Crystallization of Polytetrafluoroethylene in a Wide Range of Cooling Rates: Nucleation and Diffusion in the Presence of Nanosilica Clusters.

Polytetrafluoroethylene (PTFE) is a polymer that displays exceptional properties. This synthetic fluoropolymer is also known to crystallize very fast upon cooling. The present work highlights for the first time the influence of nanosilica clusters on PTFE crystallization at fast cooling rates (up to 5000 K·s-1). The silica was synthesized from aqueous silicate solution and the surface modification was performed using TriEthoxyFluoroSilane (TEFS). In order to understand the crystallization behavior of PTFE/silica nanocomposite at a fast cooling rate, the measurements were carried out by Fast Scanning Calorimetry (FSC). The data were consequently combined with the measurements performed by conventional Differential Scanning Calorimetry (DSC). Interestingly, the results displayed variation of the crystallization behavior for the nanocomposite at fast cooling rates compared to slow cooling rates. The differences in crystal morphologies were then observed by Scanning Electron Microscopy (SEM) after slow and fast cooling rates. Finally, the effective activation energies (Eα) obtained from the crystallization under various cooling rates were combined in order to obtain one set of Hoffman-Lauritzen parameters. This procedure allowed us to show that the crystallization of PTFE in the presence of silica is promoted or hampered according to the cooling rates employed.

Chemical insights into the formation of Cu2ZnSnS4 films from all-aqueous dispersions for low-cost solar cells.

Cu2ZnSnS4 (CZTS) shows great potential for photovoltaic application because of its non-toxic earth-abundant components and good optoelectronic properties. Combining low-cost and environmentally friendly routes would be the most favorable approach for the development of CZTS solar cells. In this context, development of Cu2ZnSnS4 (CZTS) films from all-aqueous CZTS nanocrystals inks represents an interesting challenge. Here, we have highlighted a condensation regulation by the alkali ion size observed in the alkali series Li+ < Na+ < K+ < Rb+ < Cs+, and demonstrated the chemical stability of Cu2ZnSnS4 surfaces in basic aqueous dispersions. Data such as optimal nanocrystal size, critical cracking thickness and average thickness to fabricate micron crack-free films from all-aqueous chalcogenide nanocrystals dispersions were determined. From these results, a proof of concept for the formation of a crack-free film of 2.2 µm formed from an all-aqueous CZTS nanocrystals ink is given. When employing low-cost materials, removal of carbon impurities represents another important challenge. With the objective to fabricate residue-free films, a specific annealing strategy is proposed involving a high temperature purification step under Se partial pressure. Carbon removal is thus achieved via the CSe2 gas formation, simultaneously to the amorphous domains crystallization as demonstrated by Raman spectroscopy. These source data favoring the formation of residue-free, crack-free, annealed films should assist the large scale development of CZTS solar cells from low-cost and environmentally friendly, all -aqueous inks.

Adsorption of nickel ions by oleate-modified magnetic iron oxide nanoparticles.

In this work, magnetic nanoparticles of iron oxide (MNPs) were synthesized, and then the surface was recovered with an oleate double layer in order to investigate the ability of this material to adsorb nickel ions. First, the solution chemistry of oleate ions was investigated in order to determine the critical micellar concentration (CMC) value and the arrangements of ions above the CMC. Then, the synthesized oleate-modified MNP was characterized (TEM, DLS, XRD, FTIR, zeta potential, magnetometry). Finally, adsorption experiments were carried out as a function of pH and as a function of nickel concentration in 0.1 g L-1 suspensions of oleate-modified MNP. The results show that CMC of oleate ranges from 1 to 2.5∙10-3 mol L-1. Above CMC, arrangement of oleate ions as droplets, vesicles, or micelles depends on pH and influences the average size and solution absorbance. Potentiometric titrations allowed determining a pKa value of 7.8 for sodium oleate. The high stability in aqueous suspensions and characterization of oleate-modified MNP confirm that oleate ions are arranged as a bilayer coating at the surface of MNP. Retention of nickel was found to be highly dependent on pH, with a maximum adsorption (90%) beginning from pH = 7.5. The sorption isotherms were well fitted with the Langmuir model and the maximum nickel adsorption capacities were found to be 44 and 80 mg g-1 for pH = 6.8 and 7.2, respectively. The efficient removal of nickel combined with the magnetic properties of the NMP make the oleate-modified MNP an interesting water purification tool.

Melt and glass crystallization of PDMS and PDMS silica nanocomposites.

Silica nanoclusters were homogeneously dispersed into an end-linked PolyDiMethylSiloxane (PDMS) matrix. Dynamic relaxation, glass and melt crystallization of end-linked PDMS-silica nanocomposites (PDMSnanoSi) were compared to those of PDMS. A particular emphasis is made on the kinetic aspects of these transitions by corroborating investigations conducted by means of Dynamic Mechanical Analysis (DMA) and Differential Scanning Calorimetry (DSC). Addition of silica nanoclusters does not modify the relaxation behavior of the amorphous phase and the glass transition kinetics. However, melt and glass crystallizations are significantly promoted in the presence of silica nanoclusters. The secondary crystallization process is more pronounced for PDMSnanoSi and higher crystal perfection due to structuring effects of silica nanoclusters is also highlighted. For the two systems, one set of Hoffman-Lauritzen parameters have been evaluated by combining melt and glass crystallization kinetic data.

PAH contaminated soil remediation by reusing an aqueous solution of cyclodextrins.

We studied the possibility to re-use an aqueous solution of methyl-beta-cyclodextrin (bpmCD) in order to decontaminate a soil polluted by phenanthrene and pyrene. The loss of bpmCD in the soil was insignificant. In order to eliminate polycylic aromatic hydrocarbons (PAHs) from the contaminated aqueous solution, on one hand we tested their photodegradation using TiO(2) suspensions. But it was inefficient, because of the stabilisation of PAHs within the cavity of bpmCD. On the other hand, we removed PAHs by liquid-liquid extraction with colza oil. This allowed the regeneration of cyclodextrins, by concentrating the pollutants in the organic phase with a small loss of carrier. Contaminated soils were almost completely de-polluted after 2d of re-circulation, using a 10mM solution of bpmCD. To reduce the amount of bpmCD loss in the oil phase, we set the fraction of colza oil low, by using a micro-emulsion or by impregnating an organic membrane with the oil. We found this last possibility more interesting.

How do colloidal aggregates yield to compressive stress?

Aqueous dispersions of silica nanoparticles have been aggregated through the addition of Al13 polycations and then submitted to osmotic compression. The structures of these dispersions have been determined through small-angle neutron scattering, before and after compression. Some dispersions consisted of mixtures of aggregated and nonaggregated particles--actually a few aggregates dispersed in a "sea" of nonaggregated particles. In these dispersions, it was found that the resistance to osmotic compression originated from the ionic repulsions of the nonaggregated particles; the compression law that related the applied osmotic pressure Pi to the silica volume fraction Phi was Pi approximately [Phi/(1-Phi)]2. Other dispersions were fully aggregated, with all particles forming a fractal network that extended throughout the available volume. In these dispersions, it was found that the resistance to compression originated from surface-surface interparticle bonds. The application of low osmotic pressures (<50 kPa) resulted in compression at macroscopic scales only (>300 nm), while the structure of the network at local and mesoscopic scales was unchanged. Accordingly, few interparticle bonds were broken, and the deformation was primarily elastic. The compression law for this elastic deformation was in agreement with the predicted scaling law Pi approximately Phi4. The application of higher osmotic pressures (>50 kPa) resulted in compression at macroscopic and mesoscopic scales (30-300 nm), while the local structure was still retained. Accordingly, many more interparticle bonds were broken. The compression law for this plastic deformation was in agreement with a scaling prediction of Pi approximately Phi1.7. The location of the elastic-plastic transition indicated that the strength of the interparticle bonds was on the order of 5 times the thermal energies at ambient temperature.

Electrorheological properties and microstructure of silica suspensions.

We present experimental and theoretical results on the electrorheological response and microstructure of colloidal suspensions composed of silica nanoparticles dispersed in a silicon oil, as a function of electric field strength and silica water content. Using small-angle neutrons scattering experiments, we determined the evolution of the static structure factor of the suspensions when an electric field is applied. Experimental data were fitted with model calculations using the Percus-Yevick solution for Baxter's hard-sphere adhesive potential. The obtained stickiness parameter is directly related to the polarization interactions that depend on the water content of silica particles. The influence of the polarization interparticle potential on the rheology of the silica dispersions was investigated in a second time. A microscopic theory for the shear viscosity of adhesive hard-sphere suspensions was successfully used which describes the steady shear viscosity of suspension in terms of the fractal concept.