Fabrication of fluorinated polymer composite materials with a perfluoroalkylated POSS filler to reduce the refractive index

In this study, we designed and synthesized a filler comprising a polyoctahedral oligomeric silsesquioxane (POSS) core and perfluoroalkyl side chains. We found that by blending POSS fillers into a fluorinated polymer, the refractive indices of fluorinated polymer films can be efficiently lowered. The perfluoroalkyl side chains increase the compatibility with fluorinated polymers, and the dendrimer-like structure of POSS creates a large space in the material. The degree of packing was quantitatively evaluated by using the packing coefficient (kp), which was decomposed into two components: the fluorinated polymer (kp,1) and the POSS (kp,2). Owing to the small packing coefficient of the POSS component (kp,2 = 0.430–0.513), the refractive index (n) of the composite material decreased to n = 1.379 from that of the pristine fluorinated polymer (n = 1.424, kp,1 = 0.68–0.76). This study represents a milestone for further reducing the refractive indices of various fluorinated polymers currently in practical use. Fluorinated polymer composite materials of PVDF-HFP and a perfluoroalkylated POSS (F7POSS) filler were manufactured. The refractive indices of the resulting composite films decreased as the content of the POSS filler increased, and the thermal stability remained sufficient. The degree of packing was quantitatively evaluated by using the packing coefficient from the Lorentz–Lorenz equation.