Thermal Stability of Polymers Confined in Graphite Oxide

In this study, polymer graphite oxide (GO) interactions are demonstrated to induce thermal instability in both the polymer and GO phase in conditions of maximum polymer uptake, in which the polymer chains are either intercalated into the GO interlayer or adsorbed on the GO sheets. We investigate the thermal stability of poly(ethylene oxide) (PEO) intercalated in GO (PEO/GO) in detail, using a combination of techniques including X-ray diffraction (XRD), thermogravimetry (TGA) and TGA-mass spectroscopy (TGA-MS) in dynamic (nonisothermal) and static (isothermal) modes. Our results show that intercalated PEO decomposes at 160 degrees C lower than neat PEO, and that GO decomposes at 50 G lower than pristine GO, due to a synergistic instability between GO and intercalated PEO upon heating. Other hydrophilic polymers-such as poly(vinyl methyl ether) (PVME), poly(vinyl alcohol) (PAA), poly(vinylpyrrolidone) (PVP) and poly(acrylic acid) (PAA)-forming polymer-adsorbed GO structures are also observed to decompose at noticeably lower temperatures than either pristine GO or their own corresponding neat polymers. Unlike PEO/GO intercalation compounds, the excess of PEO phase in PEO GO composites decomposes at temperatures close to that of neat PEO, which demonstrates that those polymer chains far from the adsorbing GO surface are not significantly affected by the presence of GO sheets. Furthermore, isothermal TGA data for PEO/GO intercalation compounds with PEO chains from 5 to 2135 monomeric units are well described by an autocatalytic model similar to that we found for pristine GO in a previous study, which suggests that the overall decomposition of PEO/GO is dominated by the reduction and exfoliation of GO. However, when PEO is adsorbed on graphene substrate, which is thermally stable at the studied temperatures, the kinetic mechanisms follow a first-order reaction similar to neat PEO, but the decomposition occurs with a considerably lower activation energy.