Thermal Behavior of Polyvinylpyrrolidone on Reduced Graphene Oxide

Temperature-modulated differential scanning calorimetry (TMDSC) and Fourier transform infrared (FTIR) spectroscopy were used to study the interphase behavior of polyvinylpyrrolidone (PVP) on reduced graphene oxide (rGO). In this study, in situ prepared rGO was used due to its thermal stability around the glass transition temperature of bulk PVP (176°C). Based on FTIR results, it was found that the PVP molecules cover many of the residual functional groups of the rGO surface and H–bonding occurs. The H–bonding interaction produces tightly bound PVP on the rGO surface. Unfortunately, the thermal signature of the tightly bound segments of PVP on the rGO was weak and not directly quantifiable from the TMDSC thermograms of the composite samples. This was in contrast to the cases for either poly(methyl methacrylate) (PMMA) or poly(vinyl acetate) (PVAc) on silica where the thermal activity (peaks) from the tightly bound polymer were quantifiable. Therefore, a different analysis was conducted to indirectly estimate the amount of tightly bound PVP absorbed on the rGO from the “missing” PVP thermal signal. Linear regression of the data fitted to this model provided an estimate of the amount of tightly bound PVP on the rGO surface as 0.84 ± 0.03 (SD) mg PVP/m² rGO. Above this bound amount, the tightly bound fractions decreased with increased adsorbed amounts, as expected. This result was also similar in amount to the previously reported H–bonded systems mentioned above on silica, where the signature of a tightly bound polymer was directly observed, and the tightly bound amount was on the order of 1 mg/m² (for PMMA, 1.21 and PVAc, 0.78 mg/m²). This work also highlights the need for caution when interpreting the bulk-like intense DSC peaks from thermal analysis as the only indication of the strength of the polymer surface interaction.