Enhanced thermal conductivity of polyamide-based nanocomposites containing graphene oxide sheets decorated with compatible polymer brushes

Polyamide-based nanocomposites containing graphene platelets decorated with poly(acrylamide) brushes were prepared and characterized. The brushes were grafted from the surface of graphene oxide (GO), a thermally conductive additive, using atom transfer radical polymerization, which led to the formation of the platelets coated with covalently tethered polymer layers (GO_PAAM), accounting for ca. 31% of the total mass. Polyamide-6 (PA6) nanocomposites containing 1% of GO_PAAM were formed by extrusion followed by injection molding. The thermal conductivity of the nanocomposite was 54% higher than that of PA6 even for such a low content of GO. The result was assigned to strong interfacial interactions between the brushes and PA6 matrix related to hydrogen bonding. Control nanocomposites containing similarly prepared GO decorated with other polymer brushes that are not able to form hydrogen bonds with PA6 revealed no enhancement of the conductivity. Importantly, the nanocomposite containing GO_PAAM also demonstrated larger tensile strength without deteriorating the elongation at break value, which was significantly decreased for the other coated platelets. The proposed approach enhances the interfacial interactions thanks to the covalent tethering of dense polymer brushes on 2D fillers and may be used to improve thermal properties of other polymer-based nanocomposites with simultaneous enhancement of their mechanical properties

Airborne magnetic technoparticles in soils as a record of Anthropocene

Airborne magnetic particles in soils were studied in sites located in various distances to industrial plants. Chemical and mineral composition of soil samples were analysed. The highest values of the Pollution Load Index (PLI) calculated for several elements were noted for sites relatively distant from industrial plants. Soil samples exposed for the deposition of airborne particles were examined by means of magnetization versus an external applied magnetic field as well as ^{52}Fe Mössbauer spectroscopy measurements showing a lack of correlation of the magnetic properties with total Fe content, which points toward a strong impact of the industrial activities. Magnetic fraction was extracted and studied using scanning electron microscopy with energy dispersive spectrometry. Two types of spherical particles were noted. Massive ones occurring in all sites and particles with a complex internal structure more abundant in sites situated closer to industrial plants. The presence of spherical magnetic particles formed in high temperature processes indicate their relatively long range transport in the atmosphere. Broad distribution, characteristic and easy for identification spherical form of these magnetic technoparticles, suggest that one can consider them as a record of the Anthropocene. Accumulation of such technoparticles is related to rapid industrialization in the post-World War II period

Hydrophobically coated superparamagnetic iron oxides nanoparticles incorporated into polymer-based nanocapsules dispersed in water

This paper reports the characterization of iron oxide magnetic nanoparticles obtained via the thermal decomposition of an organometallic precursor, which were then loaded into nanocapsules prepared via the emulsification process in the presence of an amphiphilic derivative of chitosan. The applied synthetic method led to the formation of a hydrophobic layer on the surface of nanoparticles that enabled their loading in the hydrophobic liquid inside of the polymer-based capsules. The average diameter of nanoparticles was determined to be equal to 15 nm, and they were thoroughly characterized using X-ray diffraction (XRD), magnetometry, and Mössbauer spectroscopy. A core–shell structure consisting of a wüstite core and maghemite-like shell was revealed, resulting in an exchange bias effect and a considerable magnetocrystalline anisotropy at low temperatures and a superparamagnetic behavior at room temperature. Importantly, superparamagnetic behavior was observed for the aqueous dispersion of the nanocapsules loaded with the superparamagnetic nanoparticles, and the dispersion was shown to be very stable (at least 48 weeks). The results were analyzed and discussed with respect to the potential future applications of these nanoparticles and nanocapsules based on biopolymers as platforms designed for the magnetically navigated transport of encapsulated hydrophobic substances

Lateral resolution in focused electron beam-induced deposition: scaling laws for pulsed and static exposure

In this work, we review the single-adsorbate time-dependent continuum model for focused electron beam-induced deposition (FEBID). The differential equation for the adsorption rate will be expressed by dimensionless parameters describing the contributions of adsorption, desorption, dissociation, and the surface diffusion of the precursor adsorbates. The contributions are individually presented in order to elucidate their influence during variations in the electron beam exposure time. The findings are condensed into three new scaling laws for pulsed exposure FEBID (or FEB-induced etching) relating the lateral resolution of deposits or etch pits to surface diffusion and electron beam exposure dwell time for a given adsorbate depletion state

Hyperfine interactions and irreversible magnetic behavior in multiferroic aurivillius compounds

In this work investigations of structure and magnetic properties of conventionally sintered Bim+1Ti3Fem–3O3m+3 compounds with 4 ≤ m ≤ 8 were performed using X-ray diffraction, Mössbauer spectroscopy and vibrating sample magnetometry. Room-temperature Mössbauer spectra of the compounds correspond to a paramagnetic state, however, low temperature measurements (80 K) reveal the antiferromagnetic state with a residual paramagnetic phase. Temperature dependencies of magnetic susceptibility, χσ(T), provided magnetic ordering temperatures and revealed an irreversibility in Aurivillius compounds with m ≥ 5. In the case of Bi5Ti3FeO15 compound the χσ(T) dependence shows a paramagnetic behavior down to 2 K. The Bi6Ti3Fe2O18 compound reveals a magnetic ordering at 11 K. The compounds with m = 6–8 show a magnetic ordering at temperatures higher than 200 K. Highly irreversible character of their temperature dependencies of χσ indicates a spin-glass type disordered magnetism with frustration due to a random distribution of Fe on Ti at their sites

T1−T2T_{1}-T_{2} dual-modal MRI contrast agents based on superparamagnetic iron oxide nanoparticles with surface attached gadolinium complexes

Dual-mode MRI contrast agents consisting of superparamagnetic iron oxide nanoparticle (SPION) cores and gadolinium ions associated with the ionic chitosan protecting layer were synthesized and studied. Gadolinium ions were introduced into the coating layer via direct complex formation on the nanoparticles surface, covalent attachment or electrostatically driven deposition of the preformed Gd complex. The modified SPIONs having hydrodynamic diameters ca. 100 nm form stable, well-defined dispersions in water and have excellent magnetic properties. Physiochemical prop- erties of those new materials were characterized using e.g., FTIR spectroscopy, dynamic light scattering, X-ray fluorescence, TEM, and vibrating sample mag- netometry. They behave as superparamagnetics and shorten both T 1 and T 2 proton relaxation times, thus influencing both r 1 and r 2 relaxivity values that reach 53.7 and 375.5 mM - 1 s - 1 , respectively, at 15 MHz. The obtained materials can be considered as highly effective contrast agents for low-field MRI, particu- larly useful at permanent magnet-based scanners

Dynamics of superparamagnetic iron oxide nanoparticles with various polymeric coatings

In this article, the results of a study of the magnetic dynamics of superparamagnetic iron oxide nanoparticles (SPIONs) with chitosan and polyethylene glycol (PEG) coatings are reported. The materials were prepared by the co-precipitation method and characterized by X-ray diffraction, dynamic light scattering and scanning transmission electron microscopy. It was shown that the cores contain maghemite, and their hydrodynamic diameters vary from 49 nm for PEG-coated to 200 nm for chitosan-coated particles. The magnetic dynamics of the nanoparticles in terms of the function of temperature was studied with magnetic susceptometry and Mössbauer spectroscopy. Their superparamagnetic fluctuations frequencies, determined from the fits of Mössbauer spectra, range from tens to hundreds of megahertz at room temperature and mostly decrease in the applied magnetic field. For water suspensions of nanoparticles, maxima are observed in the absorption part of magnetic susceptibility and they shift to higher temperatures with increasing excitation frequency. A step-like decrease of the susceptibility occurs at freezing, and from that, the Brown’s and Néel’s contributions are extracted and compared for nanoparticles differing in core sizes and types of coating. The results are analyzed and discussed with respect to the tailoring of the dynamic properties of these nanoparticle materials for requirements related to the characteristic frequency ranges of MRI and electromagnetic field hyperthermia