Folke Tölle

Influence of graphene on the cell morphology and mechanical properties of extruded polystyrene foam

The incorporation of nanoparticles to polymer foams not only reinforces the cell walls and struts but can also lead to a decrease of cell size and enhanced cell morphology which in turn, yield foams with superior mechanical properties. For this purpose, several studies have focused on identifying close-to-ideal nucleating agents as well as understanding the influence of processing parameters on foam cell morphology. This research provides a systemic approach to low-density polystyrene foams produced with graphene (thermally reduced graphite oxide), talc and carbon nanotubes (MWCNTs) via foam extrusion. Remarkably, the cell morphologies of polystyrene/thermally reduced graphite oxide foams show enhanced cell homogeneity with a tremendous increase of the cell densities by more than one order of magnitude compared to neat polystyrene and its counterparts.

Nitrogenated graphene and carbon nanomaterials by carbonization of polyfurfuryl alcohol in the presence of urea and dicyandiamide

Most of the numerous emerging synthetic routes toward functionalized few- and multilayer graphene employ fossil resources such as graphite together with polar solvents. Herein we report on the solvent-free preparation of nitrogenated graphene and green carbon nanomaterials exploiting furfuryl alcohol (FA) and urea as renewable feedstocks. Typically, polyfurfuryl alcohol (PFA) reactor blends, prepared by cationic polymerization of furfuryl alcohol in the presence of urea or dicyandiamide (DICY), respectively, are carbonized by thermolysis. In sharp contrast to conventional PFA thermolysis, as verified by a microscopy (SEM, TEM, AFM) and WAXS, only PFA thermolysis in the presence of urea and DICY affords nitrogenated functionalized multilayer graphene. This is attributed to the in situ formation of carbon nitride nanosheets as thermally degradable template. Variations of process parameters such as the temperature ramp and the FA/urea and FA/DICY molar ratios enable control of graphene morphology development and properties. Thus, our green carbon approach toward functionalized multilayer graphene holds promise for fabricating renewable carbon nanosheet materials, meeting the demands of manifold applications.

Stable aqueous dispersions of functionalized multi-layer graphene by pulsed underwater plasma exfoliation of graphite

A process was developed for graphite particle exfoliation in water to stably dispersed multi-layer graphene. It uses electrohydraulic shockwaves and the functionalizing effect of solution plasma discharges in water. The discharges were excited by 100 ns high voltage pulsing of graphite particle chains that bridge an electrode gap. The underwater discharges allow simultaneous exfoliation and chemical functionalization of graphite particles to partially oxidized multi-layer graphene. Exfoliation is caused by shockwaves that result from rapid evaporation of carbon and water to plasma-excited gas species. Depending on discharge energy and locus of ignition, the shockwaves cause stirring, erosion, exfoliation and/or expansion of graphite flakes. The process was optimized to produce long-term stable aqueous dispersions of multi-layer graphene from graphite in a single process step without requiring addition of intercalants, surfactants, binders or special solvents. A setup was developed that allows continuous production of aqueous dispersions of flake size-selected multi-layer graphenes. Due to the well-preserved sp(2)-carbon structure, thin films made from the dispersed graphene exhibited high electrical conductivity. Underwater plasma discharge processing exhibits high innovation potential for morphological and chemical modifications of carbonaceous materials and surfaces, especially for the generation of stable dispersions of two-dimensional, layered materials.