Static and Dynamic Behavior of Polymer/Graphite Oxide Nanocomposites before and after Thermal Reduction

Varying the degree of oxidation of graphite: effect of oxidation time and oxidant mass

In this work, we employ a fast and less toxic modified Hummers' method to develop graphene oxide (GO) with varying degrees of oxidation and investigate the effect of the latter on the structure and the thermal properties of the synthesized materials. Two different key parameters, the time of the oxidation reaction and the mass of the oxidation agent, were systematically altered in order to fine tune the oxidation degree. All graphene oxides were characterized by a plethora of experimental techniques, like X-ray diffraction (XRD), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) as well as infrared spectroscopy (IR) and X-ray photoelectron spectroscopy (XPS) for their structural, thermal and chemical identification. The results revealed that for a certain amount of oxidant, the time does not affect the final degree of oxidation of the materials, at least for the examined reaction times, because very similar structural patterns and thermal properties were obtained. At the same time, the oxygen-containing functional groups were found very similar. On the other hand, the degree of oxidation was found highly dependent on the mass of the oxidizing agent. XRD analysis showed a systematic increase of the interlayer distance of the synthesized GOs with the increase of the oxidant mass, whereas both the enthalpy of reduction and the % weight loss were increased. Moreover, XPS measurements provided a quantitative evaluation of the amount of carbon and oxygen in the materials; the increase of the oxidant mass led to a decrease of the total carbon content with the concurrent increase of the total oxygen amount.

Dynamics of Polymer Chains in Poly(ethylene oxide)/Silica Nanocomposites via a Combined Computational and Experimental Approach

The dynamics of polymer chains in poly(ethylene oxide)/silica (PEO/SiO₂) nanoparticle nanohybrids have been investigated via a combined computational and experimental approach involving atomistic molecular dynamics simulations and dielectric relaxation spectroscopy (DRS) measurements. The complementarity of the approaches allows us to study systems with different polymer molecular weights, nanoparticle radii, and compositions across a broad range of temperatures. We study the effects of spatial confinement, which is induced by the nanoparticles, and chain adsorption on the polymer's structure and dynamics. The investigation of the static properties of the nanocomposites via detailed atomistic simulations revealed a heterogeneous polymer density layer at the vicinity of the PEO/SiO₂ interface that exhibited an intense maximum close to the inorganic surface, whereas the bulk density was reached for distances ∼1-1.2 nm away from the nanoparticle. For small volume fractions of nanoparticles, the polymer dynamics, probed by the atomistic simulations of low-molecular-weight chains at high temperatures, are consistent with the presence of a thin adsorbed layer that exhibits slow dynamics, with the dynamics far away from the nanoparticle being similar to those in the bulk. However, for high volume fractions of nanoparticles (strong confinement), the dynamics of all polymer chains were predicted slower than that in the bulk. On the other hand, similar dynamics were found experimentally for both the local β-process and the segmental dynamics for high-molecular-weight systems measured at temperatures below the melting temperature of the polymer, which were probed by DRS. These differences can be attributed to various parameters, including systems of different molecular weights and nanoparticle states of dispersion, the different temperature range studied by the different methods, the potential presence of a reduced-mobility PEO/SiO₂ interfacial layer that does not contribute to the dielectric spectrum, and the presence of amorphous-crystalline interfaces in the experimental samples that may lead to a different dynamical behaviors of the PEO chains.