Fabrication of composite films with poly(methyl methacrylate) and incompletely condensed cage‐silsesquioxane fillers

Polyhedral oligomeric silsesquioxane (POSS) is a promising nanofiller for tuning properties of optically transparent polymer materials. On the other hand, traditional completely condensed POSS (CC‐POSS) has a fundamental problem for fabricating optically transparent composite films; CC‐POSS has high crystallinity due to its symmetrical structure, resulting in aggregation in the polymer matrices. In this work, we have demonstrated that incompletely‐condensed POSS (IC‐POSS), which has an open‐cage structure, can be well dispersed in a poly(methyl methacrylate) (PMMA) matrix. IC‐POSS with various substituents were readily synthesized, and their composite films of PMMA were fabricated by solution‐casting method. High transparency was achieved with up to 30 wt % of the IC‐POSS fillers, while the CC‐POSS analogues caused phase separation with 10 wt % loading. Addition of the IC‐POSS fillers can improve thermal stability and control glass transition temperature by the substituent structure. Additionally, refractive index was tuned from 1.485 to 1.513. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46033.