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.

Incompletely condensed polyhedral oligomeric silsesquioxane (IC-POSS), which has an open-cage structure, exhibits high optical transparency even incorporating 30 wt% of the IC-POSS fillers in a poly(methyl methacrylate) (PMMA) matrix dependent on their substituents. However, real understanding of the substituent-dependent dispersibility of the IC-POSS fillers is unclear. In this work, we studied structure-dependent bulk thermal properties of the IC-POSS derivatives substituting isobutyl, phenyl and cyclohexyl groups at the Si corners and dimethylsilyl, trimethylsilyl, dimethylethylsilyl, dimethylvinylsilyl, dimethylphenylsilyl, and dimethylethylcyclohexyl groups at the opening moieties. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) measurement revealed that both the substituents at the corners and opening moieties significantly affected the thermal properties of the IC-POSS derivatives. We found volcano-type dependence of the phase transition temperature on the molecular weights of the substituents at the opening moieties in all the tested IC-POSS derivatives. The trimethylsilyl groups and the dimethylphenylsilyl groups at the open moieties showed highest and lowest phase transition temperatures, respectively. Especially, the phenyl groups at the opening moieties lead to low melting point and high thermal stability.

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