Biswajit Sarkar

Polyhedral Oligomeric Silsesquioxane (POSS)-Containing Polymer Nanocomposites

Hybrid materials with superior structural and functional properties can be obtained by incorporating nanofillers into polymer matrices. Polyhedral oligomeric silsesquioxane (POSS) nanoparticles have attracted much attention recently due to their nanometer size, the ease of which these particles can be incorporated into polymeric materials and the unique capability to reinforce polymers. We review here the state of POSS-containing polymer nanocomposites. We discuss the influence of the incorporation of POSS into polymer matrices via chemical cross-linking or physical blending on the structure of nanocomposites, as affected by surface functional groups, and the POSS concentration.

Micellization of Alkyl-Propoxy-Ethoxylate Surfactants in Water−Polar Organic Solvent Mixtures

The effects of cosolvents (glycerol, ethanol, and isopropanol) on the self-assembly of novel alkyl-propoxy-ethoxylate surfactants in aqueous solutions have been investigated with a focus on the (i) quantification of solvent effects on the critical micelle concentration (cmc), (ii) free-energy contributions to micellization, (iii) local environment in the micellar solution, and (iv) structure of the micelles. The introduction of the polar organic solvents considered in this work into water decreases cohesive forces in the solvent mixture, resulting in an increase in the solubility of the surfactant molecules. As a result, micelle formation becomes less favorable and the cmc increases. The contribution of the cosolvent to the free energy of micellization is positive, and the data for different mixed solvents collapse onto a single straight line when plotted versus a function of the solubility parameters of the surfactant alkyl chains and the mixed solvents. The behavior of the poly(propylene oxide) part of the alkyl-propoxy-ethoxylate surfactants is hydrophilic, albeit less so in the ethanol-water mixed solvent than in plain water. Pyrene fluorescence emission I(1)/I(3) data suggest that the microenvironment in micellar solutions is affected mainly by the cosolvent concentration, not the surfactant degree of ethoxylation. Small-angle X-ray scattering data for both water and ethanol-water surfactant solutions are consistent with oblate ellipsoid micelles and reveal that the introduction of 20% ethanol decreases the micelle long axis by 10-15%.

Micellization of amphiphilic block copolymers in binary and ternary solvent mixtures

Amphiphilic block copolymers of the poly(ethylene oxide)-poly(propylene oxide) (PEO-PPO) family (commercially available as Pluronics or Poloxamers) are well-known for self-assembling in water (selective solvent for PEO) into micelles with a PPO-rich core and a hydrated PEO corona. The micellization of two PEO-PPO-PEO block copolymers (Pluronic P105: EO(37)PO(56)EO(37) and Pluronic F127: EO(100)PO(65)EO(100)) has been studied in binary mixed solvents consisting of water and one of the following organic solvents: ethanol, glycerol, D(+)-glucose monohydrate, propylene carbonate, or triacetin, and also in ternary mixtures of water with 50/50 wt% ethanol+glycerol or 50/50 wt% ethanol+propylene carbonate. Glycerol, glucose, propylene carbonate and triacetin were found to promote micellization when added to water. Glycerol and glucose interact favorably with water, and reduce the block copolymer critical micelle concentration (cmc) by dehydrating the PEO-PPO interface as well as changing the bulk solvent properties. Propylene carbonate and triacetin act by locating at the PEO-PPO interface and increasing its hydrophobicity. The addition of ethanol to water provides better solvent conditions for the block copolymers compared to plain water, and disfavors the formation of micelles. In the case of ternary solvents consisting of water, ethanol (that prevents micelle formation), and glycerol or propylene carbonate (that favor micelle formation), the observed changes in the cmc are subtle. For Pluronic P105, the cmc increase is greater for ethanol+propylene carbonate (50/50 wt%) than for ethanol+glycerol (50/50 wt%). For Pluronic F127, the cmcs remain the same as in plain water, i.e., the effects of the two organic solvents compensate each other. The difference between the free energy of micellization in plain water and that in solvent mixtures varies linearly with the cosolvent concentration, and collapses into a single line for each solvent mixture type when normalized with the number of the block copolymer PO units (N(PO)), indicating that the micelle core is mainly affected by varying solvent condition for different PEO/PPO ratios.

Self-assembled block copolymer-nanoparticle hybrids: Interplay between enthalpy and entropy

The dispersion of nanoparticles in ordered block copolymer nanostructures can provide control over particle location and orientation, and pave the way for engineered nanomaterials that have enhanced mechanical, electrical, or optical properties. Fundamental questions pertaining to the role of enthalpic and entropic particle-polymer interactions remain open and motivate the present work. We consider here a system of 10.6 nm silica nanoparticles (NPs) dispersed in ordered cylinders formed by hydrated poly(ethylene oxide)-poly(propylene oxide) block copolymers (Pluronic P105: EO(37)PO(56)EO(37)). Protonation of silica was used to vary the NP-polymer enthalpic interactions, while polar organic solvents (glycerol, DMSO, ethanol, and DMF) were used to modulate the NP-polymer entropic interactions. The introduction of deprotonated NPs in the place of an equal mass of water did not affect the lattice parameter of the PEO-PPO-PEO block copolymer hexagonal lyotropic liquid crystalline structures. However, the dispersion of protonated NPs led to an increase in the lattice parameter, which was attributed to stronger NP-polymer hydrogen bonding (enthalpic) interactions. Dispersion of protonated NPs into cylindrical structures formed by Pluronic P105 in 80/20 water/organic solvents does not influence the lattice parameter, different from the case of protonated NP in plain water. Organic solvents appear to screen the NP-polymer hydrogen bonding interactions.

Nanoparticle surface modification by amphiphilic polymers in aqueous media: Role of polar organic solvents

We investigate the role of three polar organic solvents (dimethyl formamide (DMF), dimethyl sulfoxide (DMSO), and glycerol) on the interfacial behavior of Pluronic P105 poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) block copolymers on protonated silica nanoparticles in an aqueous dispersion. The polymer adsorption and self-assembly have been assessed from critical surface micelle concentration (csmc, measured by pyrene fluorescence spectroscopy) and adsorbed layer thickness (measured by capillary viscometry) data. Above its csmc, PEO-PPO-PEO block copolymers form hydrophobic domains on the nanoparticle surface. Below a critical concentration in water (known as critical displacer concentration, cdc), organic solvents act as displacers (molecules that can displace adsorbed polymer from a solid surface). The critical displacer concentration is obtained from the csmc and the polymer adsorbed layer thickness data. The cdc is found to be dependent on both the amount of nanoparticles present in the system as well as the nature of the displacer. Below the cdc, the csmc increases and the adsorbed polymer layer thickness decreases with increasing organic solvent concentration. Interfacial free energy calculations suggest that DMF, DMSO, and glycerol can adsorb onto the silica particles by displacing adsorbed PEO. These calculations are consistent with the experimental results in that, as a displacer, glycerol is the most effective and DMF is the least effective. Above the cdc, the influence of glycerol or DMSO on csmc is opposite to that of DMF which is attributed to the cosolvent effect.

In depth analysis of alumina removal from iron ore fines using teetered bed gravity separator

Abstract The performance of floatex density separator (FDS) for alumina removal from iron ore fines of size <1˙0 mm has been studied. Screw classifier feed containing 4˙28% alumina has been used as the raw feed material. Desliming in hydrocyclone helps to reduce the alumina content down to 3˙39% by removing high alumina bearing ultrafine particles. Experimental campaign was undertaken considering a factorial design of experiments with three factors, namely teeter water rate, bed pressure and feed pulp density, to quantify the influence of various parameters. It was found that in single stage processing in FDS, 72% of the feed alumina could be removed. A concentrate containing 1˙66% alumina could be achieved at a yield of ∼57% in FDS. Higher teeter water was found to improve alumina removal albeit with a small decrease in iron recovery. It was observed that higher bed pressure and lower pulp density are favourable for alumina rejection. The present study established that underflow moisture is a good indicator of FDS performance. The misplacement of ultrafines in FDS underflow product increases linearly with underflow moisture content. It has been suggested that a feed with a narrow size distribution would be better suited for processing in the FDS. Splitting the feed into a coarse and a fine size fraction and treating them separately would be beneficial in obtaining still better alumina removal.

Selective Hydrogenolysis of Glycerol to 1,2-Propanediol Over Bimetallic Cu-Ni Catalysts Supported on γ-Al2O3

A series of Cu or Ni monometallic and Cu-Ni bimetallic catalysts supported on γ-Al2O3 were synthesized by incipient wetness impregnation method. X-ray diffraction results exhibited the formation of bimetallic Cu-Ni phase in the reduced Cu-Ni(1:1)/γ-Al2O3 catalyst. Among the catalyst examined for hydrogenolysis of glycerol, bimetallic catalysts exhibited higher catalytic activity than monometallic catalysts due to synergetic effect of Cu-Ni bimetal. Cu-Ni(1:1)/γ-Al2O3 catalyst displayed a maximum glycerol conversion of 71.6% with 92.8% selectivity to 1,2-propanediol at 210 °C and 4.5 MPa hydrogen pressure. The superior performance of Cu-Ni(1:1)/γ-Al2O3 catalyst was attributed to the formation of bimetallic Cu-Ni phase, high active metal surface area, small Cu-Ni particle size, and high acidic strength of the catalyst. Stability and reusability of Cu-Ni(1:1)/γ-Al2O3 catalyst was performed and detailed characterization results of fresh and used catalysts suggested that bimetallic Cu-Ni phase remained stable after reuses.

Adsorption of poly(ethylene oxide)-containing amphiphilic polymers on solid-liquid interfaces: Fundamentals and applications

The adsorption of amphiphilic molecules of varying size on solid-liquid interfaces modulates the properties of colloidal systems. Nonionic, poly(ethylene oxide) (PEO)-based amphiphilic molecules are particularly useful because of their graded hydrophobic-hydrophilic nature, which allows for adsorption on a wide array of solid surfaces. Their adsorption also results in other useful properties, such as responsiveness to external stimuli and solubilization of hydrophobic compounds. This review focuses on the adsorption properties of PEO-based amphiphiles, beginning with a discussion of fundamental concepts pertaining to the adsorption of macromolecules on solid-liquid interfaces, and more specifically the adsorption of PEO homopolymers. The main portion of the review highlights studies on factors affecting the adsorption and surface self-assembly of PEO-PPO-PEO block copolymers, where PPO is poly(propylene oxide). Block copolymers of this type are commercially available and of interest in several fields, due to their low toxicity and compatibility in aqueous systems. Examples of applications relevant to the interfacial behavior of PEO-PPO-PEO block copolymers are paints and coatings, detergents, filtration, and drug delivery. The methods discussed herein for manipulating the adsorption properties of PEO-PPO-PEO are emphasized for their ability to shed light on molecular interactions at interfaces. Knowledge of these interactions guides the formulation of novel materials with useful mesoscale organization and micro- and macrophase properties.

Competitive Adsorption Between PEO-Containing Block Copolymers and Homopolymers at Silica

The ability to manipulate polymer adsorption is useful for applications involving colloidal stabilization, for example, paints, cosmetics, lubricants, and mineral and waste-water treatment. We have an ongoing interest on the use of organic molecules for modulating the aqueous solution and adsorption properties of poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (PEO–PPO–PEO) block copolymers. In the present study, the influence of low molecular weight PEO homopolymer on the adsorption of a representative PEO–PPO–PEO block copolymer (Pluronic P105: EO37-PO56-EO37) at the surface of protonated silica nanoparticles dispersed in water is investigated. Pluronic P105 forms hydrophobic domains on the surface of protonated silica at a critical surface micelle concentration, csmc, of 0.02 wt% in the presence of 0.1 wt% silica nanoparticles in water, well below the cmc of Pluronic P105 in water (0.6 wt%). Dye solubilization experiments reveal an increase in the PEO–PPO–PEO block copolymer csmc with increasing amounts of added PEO homopolymer. The resulting critical displacer concentration for PEO homopolymer of molecular weights 200 and 600 Da was measured to be 0.1 wt% and 0.07 wt%, respectively, in the presence of 0.1 wt% silica nanoparticles. Capillary viscometry measurements indicate a decrease in the adsorbed layer thickness at the protonated silica surface with increasing PEO homopolymer concentration. The data presented herein are consistent with a physical model which considers “patches” of PEO–PPO–PEO block copolymer and PEO homopolymer adsorbed at the silica surface.

Performance prediction of floatex density separator in processing iron ore fines – a relative velocity approach

Abstract Effective use of a Floatex density separator (FDS), a continuous teetered bed separator, in beneficiating iron ore fines in terms of alumina and silica removal has been investigated. Particle behaviour in an FDS is described using steady state force balance on the particle. A relative velocity approach coupled with the mass balance has been adopted for theoretical performance predictions of multispecies particulate systems having density and size variations. Prediction of suspension density has been identified as most crucial due to the strong dependence of FDS performance on it. It was established that a threshold bed pressure exists below which the unit acts merely as a size classifier. Above this threshold pressure the bed develops significantly, formation of a proper fluidised suspension takes place and concentration effect dominates. Higher teeter water rates enhance hydraulic transport but reduce the yield to the underflow. Predicted separation performance has been validated for fines from one Indian iron ore. The effect of superficial teeter water velocity and bed pressure on process performance has been studied from a theoretical as well as an experimental stand point. Apart from the process variables, liberation characteristics of the ore have significant effect on the performance of the unit. It is established that the FDS has great potential to remove substantial alumina and silica even in single stage operation. This is very significant in an Indian context where high alumina in iron ore fines poses a major problem in downstream operations.