Phase separation of polymer blends in solution near adsorbing surface

In this paper, we consider a mixture of two polymers A and B of different chemical nature, dissolved in a common good solvent, in contact with an interacting surface. We start from a mixture of two incompatible homopolymers A and B in the molten state, and assume that the surface adsorbs strongly one or both polymer species at high temperature. It is assumed that this is a strong adsorption, so that chains cannot desorb once they are linked to the surface. This constrains the system to a quenched composition on the surface. Once the adsorption process is finished, a quantity of a good solvent is added to get a semi-dilute solution. We assume that demixing transition in the presence of solvent occurs at lower temperature. The purpose is to discuss the influence of the quenched surface fluctuations on the critical properties of the mixture. Within the framework of the so-called blob model, we determine the exact shape of the composition profile as a function of the distance z to the surface, for any value of the relevant parameters, namely, the temperature T, the molecular weight M, the monomer concentration c and the surface composition x₀. Our analysis reveals a universal character of the composition profile for , where the characteristic size D is some known length depending on the relevant parameters of the problem, and not on temperature, and is the thermal correlation length. Near surface, for (a is the monomer size), the profile is no longer universal, and in particular, it is sensitive to the boundary condition. Far from the surface, that is , the profile tends exponentially to its bulk value. We show that the length Dapproaches its lowest value as the surface composition reaches its saturated value l. In this limit, we find that the profile shape is a characteristic of critical adsorption in simple binary fluid mixtures. Finally, this work must be regarded as a natural extension of a previous one, which was concerned with the same problem, but in the absence of solvent. Received 24 June 1999 and Received in final form 5 November 1999     

Continuous synthesis of colloidal silver nanoparticles by electrochemical discharge in aqueous solutions

This article presents an electrochemical discharge (ECD) method that consists of a combination of chemical methods and electric arc discharges. In the method, 140 V is applied to an Ag electrode from a DC power supply. The arc-discharge between the electrodes produces metallic silver nanoparticles and silver ions in the aqueous solution. Compared with the original arc discharge, this ECD method creates smaller nanoparticles, prevents clumping of the nanoparticles, and shortens the production time. The citrate ions also reduce the silver ions to silver nanoparticles. In addition, the citrate ions cap the surface of the produced silver nanoparticles and the zeta potential increases. In this article, the weight loss of the electrodes and the reduction of silver ions to silver nanoparticles as a function of citrate concentration and electric conductivity of the medium are discussed. Furthermore, the properties of the colloidal silver prepared with ECD are analyzed by UV–Vis spectroscopy, dynamic light scattering, electrophoresis light scattering, and scanning electron microscopy. Finally, a continuous production apparatus is presented for the continuous production of colloidal silver.     

Depletion interactions between colloidal particles in polymer solutions: density functional approach

A density functional theory for colloid–polymer mixtures based on the weighted-density approximation has been developed to investigate the depletion effects acting between two colloids immersed in a bath of polymers and the depletion effects for a colloid near a planar hard wall. The theoretical results for the polymer-induced depletion interactions and the local polymer density distributions are in good agreement with the computer simulations. The calculation shows that the depletion interaction for a colloid near a planar hard wall is much stronger than that between two colloids in a polymer solution because of the strong confinement effect. The behaviour of the depletion interactions has been analysed as a function of the polymer density, the polymer chain length, and the colloid/polymer size ratio. Strong depletion effects appear in short-chain systems and with large colloid/polymer size ratios.     

Magnetooptical studies of aggregates of nanoparticles in colloidal solutions of magnetite

The birefringence in a colloidal solution of nanosized magnetite particles in kerosene exposed to constant, alternating, and pulsed magnetic fields is studied. Data on the birefringence kinetics in nonstationary magnetic fields is used to determine the hydrodynamic radius of particle aggregates in solutions. The permanent dipole moment of aggregates and the anisotropy of the magnetic susceptibility are calculated based on the data of magnetooptical experiments. It is shown that the induced dipole moment plays a significant role in an orientation of aggregates of magnetic nanoparticles under the effect of a field.     

Phase separation in mixtures of colloidal platelets and non-adsorbing polymer: a scaled particle treatment

Scaled particle theory is used to calculate the equation of state for and the free volume accessible to a non-adsorbing polymer, represented by spheres, in a system of colloidal platelets, represented by cut spheres. At low densities the predictions agree very well with computer simulations. For infinitely thin discs, the predictions agree with existing simulation data for the isotropic phase. The analytical results are used to predict the isotropic-isotropic phase boundary as a function of platelet aspect ratio and polymer/colloid size ratio.     

Dispersion of nanoparticles: From organic solvents to polymer solutions

This work is devoted to a systematic study of nanoparticle dispersion by ultrasonication in different solutions: from organic solvents to polymer solutions. The cluster size of nanoparticles at different concentrations in both organic solvents and polymer solutions were directly characterized by Dynamic Light Scattering to study the effect of solid concentration, surfactant and polymer on the dispersion. It reveals that in stabilized suspensions, the smallest attainable size or aggregate size of nanoparticles is independent of solvent type and solid content over the tested range. Furthermore, nanoparticles in simple solvent and in polymer solutions had the similar evolution of cluster size and almost the same final size, which could be very helpful to optimize the dispersion of nanofillers in polymer solutions and nanocomposites. It is also shown that, with appropriate sonication amplitudes, the dispersion procedure developed for very dilute suspensions could be transferred to higher concentration suspensions or even to polymer suspensions.     

Silver nanoparticles in complex biological media: assessment of colloidal stability and protein corona formation

Engineered silver nanoparticles (AgNPs) are among the most used nanomaterials in consumer products, therefore concerns are raised about their potential for adverse effects in humans and environment. Although an increasing number of studies in vitro and in vivo are being reported on the toxicity of AgNPs, most of them suffer from incomplete characterization of AgNPs in the tested biological media. As a consequence, the comparison of toxicological data is troublesome and the toxicity evaluation still remains an open critical issue. The development of a reliable protocol to evaluate interactions of AgNPs with surrounding proteins as well as to assess their colloidal stability is therefore required. In this regard, it is of importance not only to use multiple, easy-to-access and simple techniques but also to understand limitations of each characterization methods. In this work, the morphological and structural behaviour of AgNPs has been studied in two relevant biological media, namely 10 % FBS and MP. Three different techniques (Dynamic Light Scattering, Transmission Electron Microscopy, UV–Vis spectroscopy) were tested for their suitability in detecting AgNPs of three different sizes (10, 40 and 100 nm) coated with either citrate or polyvinylpyrrolidone. Results showed that UV–Vis spectroscopy is the most versatile and informative technique to gain information about interaction between AgNPs and surrounding proteins and to determine their colloidal stability in the tested biological media. These findings are expected to provide useful insights in characterizing AgNPs before performing any further in vitro/in vivo experiment.     

Laser synthesis of aluminium nanoparticles in biocompatible polymer solutions

Pulsed laser ablation of Aluminium (Al) in pure water rapidly forms a thin alumina (Al₂O₃) layer which drastically modifies surface plasmon resonance (SPR) absorption characteristics in deep-UV region. Initially, pure aluminium nanoparticles (NPs) are generated in water without any stabilizers or surfactants at low laser fluence which gradually transform to stable Al-Al₂O₃ core-shell nanostructure with increasing either residency time or fluence. The role of laser wavelength and fluence on the SPR properties and oxidation characteristics of Al NPs has been investigated in detail. We also present a one-step in situ synthesis of oxide-free stable Al NPs in biocompatible polymer solutions using laser ablation in liquid method. We have used nonionic polymers (PVP, PVA and PEG) and anionic surfactant (SDS) stabilizer to suppress the Al₂O₃ formation and studied the effect of polymer functional group, polymeric chain length, polymer concentration and anionic surfactant on the incipient embryonic aluminium particles and their sizes. The different functional groups of polymers resulted in different oxidation states of Al. PVP and PVA polymers resulted in pure Al NPs; however, PEG and SDS resulted in alumina-modified Al NPs. The Al nanoparticles capped with PVP, PVA, and PEG show a good correlation between nanoparticle stability and monomeric length of the polymer chain.     

Processes of electronic excitation energy transfer between luminescent dye molecules in microheterogeneous systems based on polymer micellar solutions

The processes of electronic excitation energy transfer (EEET) between different-type dye molecules in polymer micellar solutions with different polyelectrolyte concentration have been analyzed. The possibility of regulating the EEET efficiency between dye molecules due to change of the structure of polymer micellar complexes has been established. The results presented are explained from the viewpoint of the fractal character of distribution of interacting particles. The experimental dependence of the fractal dimension of the distribution of dye molecules in polymer micellar solutions on the polyelectrolyte concentration has been obtained.     

The colloidal stability and core-shell structure of magnetite nanoparticles coated with alginate

The adsorption of alginate (Alg) onto the surface of in water dispersed Fe₃O₄ nanoparticles and zeta potential of alginate-coated Fe₃O₄ nanoparticles have been investigated to optimize the colloidal stability of Alg-coated Fe₃O₄ nanoparticles. The adsorption amount of Alg increased with the decrease of adsorption pH. The zeta potential of Fe₃O₄ nanoparticles shifted to a lower value after adsorption of Alg. The lower adsorption pH was the lower zeta potential of Fe₃O₄ nanoparticles became. The Alg-coated Fe₃O₄ nanoparticles were found to be stabilized by steric and electrostatic repulsions. Those prepared at pH 6 were not stable around pH 5, and those prepared at pH 4 became unstable at pH below 3.5. Alg of M_w 45 kDa was a little bit more adsorbed onto nanoparticles surface than that of M_w 24 kDa. An average Fe₃O₄ core size of 9.3 ± 1.7 nm was found by transmission electronic microscopy. An average hydrodynamic diameter of 30-150 nm was measured by photon correlation spectroscopy. However, an average core size of 10 nm and an average hydrodynamic diameter of 38 nm were estimated from the magnetization curve of the concentrated magnetic fluids (MFs). The maximum available saturation magnetization of MFs was about 3.5 kA/m.     

Phase transfer of Au nanoparticles using one chemical inducer: DDAB

Au nanoparticles in aqueous medium were transferred to organic solution via a remarkably simple one-step route under the effect of phase transfer inducer dimethyldioctadecylammonium bromide (DDAB). The feature of our method is that only one reagent (DDAB) was added purposely for the phase transfer. This is different from the literatures, where two reagents were used for the phase transfer of Au nanoparticles. One is an Au nanoparticle stabilizer; the other is a phase transfer agent. The effect of the inducer DDAB concentration was checked, and it was found that the optimum concentration of DDAB was 1 × 10⁻⁵ mol dm⁻³ for phase transfer of Au nanoparticles. The organosol, Au nanoparticle, and phase transfer were characterized by UV–vis spectra, TEM, EDS, ED, and FT-IR. Molecular structure of quaternary ammonium salt was used to explain the unique effect of DDAB for the phase transfer of nanoparticles.     

Formation mechanism of carbon nanotubes in the gas-phase synthesis from colloidal solutions of nanoparticles

Single- and multi-wall carbon nanotubes have been synthesized by the gas-phase catalytic reaction of colloidal solutions of metal nanoparticles using a vertical flow reactor. The reverse micelle solution of the Co–Mo nanoparticles with the mean diameter of 11 nm dissolved in toluene was injected directly into the reactor maintained at 1200 °C. The nanoparticles and the solvent act as the catalyst and carbon source, respectively. When the concentration of the thiophene additive is low (1 wt.%), the formation of SWNT bundles preferentially occurred. The SWNT bundles were present together with the relatively small metal nanoparticles with the diameter of 0.5–5.5 nm. It is likely that the original nanoparticles with the diameter of 11 nm break into smaller ones, 1–2 nm diameters, which is suitable for the SWNT growth. The synactic effect of Co and Mo was also observed.     

Surface coating of Al nanoparticles by using a wet ball milling method: A facile synthesis and characterization of colloidal stability

A facile surface coating of aluminum (Al) nanoparticles with various dispersants by using a wet ball milling method is reported. Various mixtures of Al nanoparticles (d = 30–130 nm) and dispersants in solvent were ball milled. The excellent surface coating was observed with coating thickness ranging from 10 to 13 nm. The resulting good colloidal stability confirmed by both visual inspection of colloidal precipitation and Turbiscan backscattering was attributed to a stable dispersant organic layer formed on Al nanoparticle surfaces after ball milling as observed in HRTEM images. This method can be extended to the synthesis of a variety of any other metallic nano-colloidal solutions.     

Agglomeration,colloidal stability,and magnetic separation of magnetic nanoparticles: collective influences on environmental engineering applications

Magnetic nanoparticles (MNPs) which exhibit magnetic and catalytic bifunctionalities have been widely accepted as one of the most promising nanoagents used in water purification processes. However, due to the magnetic dipole-dipole interaction, MNPs can easily lose their colloidal stability and tend to agglomerate. Thus, it is necessary to enhance their colloidal stability in order to maintain the desired high specific surface area. Meanwhile, in order to successfully utilize MNPs for environmental engineering applications, an effective magnetic separation technology has to be developed. This step is to ensure the MNPs that have been used for pollutant removal can be fully reharvested back. Unfortunately, it was recently highlighted that there exists a conflicting role between colloidal stability and magnetic separability of the MNPs, whereby the more colloidally stable the particle is, the harder for it to be magnetically separated. In other words, attaining a win-win scenario in which the MNPs possess both good colloidal stability and fast magnetic separation rate becomes challenging. Such phenomenon has to be thoroughly understood as the colloidal stability and the magnetic separability of MNPs play a pivotal role on affecting their effective implementation in water purification processes. Accordingly, it is the aim of this paper to provide reviews on (i) the colloidal stability and (ii) the magnetic separation of MNPs, as well as to provide insights on (iii) their conflicting relationship based on recent research findings.

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Graphical abstract Interrelationship of agglomeration, colloidal stability, and magnetic separability of nanoparticles

     

Elasticity of polymer solutions in Couette flow measured by fluorescence resonance energy transfer (FRET)

Polymer solutions are complex fluids that show elasticity and deformation in response to shear flows. A fluorescence resonance energy transfer (FRET) technique has been applied to measure the end-to-end distances of individual polymer molecules in Couette flow, using end-tagged reversible-addition fragmentation chain transfer (RAFT) polymerised poly(methyl methacrylate) (PMMA). Real-time rheofluorescence measurements on these polymers in solution above the critical overlap concentration are reported at several shear rates. The PMMA in Couette flow shows a systematic decrease in fluorescence, corresponding to a reduction in end-to-end distance of the polymer molecules with shear exposure. Full reversibility of the fluorescence signal is observed after the cessation of shear. These results show that polymer solution elasticity arises from compressive deformation of the polymer molecules in Couette flow. At polymer concentrations above the critical overlap, the polymer molecules are restricted by their neighbours and the net hydrodynamic forces are compressive rather than extensive.