher heat resistance.