Ostwald ripening in an immiscible hydrogenated polybutadiene and high-density polyethylene blend

The long-time regime coarsening in a phase-segregated blend of hydrogenated polybutadiene (HPB) with high-density polyethylene (HDPE) was studied. The blend consisted of 10 wt% of HPB in a HDPE matrix. The morphology of the system was studied by etching the HPB particles from the HDPE matrix and observing the etched specimens in a scanning electron microscope. The average volume of the HPB particles was found to increase with storage time in the melt, and to follow a temporal exponent of 1 in agreement with the predictions of the Ostwald ripening theory. This indicated that the particles coarsen by the evaporation-condensation mechanism on which the Ostwald ripening theory is based. The rate constant from the Ostwald ripening theory was calculated and compared to the rate constant determined from the experimental data. The theoretical rate constant, K, calculated from Ostwald ripening theory, was 3.6 × 10^?18 cm³/s compared to an experimentally determined rate constant of 4.8 × 10^?18 cm³/s. The agreement between the theoretical and experimental rate constants was quite good. The significance attached to the good agreement between the theoretical and experimental rate constants might be mollified to some extent by the uncertainty involved in the parameters used to calculate the theoretical rate constant; viz. the interfacial tension, mutual-diffusion coefficient, and equilibrium concentration of the HPB in the matrix phase that are not known to high accuracy. In reality, because other theories were used to determine the interfacial tension, mutual-diffusion coefficient, and equilibrium phase compositions, this study was a test of several theories simultaneously. However, the agreement of the experimental temporal exponent and rate constant with the predictions of Ostwald ripening theory strongly indicates that the HPB/HDPE system coarsens by the evaporation-condensation mechanism upon which the Ostwald ripening theory is based. © 1994 John Wiley & Sons, Inc.     

Phase ripening in particulate binary polymer blends

Particulate polymer‐in‐polymer mezodispersions show a pronounced increase in the size of the dispersed particles during melt‐phase annealing. Three ripening mechanisms have been proposed: Brownian coalescence, Ostwald ripening, and hydrodynamic coarsening. The modified Cahn–Hilliard equation predicts growth by Ostwald ripening and diffusion‐induced coalescence. Simulations of this mechanism show a self‐similar particle size distribution, but the distribution broadens with the increasing volume fraction of the minor phase. Hydrodynamic coarsening caused by concentration gradients and random Brownian forces has been simulated according to the hydrodynamic model. The simulations show that concentration‐driven hydrodynamics have little effect on the particle size distribution. Experiments have been performed to investigate the relative importance of these ripening mechanisms for polybutadiene in a polystyrene system. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 603–612, 2004     

Phase separation and the kinetics of phase coarsening in commercial impact polypropylene copolymers

The phase segregation and subsequent minor phase coarsening of a commercial impact polypropylene copolymer was studied. The major components of the impact polypropylene copolymer studied were 82.4 wt % polypropylene homopolymer and 17.6 wt % ethylene-propylene rubber (EPR). The system was artificially manipulated to ensure homogeneity by precipitation from solution with a nonsolvent. This ensured that the initial system did not exhibit large-scale phase segregation. The homogeneous initial system was subjected to storage in the melt at 193°C for a series of times. The two-phase morphology of commercial impact polypropylenes was generated in the melt state by storage in the melt for various periods of time from 5 s to 1 h. Small nuclei of particles appeared at short time and increased in volume with increasing time in the melt state. The coarsening of the minor phase EPR component was shown to follow the theoretically predicted d ～ t^1/3 and N ～ t^?1 (where d = diameter, N = number of particles, and t = time in the melt) relationships to a close approximation in accord with Ostwald ripening theory. At short times these relationships were not obeyed. The indication was that the long-time coarsening regime was not entered until several minutes elapsed in the melt state. The particle size distribution was initially quite narrow and exhibited a trend of broadening at longer times of coarsening. This may be due to a shift from the short-time regime to the long-time coarsening regime. The initial polymer, which was precipitated from solution, was shown not to have undergone large-scale phase segregation in that it exhibited a one-phase morphology (i.e., no particles with > 0.1 μm diameter) as determined by ¹²⁹Xe NMR spectroscopy. The precipitated blend produced incipient particle nuclei (> 0.1 μm diameter) after a very short time (5 s) in the melt state. © 1994 John Wiley & Sons, Inc.     

Particulate growth in phase-separated polymer blends

Particle coarsening was investigated in polymer blends containing a minor phase volume of 23% produced by compositional quenching. The scaling exponents for three binary blends (polystyrene/polybutadiene, polystyrene/polyisoprene, and polystyrene/S-B random copolymer) were in reasonable agreement with the expected value of 0.33. The scaling exponent for a ternary blend containing an amphiphile (polystyrene/polybutadiene/S-B block copolymer) was substantially lower at 0.14. The particle size distributions for all the blends were broader than the self-similar distribution expected for Ostwald ripening and became increasingly broad with time. These distributions fit a two parameter coalescence model in which the probability of coalescence is proportional to the particle diameter. However, Ostwald ripening appears to make some contribution, particularly at early times. © 1998 John Wiley & Sons, Inc. J. Polym. Sci. B Polym. Phys. 36: 2191–2196, 1998     

Properties of amorphous and crystallizable hydrocarbon polymers. IV. Melt rheology of linear and star-branched hydrogenated polybutadiene

Some results on the melt rheology of hydrogenated polybutadiene (HPB) with narrow-molecular-weight distribution are reported and compared with the corresponding properties of the precursor polybutadienes (PBD) and fractions of linear polyethylene (PE). In linear samples the dynamic moduli obeyed frequency-temperature superposition. The relationship between melt viscosity and intrinsic viscosity at 190°C for HPB was indistinguishable from that for PE, but their flow activation energies were slightly different (E_a = 7.2 kcal for HPB and 6.4 kcal for PE). Like PE, but unlike the PBD precursors, the dynamic storage modulus at low frequencies was anomalous. Otherwise, the dynamic moduli of HPB and its PBD precursor were essentially superposable. Plateau moduli from different samples were somewhat variable around an average of G = 2.3₁ × 10⁷ dyn/cm². The dynamic moduli for the HPB stars, unlike their PBD precursors, did not obey temperature-frequency superposition. At high frequencies the temperature coefficient approached that for linear HPB, but it increased with decreasing frequency, reaching limiting values which depended on the arm length. The flow activation energy ranged from 9 kcal to more than 15 kcal as arm length increased.     

Coarsening of the phase structure in immiscible polymer blends. Coalescence or ostwald ripening?

The results of theories of the Ostwald ripening and coalescence applied to molten quiescent polymer blends with dispersed phase structure were analyzed. From a comparison of predictions of the theories with available experimental results follows that coarsening of the phase structure in quiescent polymer blends with medium or high interfacial tension is induced by the coalescence. Both the mechanisms play a role in coarsening of the phase structure in blends with low interfacial tensions. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 181–187, 1999     

Sterically stabilized emulsion polymerization of styrene: Pseudo‐semicontinuous approach

The sterically stabilized emulsion polymerization of styrene initiated by a water‐soluble initiator at different temperatures has been investigated. The rate of polymerization (R_p) versus conversion curve shows the two non‐stationary‐rate intervals typical for the polymerization proceeding under non‐stationary‐state conditions. The shape of the R_p versus conversion curve results from two opposite effects—the increased number of particles and the decreased monomer concentration at reaction loci as the polymerization advances. At elevated temperatures the monomer emulsion equilibrates to a two‐phase or three‐phase system. The upper phase is transparent (monomer), and the lower one is blue colored, typical for microemulsion. After stirring such a multiphase system and initiation of polymerization, the initial coarse polymer emulsion was formed. The average size of monomer/polymer particles strongly decreased up to about 40% conversion and then leveled off. The initial large particles are assumed to be highly monomer‐swollen particles formed by the heteroagglomeration of unstable polymer particles and monomer droplets. The size of the “highly monomer” swollen particles continuously decreases with conversion, and they merge with the growing particles at about 40–50% conversion. The monomer droplets and/or large highly monomer‐swollen polymer particles also serve as a reservoir of monomer and emulsifier. The continuous release of nonionic (hydrophobic) emulsifier from the monomer phase increases the colloidal stability of primary particles and the number of polymer particles, that is, the particle nucleation is shifted to the higher conversion region. Variations of the square and cube of the mean droplet radius with aging time indicate that neither the coalescence nor the Ostwald ripening is the main driving force for the droplet instability. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 804–820, 2003     

Mechanism of silver particle formation during photoreduction using in situ time-resolved SAXS analysis

Formation mechanisms of silver (Ag) particles in an aqueous ethanol solution of poly(N-vinyl-2-pyrrolidone) (PVP) by the photoreduction of AgClO(4) were investigated by means of in situ small-angle X-ray scattering (SAXS) measurements. The kinetics of association process (nucleation, growth, and coalescence) of Ag(0) atoms to produce Ag particles was successfully revealed by the quantitative SAXS analysis for the number-average of radius (R(0)), number of particles (n(Ag)), reduced standard deviation (σ(R)/R(0)), and volume fraction (?(Ag)) of Ag particles produced by the photoreduction. The rate of nucleation and growth process during Ag particle formation strongly depend on the initial metal concentration. The time evolution of radius and number of Ag particles indicates that a mechanism of Ag particle formation is composed of different three processes, that is, reduction-nucleation, Ostwald ripening, and particle coalescence. In a rapid reduction-nucleation process, small nuclei or particles (average radius ～2.5 nm) are produced by an autocatalytic reduction. After the formation of small nuclei or particles proceeds, Ostwald ripening and particle coalescence, predicted by the Lifshitz-Slyozov-Wagner theory (LSW theory), subsequently occur, resulting in the particle growth (average radius ～11.5 nm).     

Emulsion Ripening through Molecular Exchange at Droplet Contacts

Two coarsening mechanisms of emulsions are well established: droplet coalescence (fusion of two droplets) and Ostwald ripening (molecular exchange through the continuous phase). Here a third mechanism is identified, contact ripening, which operates through molecular exchange upon droplets collisions. A contrast manipulated small‐angle neutron scattering experiment was performed to isolate contact ripening from coalescence and Ostwald ripening. A kinetic study was conducted, using dynamic light scattering and monodisperse nanoemulsions, to obtain the exchange key parameters. Decreasing the concentration or adding ionic repulsions between droplets hinders contact ripening by decreasing the collision frequency. Using long surfactant chains and well‐hydrated heads inhibits contact ripening by hindering fluctuations in the film. Contact ripening can be controlled by these parameters, which is essential for both emulsion formulation and delivery of hydrophobic ingredients.     

Ligand effect on the growth and the digestion of Co nanocrystals

The reaction product of cobalt carbonyl decomposition depends on the concentration of the oleic acid ligand. With a low concentration of ligand, nanocrystals nucleate and grow to large ferromagnetic particles through the process of Ostwald ripening and coalescence coarsening. With a high concentration of ligand, stable cluster complexes are formed. Addition or removal of ligand from the reaction products can interchange the formation of cluster complexes and nanocrystals.     

Thermal coarsening of supported palladium combustion catalysts

An essential property of combustion catalysts is long-term (>8000 h) stability at high temperatures in an environment (approximately 1 atm of both oxygen and water vapor) that aggressively promotes sintering of the supporting oxide and coarsening of the active component. Extrapolation of accelerated coarsening rate measurements, determined from shorter exposures at higher temperatures, can be made with more confidence if the physical processes of the coarsening and sintering processes were well understood. The current work examines in detail the coarsening of a high-weight-loaded palladium catalyst supported by silica-stabilized alumina at 900 degrees C in such an aggressive environment. The results of this investigation showed that the Pd particle size distribution was consistently log-normal for time periods from 100 to 4000 h, the mean particle growth rate was roughly inverse second-order in mean particle diameter, and the support not only sintered but also underwent phase transformation. The results implicate both coalescence and Ostwald ripening as important coarsening processes.     

Preparation of water-in-diesel oil nano-emulsion using nonionic surfactants with enhanced stability and flow properties

Formation of water-in-diesel oil (w/o) nano-emulsion has been achieved by a low-energy emulsification method by stabilizing a new combination of nonionic sorbitan esters surfactants, that is PEG20-sorbitan monostearate and sorbitan monooleate in mixed proportions. Different combinations of the surfactants (T6?+?S8) have been tested and the best possible combination of mixed surfactants is found at a surfactants ratio of 35:65 (wt/wt) for T6:S8 at hydrophile–lipophile balance (HLB)?=?8.01, which resulted in smaller droplet size of 44.87?nm. A phase diagram study is performed to identify the zones of formation of transparent, translucent, and opaque emulsions (44?nm??27?m³?·?s^?1. Comparison of Ostwald ripening rate with other sets of surfactants obtained by different authors showed the lowest rate among them, indicative of enhanced stability. A rheological study of the tested set of nano-emulsions depicts the Newtonian behavior (1.0371?≤?n?≤?1.0826) over a wider range of shear rates (10–1000?s^?1) at different temperatures (25–40°C).     

Nanoreactors for studying single nanoparticle coarsening

The ability to observe intermediate structures as part of coarsening processes that lead to the formation of single nanoparticles (NPs) is important in gaining fundamental insight pertaining to nanostructure growth. Here, we use scanning probe block copolymer lithography (SPBCL) to create "nanoreactors" having attoliter volumes, which confine Au NP nucleation and growth to features having diameters <150 nm on a substrate. With this technique, one can use in situ TEM to directly observe and study NP coarsening and differentiate Ostwald ripening from coalescence processes. Importantly, the number of metal atoms that can engage in coarsening can be controlled with this technique, and TEM "snapshots" of particle growth can be taken. The size of the resulting nanostructures can be controlled in the 2-10 nm regime.     

Distribution kinetics of Ostwald ripening at large volume fraction and with coalescence

Condensation phase transitions from metastable fluids occur by nucleation with accompanying particle growth and eventual Ostwald ripening. During ripening the subcritical particles dissolve spontaneously while larger particles grow and possibly coalesce if their volume fraction is large enough. The classical diffusion-influenced rates are also affected by large particle concentrations and are here described by mass-dependent rates. We represent the kinetics of ripening through growth, dissolution, and biparticle coalescence by a new population dynamics equation for the particle size distribution (PSD). Numerical solutions of the scaled governing equations show that coalescence plays a major role in influencing the PSD when the scaled mass concentration (volume fraction) or number concentration is relatively large. The solution describes the time range from initial conditions to the final narrowing of polydispersity. We show that the time dependence of the average particle mass in the asymptotic period of ripening has a power-law increase dependent on rate expressions for particle growth and coalescence at large values of volume fraction.     

Premix Membrane Emulsification: Preparation and Stability of Medium-Chain Triglyceride Emulsions with Droplet Sizes below 100 nm

Premix membrane emulsification is a promising method for the production of colloidal oil-in-water emulsions as drug carrier systems for intravenous administration. The present study investigated the possibility of preparing medium-chain triglyceride emulsions with a mean particle size below 100 nm and a narrow particle size distribution using sucrose laurate as an emulsifier. To manufacture the emulsions, a coarse pre-emulsion was repeatedly extruded through alumina membranes (Anodisc^™) of 200 nm, 100 nm and 20 nm nominal pore size. When Anodisc^™ membranes with 20 nm pore size were employed, nanoemulsions with z-average diameters of about 50 nm to 90 nm and polydispersity indices smaller than 0.08 could be obtained. Particle growth due to Ostwald ripening was observed over 18 weeks of storage. The Ostwald ripening rate linearly depended on the emulsifier concentration and the concentration of free emulsifier, indicating that micelles in the aqueous phase accelerated the Ostwald ripening process. Long-term stability of the nanoemulsions could be achieved by using a minimised emulsifier concentration or by osmotic stabilisation with soybean oil added in a mass ratio of 1:1 to the lipid phase.     

Der Sintermechanismus von hochdispersem Pt auf Al2O3 in Sauerstoff

Mechanism of Sintering of Highly Dispersed Pt on Al₂O₃ in Oxygen Sintering of highly dispersed Pt on alumina in O₂ consists in a slow thermal decomposition of a Pt^IV surface compound, followed by a fast coalescence of the highly mobile, zerovalent Pt, formed during the decomposition. Sintering according to this mechanism is inhibited by O₂. The slower Ostwald ripening, promoted by O₂, comes into effect only after complete decomposition of the Pt^IV surface compound.     

Solubility‐Parameter‐Guided Solvent Selection to Initiate Ostwald Ripening for Interior Space‐Tunable Structures with Architecture‐Dependent Electrochemical Performance

Despite significant advancement in preparing various hollow structures by Ostwald ripening, one common problem is the intractable uncontrollability of initiating Ostwald ripening due to the complexity of the reaction processes. Here, a new strategy on Hansen solubility parameter (HSP)‐guided solvent selection to initiate Ostwald ripening is proposed. Based on this comprehensive principle for solvent optimization, N,N‐dimethylformamide (DMF) was screened out, achieving accurate synthesis of interior space‐tunable MoSe₂ spherical structures (solid, core–shell, yolk‐shell and hollow spheres). The resultant MoSe₂ structures exhibit architecture‐dependent electrochemical performances towards hydrogen evolution reaction and sodium‐ion batteries. This pre‐solvent selection strategy can effectively provide researchers great possibility in efficiently synthesizing various hollow structures. This work paves a new pathway for deeply understanding Ostwald ripening.     

Fractal analysis of the coalescence process of Au nano-meter particles dispersed in 2-propanol

The coalescence process of Au small particles (11 nm in radius) dispersed in 2-propanol is studied via fractal analysis. It was found that the particle network grows one-dimensionally at an initial stage and three-dimensionally in later stage accompanying with the increase of the size of a constituent particle. Ostwald ripening was found at the beginning of the growth process. The intensity of the absorption band at around 750 nm gradually increases in concurrence with the growth of particle network at the expense of the decrease of the intensity of 523 nm plasmon band of isolated particles.     

Synthesis of ZnO and TiO2 nanoparticles

We report on the synthesis of ZnO and TiO₂ nanoparticles by solution-phase methods, with a particular focus on the influence of experimental parameters on the kinetics of nucleation and coarsening. The nucleation rate of ZnO from the reaction between ZnCl₂ and NaOH in ethanol was found to increase with increasing precursor concentration, while the coarsening rate is independent of precursor concentration up to a threshold concentration. The nucleation rate of ZnO from Zn(OOC-CH₃)₂ and NaOH in n-alkanols was found to decrease with decreasing chain length, which is explained by the increase of the dielectric constant of the solvent. Due to the larger solubility of ZnO, nucleation is significantly slower than that observed in the case of TiO₂. TiO₂ nanoparticles coarsen according to the Lifshitz-Slyozov-Wagner model for Ostwald ripening. We also show that using amorphous titania as a base material, pure anatase and brookite nanoparticles can be synthesized.     

Ostwald ripening of protein-stabilized emulsions: effect of transglutaminase crosslinking

Ostwald ripening in n-alkane oil-in-water emulsions stabilized by sodium caseinate at neutral pH has been studied by monitoring time-dependent changes in the number-average droplet diameter and the droplet-size distribution. In qualitative agreement with theory, the destabilization rate has been shown to increase with reduction of the n-alkane chain length and on addition of ethanol to the aqueous phase. Replacement of caseinate by β-lactoglobulin also leads to improved stability, but addition of calcium ions does not. The potential use of transglutaminase-induced crosslinking of adsorbed protein as a way of inhibiting the Ostwald ripening of caseinate-stabilized emulsions has been examined. It is shown that enzymic crosslinking before emulsification can lead to a modest reduction in the coarsening rate at long storage times. Crosslinking of caseinate after emulsification produces enhanced stability at short times, but there is a catastrophic loss of stability at long times due to droplet coalescence.