Ralf Thomann

Functionalized Graphenes and Thermoplastic Nanocomposites Based upon Expanded Graphite Oxide

Exfoliation of expanded GO represents an attractive route to functionalized graphenes as versatile 2D carbon nanomaterials and components of a wide variety of polymer nanocomposites. Thermally reduced graphite oxides (TrGO) with specific surface areas of 600 to 950 m(2)  · g(-1) were obtained by oxidation of graphite followed by thermal expansion at 600 °C. Thermal post treatment at 700 °C and 1 000 °C increased carbon content (81 to 97 wt.-%) and lowered resistivity (1 600 to 50 Ω · cm). During melt extrusion with PC, iPP, SAN and PA6, exfoliation afforded uniformly dispersed graphenes with aspect ratio > 200. In comparison to conventional 0D and 1D carbon nanoparticles, TrGO afforded nanocomposites with improved stiffness and lower percolation threshold. Recent progress and new strategies in development of functionalized graphenes and graphene-based nanocomposites are highlighted.

Polyolefin nanocomposites formed by melt compounding and transition metal catalyzed ethene homo- and copolymerization in the presence of layered silicates

Nanocomposites of high density polyethylene (HDPE), linear low density polyethylene (LLDPE) and highly branched polyethylene rubbers were prepared both by means of melt compounding and ethene homo- and copolymerization in the presence of layered silicates which were rendered organophilic via ion exchange with various quaternary alkyl ammonium cations. In comparison to melt compounding, in-situ ethene homo- and copolymerization, catalyzed with MAO-activated zirconocene (MBI), nickel (DMN) and palladium (DMPN) catalysts, proved more effective in nanocomposite formation, as evidenced by larger interlayer spacings and formation of exfoliated anisotropic nanosilicates with high aspect ratio.

Hybrids of silver nanoparticles with amphiphilic hyperbranched macromolecules exhibiting antimicrobial properties

Hybrids of silver particles of 1 to 2 nm in size with highly branched amphiphilically modified polyethyleneimines adhere effectively to polar substrates providing environmentally friendly antimicrobial coatings.

Morphology and rheology of polystyrene nanocomposites based upon organoclay

A correlation between morphology development and rheology of polystyrene nanocomposites based upon organophilic layered silicates (organoclay) such as fluoromicas was found as a function of the silicate modification. Organoclay was obtained by means of ion exchange of clay with protonated amine-terminated polystyrenes with molar mass of Mn = 121 and 5 800 g/mol. Only when applying shear forces during melt compounding of organoclay modified with high molar mass polystyrene (PS), individual silicate platelets of 1 nm diameter and 600 nm length were obtained. Dispersions of such in-situ formed nanoparticles with aspect ratio of 600 accounted for unique elastic properties observed in the low frequency range of the dynamic modulus, whereas organoclay modified with low molecular PS did not exfoliate and exhibited rheological behavior very similar to that of conventional fillers.

Modification of electrical properties and performance of EVA and PP insulation through nanostructure by organophilic silicates

Nanostructured materials are attracting increased interest and applications. Exciting perspectives may be offered by electrical insulation. Polymeric nanofilled materials may find new and/or upgraded applications in the electrical and electronic industry, replacing conventional insulation to provide improved performances in electrical apparatus, as regards, e.g., reliability, environmental compatibility and power rating. This paper shows that electrical properties of nanocomposite insulating materials for DC applications, specifically space charge, conductivity and breakdown voltage, can improve significantly with respect to the basis, unfilled materials. Reference is made to two polymeric materials, i.e. poly(ethylene-covinylacetate) (EVA) and polypropylene (PP), that are widely used as electrical insulation, e.g. for cables and capacitors. The nanofiller consists of an organophilic layered silicate, specifically an extra-pure synthetic fluorohectorite modified by means of interlayer exchange of sodium cations for protonated octadecylamine NH/sub 3//sup +/ (ODA), in a weight concentration of maximum 6%. In both materials the space charge accumulation rate as a function of applied electric field is significantly reduced, while the electrical conductivity is raised. The breakdown voltage can increase for the nanofilled materials.

Microwave Irradiation for the Facile Synthesis of Transition-Metal Nanoparticles (NPs) in Ionic Liquids (ILs) from Metal–Carbonyl Precursors and Ru-, Rh-, and Ir-NP/IL Dispersions as Biphasic Liquid–Liquid Hydrogenation Nanocatalysts for Cyclohexene

Stable chromium, molybdenum, tungsten, manganese, rhenium, ruthenium, osmium, cobalt, rhodium, and iridium metal nanoparticles (M-NPs) have been reproducibly obtained by facile, rapid (3 min), and energy-saving 10 W microwave irradiation (MWI) under an argon atmosphere from their metal-carbonyl precursors [M(x)(CO)(y)] in the ionic liquid (IL) 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm][BF(4)]). This MWI synthesis is compared to UV-photolytic (1000 W, 15 min) or conventional thermal decomposition (180-250 degrees C, 6-12 h) of [M(x)(CO)(y)] in ILs. The MWI-obtained nanoparticles have a very small (<5 nm) and uniform size and are prepared without any additional stabilizers or capping molecules as long-term stable M-NP/IL dispersions (characterization by transmission electron microscopy (TEM), transmission electron diffraction (TED), and dynamic light scattering (DLS)). The ruthenium, rhodium, or iridium nanoparticle/IL dispersions are highly active and easily recyclable catalysts for the biphasic liquid-liquid hydrogenation of cyclohexene to cyclohexane with activities of up to 522 (mol product) (mol Ru)(-1) h(-1) and 884 (mol product) (mol Rh)(-1) h(-1) and give almost quantitative conversion within 2 h at 10 bar H(2) and 90 degrees C. Catalyst poisoning experiments with CS(2) (0.05 equiv per Ru) suggest a heterogeneous surface catalysis of Ru-NPs.

Nanocomposites based on a synthetic layer silicate and polyamide-12

The swelling behavior of a synthetic three-layer silicate caused by 12-aminolauric acid is studied by wide angle X-ray scattering. The swelling process as a function of the 12-aminolauric acid concentration can be separated into two regimes; i) cation exchange of inorganic cations by protonated 12-aminolauric acid and ii) further diffusion of excess zwitterionic 12-aminolauric acid into the interlayer space of the silicate. After the cation exchange an interlayer distance of the silicate of about 1.7 nm can be achieved, which can be increased to more than 2 nm after additional diffusion of zwitterionic 12-aminolauric acid. Also the distribution of the interlayer distance as a function of the swelling conditions is measured from the full width at half maximum values of the d 001 spacing of the silicate. Improvement of the mechanical properties of polyamide-12 by adding the swollen layer silicate is only achieved when the silicate is present during the polymerization. This is closely related to the morphology development. It is shown by transmission electron microscopy and atomic force microscopy measurements that defoliation of the silicate layers leading to a nanocomposite occurs only when the layer silicate is present during the polymerization process. As little as 2 wt.% of swollen layer silicate with an interlayer distance of about 2.1 nm in the nanocomposites leads to an improvement of the tensile modulus of about 1000 MPa without decreasing the impact strength.

Morphological Stability of Poly(propylene) Nanocomposites

Poly(propylene) compounds containing organophilic layered nanosilicates were prepared by means of melt extrusion at 210°C in order to investigate the influence of maleic anhydride-grafted poly(propylene) (PP-g-MA) as a compatibilizer on morphology development and rheological properties. It was found that the addition of PP-g-MA leads to the strong exfolliation of silicate layers within the matrix as investigated by WAXS. This process is associated with the build up of an unstable morphology as probed by dynamic rheology and with the violation of the time-temperature superposition principle. Annealing of the samples at extrusion temperature level leads to a coarsening of the silicate superstructure and further improvement of the exfoliation at the same time. This process results in a stable morphology with unique rheological properties indicating network-like superstructure of silicate layers.

The influence of silicate modification and compatibilizers on mechanical properties and morphology of anhydride-cured epoxy nanocomposites

Resin composition, silicate filler modification, and curing agents were varied systematically in order to improve the performance of anhydride-cured epoxy nanocomposites based upon organoclay. The filler component was fluorohectorite which was rendered organophilic by means of cation exchange of intergallery sodium cations for protonated alkylamines with chain lengths variable from butyl (C4), hexyl (C6), octyl (C8), decyl (C10), dodecyl (C12), hexadecyl (C16), octadecyl (C18) to 12-aminododecanoic acid (C12A). The alkyl chain length must exceed six C atoms to afford increased interlayer distances, accompanied by increased stiffness. Compatibilizer addition such as epoxidized and maleinated soy bean oil or dodecenylsuccinate afforded improved tensile strength without sacrificing stiffness and toughness. Morphology development, determined by means of transmission electron microscopy, and fracture behavior were examined.

Translucent acrylic nanocomposites containing anisotropic laminated nanoparticles derived from intercalated layered silicates

New acrylic nanocomposites consisting of methyl methacrylate (MMA)/n-dodecylmethacrylate (LMA) copolymers and intercalated layered silicates were prepared. The silicates were based upon bentonite which was rendered organophilic by ion exchange with N,N,N,N,-dioctadecyl dimethyl ammonium ions. Morphological, thermal, mechanical, and optical properties were examined as a function of both organophilic bentonite and LMA content. Addition of LMA improved the compatibility between the layered silicate and the acrylic matrix, thus promoting bentonite intercalation and formation of anisotropic laminated silicate nanoparticles of an average diameter of 18 nm, average length of 450 nm, and interlayer distance of 4.8 nm, as determined by WAXS, TEM, and AFM. Addition of 2–10 wt % of intercalated layered silicate accounted for improved stiffness/toughness balance, higher glass temperature, and enhanced thermal stability, with respect to the properties of the corresponding MMA/LMA copolymer. As a result of the addition of LMA, translucent acrylic nanocomposites were obtained.