Synthesis and Characterization of Cationic Dendrimer-PDMS Hybrids.

A modular strategy for the synthesis of dendron-linear polymer hybrids comprised of a flexible polydimethylsiloxane (PDMS) midblock with cationic 2,2-bis(hydroxymethyl)propionic acid (bis-MPA) dendron end groups is developed. The invention of a scalable methodology to access quaternary ammonium carboxylate building blocks and their direct use in esterification chemistry enables rapid access to cationic bis-MPA dendrons. The convergent click coupling of highly charged dendrons to hydrophobic PDMS chain-ends gives a 12-membered family of hybrids that are comprised of different dendron generations (G1-3) and quaternary ammonium alkyl chain lengths (C4 , C8 , C12 , C16 ). This provides a library of materials with variable hydrophobicity, charge density, and chain-end valency. The physical behavior of the dendron-linear PDMS hybrid copolymers significantly changes after introduction of the cationic dendron end-groups and leads to soft-solid materials as a result of inhibited chain mobility. These PDMS-dendron hybrids are expected to behave as surface-active antimicrobial additives in bulk cross-linked silicone systems.

Stress-induced surface instabilities and defects in thin films sputter deposited on compliant substrates.

Existing analyses predict that thin metal films deposited on compliant substrates are subject to a variety of surface instabilities, such as wrinkles, folds, creases, etc., that become more prominent with increased compressive residual stress. Under compressive stress, cracks have been assumed to form only when the interfacial strength is weak, allowing the film to detach from the substrate. In this work, we demonstrate that cracks also form on surfaces under compressive mismatch strain when the interface is strong. In particular, we consider metal alloy films sputter deposited under bias on elastomers with different thicknesses, curing temperatures or surface treatments. The deposition parameters created residual compressive strains and strong adhesion in the bilayers. Samples without surface treatment formed wrinkles and through-thickness cracks at 0.25-0.4% mismatch strains. Only through-thickness cracks were observed in UV treated samples. The crack spacing was found to decrease by a factor of 4 when the surface was UV treated and by a factor of 3 as the elastomer thickness decreased from 30 to 6 µm. Cracks penetrated through the elastomer, 15-30 times deeper than the film thickness, and formed in all samples with a brittle coating. A numerical model was developed to explain the formation of through-thickness cracks and wrinkles under applied compressive mismatch strains. The model suggests that cracks can initiate from the peak of wrinkles when the critical fracture strength of the coating is exceeded. For the UV treated samples, through-thickness cracks are possibly impacted by the formation of an embrittled near surface PDMS layer.

Implementing magnetically-active Sn-based nanocomposites in hexavalent chromium removal from drinking water.

A novel nanocomposite consisting of Fe3O4-loaded tin oxyhydroxy-chloride is demonstrated as an efficient adsorbent for the removal of hexavalent chromium in compliance to the new drinking water regulation. This study introduces a continuous-flow production of the nanocomposite through the separate synthesis of (i) 40 nm Fe3O4 nanoparticles and (ii) multilayered spherical arrangements of a tin hydroxy-chloride identified as abhurite, before the application of a wet-blending process. The homogeneous distribution of Fe3O4 nanoparticles on the abhurite's morphology, features nanocomposite with magnetic response whereas the 10 % loaded nanocomposite preserves a Cr(VI) uptake capacity of 7.2 mg/g for residual concentrations below 25 µg/L. Kinetic and thermodynamic examination of the uptake evolution indicates a relative rapid Cr(VI) capture dominated by interparticle diffusion and a spontaneous endothermic process mediated by reduction to Cr(III). The efficiency of the optimized nanocomposite was validated in a pilot unit operating in a sequence of a stirring reactor and a rotary magnetic separator showing an alternative and competitive application path than typical fixed-bed filtration, which is supported by the absence of any acute cellular toxicity according to human kidney cell viability tests.

Tin Oxide Nanoparticles via Solar Vapor Deposition for Hexavalent Chromium Remediation.

Tin oxide nanoparticles optimized to capture low concentrations of hexavalent chromium from water were developed through a facile, scalable, and low-cost one-step solar vapor deposition methodology. Considering the preservation of high electron donation capacity as the key to support the reduction of mobile Cr(VI) into insoluble forms, the growth of SnO nanoparticles was favored by the co-evaporation of SnO2 with Fe powders at various mass ratios. Characterization techniques indicated that the percentage and the stability of SnO is proportional to the Fe content in the target with a requirement of at least 50% wt to inhibit the formation of a passive SnO2 surface layer. The produced particles were evaluated regarding their efficiency to capture Cr(VI) under conditions similar to water treatment for drinking purposes (pH 7). It was revealed that passivation-free SnO nanoparticles deliver significant improvement in the adsorption capacity corresponding to the residual concentration of 25 µg/L, reaching a value of 1.74 mg/g for the sample prepared with 50% wt Fe in the target. The increase of water acidity was found responsible for the activation of more reduction sites on the particle surface, as reflected through the elevation of efficiency by more than 20% at pH 6.

Origin, implications and management strategies for nitrate pollution in surface and ground waters of Anthemountas basin based on a δ15N-NO3- and δ18O-NO3- isotope approach.

Nitrate pollution of surface and groundwater resources is a major worldwide environmental problem. In this study nitrogen isotopes of water, soil, fertilizer and manure were analyzed to determine the pollution sources of nitrate in the groundwater and surface waters of Anthemountas basin. The SIAR model and multivariate statistical analysis were used to determine and quantify the contribution of different NO3̄ sources in groundwater and surface water. Additionally, a detailed literature overview was carried out to identify the origin of nitrate pollution in surface and ground waters based on ΝΟ3- isotopes. The Piper diagram identified the dominant water types as Mg-Ca-HCO3 and Ca-Mg-HCO3. Nitrate concentrations reached 162.0 mg/L in groundwater and 39.0 mg/L in surface waters. The main source of nitrate in groundwater was mainly nitrified ammonium-based synthetic urea and less nitrate-based synthetic fertilizers. The correlation of SIAR results with other trace elements revealed a negative correlation between hexavalent chromium and a) nitrate-based synthetic fertilizers, and b) nitrification of urea synthetic fertilizers. However, a positive correlation was observed between hexavalent chromium and anthropogenic organic matter. The literature overview provided the basis to design a novel management protocol for nitrate pollution that includes three steps: a) fundamental research, b) management tools, c) monitoring and preservation actions. However, an integrated management protocol for nitrate pollution requires a deeper understanding of the hydro-system and the full participation of local farmers and stakeholders.

Adsorption of Se(IV) and Se(VI) species by iron oxy-hydroxides: Effect of positive surface charge density.

Batch and continuous mode experiments were used to determine the influence of physic-chemicals characteristics of iron oxy-hydroxides (FeOOHs) on selenium adsorption. Batch experiments and continuous flow rapid small-scale column tests (RSSCTs) at pH 7 and NSF (National Sanitation Foundation) water matrix, showed that the adsorption capacity of FeOOHs for Se(IV) is strongly related to positive surface charge density (PSCD), and gradually increases when synthesis pH is lowered. The highest PSCD value of 3.25 mmol [OH-]/g was observed at synthesis pH 2.5 (FeOOH/2.5) and the lowest, 0.45 mmol [OH-]/g, was observed at synthesis pH 9 (FeOOH/9). A thermodynamic study verified the endothermic (ΔΗ° 21.4 kJ/mol) chemisorption of Se(IV) by the qualified FeOOH/2.5. EXAFS data showed that Se(IV) is involved in three types of surface complexes: bidentate mononuclear edge-sharing (1E) and two types of binuclear inner-sphere (2C) linkage between the SeO32- pyramids, and Fe(O,OH)6 octahedra. The FeOOHs were evaluated by their adsorption capacity (Q10) at residual concentrations equal to the EU drinking water regulation limit of 10 µg/L, e.g. in conditions implemented in full-scale water treatment plants. The qualified FeOOH/2.5 was found to be the most effective for Se(IV) adsorption with a Q10 value 4.3 mg Se(IV)/g. In contrast, the Q10 value for Se(VI) was almost three orders of magnitude lower (10 µg Se(VI)/g) than that for Se(IV). Finally, regeneration experiments showed that FeOOHs reuse for Se(IV) removal is economically feasible and the recovery of selenium by precipitation as elemental Se contributes to green chemistry.

The Effect of Cellulose Nanocrystal Coatings on the Glass Fiber-Epoxy Interphase.

This study focuses on understanding the effect of cellulose nanocrystals (CNCs) on glass fiber/epoxy interfacial interactions. The glass fibers (GF) were coated with solutions containing cellulose nanomaterial. The parameters that were investigated were the CNC surface chemistry, concentration, and dispersing medium, i.e., aqueous solution only versus emulsions. To determine the effect of the CNC coatings on the interfacial adhesion, specimens of a single GF in an epoxy matrix were prepared for GF coating by varying the coating formulations. The interfacial shear stress (IFSS) was determined by the single fiber fragmentation test (SFFT). Following the SFFT, the samples were investigated by cross-polarized microscopy in order to understand the fracture modes which are related to the nature of the interphase. According to the SFFT data and photoelastic fracture patterns, both the emulsion and aqueous coatings containing cellulose nanocrystals functionalized with methyl(triphenyl) phosphonium (CNCPh) improve the IFSS in comparison to coated GFs without CNCs.

Environmentally available hexavalent chromium in soils and sediments impacted by dispersed fly ash in Sarigkiol basin (Northern Greece).

Hexavalent chromium is one of the most toxic and carcinogenic species known and can be released into the environment from several sources. In Sarigkiol basin (N Greece) the presence of Cr(VI) in soil, sediments and groundwater may originate from both natural (ophiolitic rocks and their weathering products) and anthropogenic (dispersed fly ash produced from lignite power plants) sources. In this study, the distribution of contents and origin of environmentally available Cr(VI) in soils, sediments, regoliths and fly ash of Sarigkiol basin is presented. Detailed geochemical and mineralogical studies were performed on soil samples (up to 1 m) and regoliths, while leaching tests were also applied to fresh and old fly ash samples. Leachable chromium from soil and sediment samples generally increased with depth and the highest concentrations were observed near to the power plant of Agios Dimitrios. The speciation of chromium in leachates revealed that Cr(VI) concentrations accounted for more than 96% of total Cr. Leaching tests of regoliths established that the natural contribution of Cr(VI) is up to 14 µg kg-1. Therefore, the measurement of higher concentrations (up to 80 µg kg-1) of environmentally available Cr(VI) in soils and sediments can be attributed to the impact/presence of dispersed fly ash in the soils and sediments of the same area. This was also supported by the low correlation recorded between environmentally available chromium and Cr-bearing minerals (mainly serpentine and talc). The influenced zone is located in the eastern part of the basin near the local power plant and surrounds an open conveyor belt that transfers fly ash to an open temporary storage pit. This zone overlies an unconfined porous aquifer thus explaining the elevated concentrations of Cr(VI) in groundwater (up to 120 µg L-1) previously reported in this area.

The use of Sn(II) oxy-hydroxides for the effective removal of Cr(VI) from water: Optimization of synthesis parameters.

The development of a novel adsorbent based on Sn(II) oxy-hydroxide nanoparticles and the optimization of main synthesis parameters was examined for the efficient removal of hexavalent chromium at low residual concentration levels. The aqueous hydrolysis of Sn(II) salts in a continuous-flow process was evaluated as an effective method to synthesize an appropriate material able to operate both as an electron donor for Cr(VI) reduction, and provide a suitable crystal structure that favors strong complexation with the formed Cr(III) species. Experimental results revealed that the main hydrolysis parameters, such as pH value and tin origin/source, can be used to determine the chemical formula of the produced materials and thereby, eventually improve their uptake capacity for Cr(VI). Among the tested sorbent materials, the synthetic nanostructured hydroromarchite, Sn6O4(OH)4, prepared by the hydrolysis of SnCl2 in a highly acidic environment (pH2), was deemed the best sorbent material and it was further investigated for its Cr(VI) uptake performance under reliable conditions (column experiments) for drinking water treatment. Specifically, Rapid Small-Scale (laboratory) Column Tests indicated that aggregates of the Sn6O4(OH)4 nanomaterial can achieve a maximum uptake capacity of around 19mg/g, keeping the levels of outflow Cr(VI) below 10µg/L during the treatment of natural-like water at pH7. The high efficiency is mainly attributed to the stabilization of Sn(II) content in nanoparticles, as well as the improved surface charge density, reaching 1.0mmol[OH-]/g, whereas the obtained thermodynamic data indicate a combined reduction-sorption process. The latter aspect was further verified by XPS, showing that even in the highly-loaded sorbent materials with adsorbed chromium, its trivalent form is the predominant one. These specific characteristics suggest that the product is a more favorable candidate for wider applications in water treatment units, regarding Cr(VI) removal, compared to other examined sorbent materials.

High Thermal and Electrical Conductivity of Template Fabricated P3HT/MWCNT Composite Nanofibers.

Nanoporous alumina membranes are filled with multiwalled carbon nanotubes (MWCNTs) and then poly(3-hexylthiophene-2,5-diyl) (P3HT) melt, resulting in nanofibers with nanoconfinement induced coalignment of both MWCNT and polymer chains. The simple sonication process proposed here can achieve vertically aligned arrays of P3HT/MWCNT composite nanofibers with 3 wt % to 55 wt % MWCNT content, measured using thermogravimetric methods. Electrical and thermal transport in the composite nanofibers improves drastically with increasing carbon nanotube content where nanofiber thermal conductivity peaks at 4.7 ± 1.1 Wm(-1)K(-1) for 24 wt % MWCNT and electrical percolation occurs once 20 wt % MWCNT content is surpassed. This is the first report of the thermal conductivity of template fabricated composite nanofibers and the first proposed processing technique to enable template fabrication of composite nanofibers with high filler content and long aspect ratio fillers, where enhanced properties can also be realized on the macroscale due to vertical alignment of the nanofibers. These materials are interesting for thermal management applications due to their high thermal conductivity and temperature stability.

Processing and Characterization of Cellulose Nanocrystals/Polylactic Acid Nanocomposite Films.

The focus of this study is to examine the effect of cellulose nanocrystals (CNC) on the properties of polylactic acid (PLA) films. The films are fabricated via melt compounding and melt fiber spinning followed by compression molding. Film fracture morphology, thermal properties, crystallization behavior, thermo-mechanical behavior, and mechanical behavior were determined as a function of CNC content using scanning electron microscopy, differential scanning calorimetry, X-ray diffraction, dynamic mechanical analysis, and tensile testing. Film crystallinity increases with increasing CNC content indicating CNC act as nucleating agents, promoting crystallization. Furthermore, the addition of CNC increased the film storage modulus and slightly broadened the glass transition region.

Poly(3-hexylthiophene) nanotube array surfaces with tunable wetting and contact thermal energy transport.

Solution casting using a sacrificial template is a simple technique to fabricate vertical arrays of polymer nanotubes. However, because of their close proximity and high aspect ratios, large capillary forces cause nanotubes to cluster as the array dries; researchers often use special drying techniques to avoid this clustering. Here, we exploit the clustering of regioregular poly(3-hexylthiophene) (rr-P3HT) nanotubes in a unique template etching process to create surfaces that exhibit tunable wetting and contact thermal energy transport. Vertical arrays of rr-P3HT nanotubes are cast from solution in nanoscale alumina templates, and a solution etching process is used to partially release the nanotubes from the template. The clustering of rr-P3HT nanotube tips upon template etching produces hierarchical surface structuring with a distinct pattern of interconnected ridges, and the spacing between the ridges increases with increased template etch times. These changes in morphology cause the water contact angle to increase from 141° to 168° as the etch time is increased from 4 to 12 min. When assembled into an interface, the morphological changes cause the thermal contact resistance of the vertical rr-P3HT nanotube arrays to increase linearly at a rate of approximately 6 mm(2)·K/W per 2 min etch interval (after 6 min of etching is surpassed). The effective thermal conductivity of the rr-P3HT nanotube arrays is 1 ± 0.2 W/mK independent of the etch time, which is approximately 5 times higher than the bulk rr-P3HT film value.

Enhanced molecular order in polythiophene films electropolymerized in a mixed electrolyte of anionic surfactants and boron trifluoride diethyl etherate.

We synthesized polythiophene (PTh) films on stainless steel electrodes using chronoamperometry in boron trifluoride diethyl etherate (BFEE) electrolyte with anionic surfactants. The presence of the anionic surfactants in BFEE reduced the oxidation potential of thiophene and increased the oxidation current during electropolymerization. The measured in-plane electrical conductivity of PTh films synthesized in the presence of anionic surfactants was up to 300% higher than that of films synthesized under similar conditions without surfactants. The observed increase in conductivity reflects the improved order and packing of polymer chains revealed by X-ray diffraction.