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

ABSTRACT 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 (αmax) follows a non-linear path with C-value due to a donor–acceptor PVP → C60 charge transfer in a C60/PVP complex. The αmax 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.