Variation of optical properties,rheology, and microstructure in fullerene/poly(vinyl pyrrolidone) nanofluids with fullerene content in n-butanol

In this study, we report non-linear variation of optical and rheology properties in fullerene (C₆₀)/poly(vinyl pyrrolidone) (PVP) nanofluids (NFs) with C₆₀-content (C) in an alcoholic medium. From the absorption spectra, we found that the absorbance maximum (α_max) follows a non-linear path with C-value due to a donor–acceptor PVP → C₆₀ charge transfer in a C₆₀/PVP complex. The α_max value reaches maximum at a specific C₆₀-content of 13.9 μM due to percolation effect and then starts decreasing with a further increase in C-value over the subsequent region covered in this experiment. Models were proposed to show donor–acceptor interaction between C₆₀ and PVP. The molar extinction coefficient also drops through a peak maximum at 13.9-µM C₆₀. Regardless of the C-values, shear-thinning behavior was found in all NFs. The shear viscosity (η) value, which decreases with shear rate value over 5 to 100 s^?1, is shown to be increasing non-linearly against the C-value. The η-value varies non-linearly with C₆₀-content and shows a dip at a point near 13.9-µM C₆₀. High-resolution transmission electron microscope images reveal that the size of C₆₀/PVP nano-assemblies increases with C-value as PVP is anchoring C₆₀ molecules in specific structures.