Phase Separation In Dispersions Research Articles

Effects of Compatibilizers on Morphology and Properties of Polyoxymethylene/Polypropylene Blends

The effects of polypropylene-graft-maleic anhydride compatibilizer on the mechanical thermal and morphological properties of polyoxymethylene/polypropylene blends were investigated. Polyoxymethylene/polypropylene blends with and without polypropylene-graft-maleic anhydride compatibilizer were prepared by an internal mixer. The morphology of polyoxymethylene/polypropylene blends clearly demonstrated a two-phase separation of dispersed phase and the matrix phase and the addition of polypropylene-graft-maleic anhydride changed the morphological characteristics of blends. Polyoxymethylene/polypropylene blends showed the decrease of mechanical properties with increasing of polypropylene content. The addition of polypropylene-graft-maleic anhydride improved Young’s modulus and storage modulus of polyoxymethylene/polypropylene blends. The incorporation of polypropylene improved the degradation temperature of polyoxymethylene.

Tethered Nanoparticle–Polymer Composites: Phase Stability and Curvature

Phase behavior of poly(ethylene glycol) (PEG) tethered silica nanoparticles dispersed in PEG hosts is investigated using small-angle X-ray scattering. Phase separation in dispersions of densely grafted nanoparticles is found to display strikingly different small-angle X-ray scattering signatures in comparison to phase-separated composites comprised of bare or sparsely grafted nanoparticles. A general diagram for the dispersion state and phase stability of polymer tethered nanoparticle-polymer composites incorporating results from this as well as various other contemporary studies is presented. We show that in the range of moderate to high grafting densities the dispersion state of nanoparticles in composites is largely insensitive to the grafting density of the tethered chains and chemistry of the polymer host. Instead, the ratio of the particle diameter to the size of the tethered chain and the ratio of the molecular weights of the host and tethered polymer chains (P/N) are shown to play a dominant role. Additionally, we find that well-functionalized nanoparticles form stable dispersions in their polymer host beyond the P/N limit that demarcates the wetting/dewetting transition in polymer brushes on flat substrates interacting with polymer melts. A general strategy for achieving uniform nanoparticle dispersion in polymers is proposed.

Onsager Revisited: Magnetic Field Induced Nematic−Nematic Phase Separation in Dispersions of Goethite Nanorods

We found, using polarization microscopy and small-angle X-ray scattering, that for goethite, a low polydispersity suffices to form two separate nematic phases, while previous theory showed that this is only possible for mixtures of particles with extremely different lengths or diameters. Applying a critical magnetic field, which induces some of the goethite nanorods to rotate, leads to sufficient excluded volume between the particles to cause macroscopic phase separation between two orthogonal nematic phases. The larger the polydispersity of the system, the broader the range of field strengths where nematic−nematic phase separation occurs. This is a new phase separation mechanism which is expected to lead to interesting interfacial phenomena.

Cationic Flocculants Carrying Hydrophobic Functionalities: Applications for Solid/Liquid Separation

The flocculation behaviors of three series of polycations with narrow molecular weight distributions carrying hydrophobic substituents on their backbones [poly(N-vinylbenzyl-N,N,N-trimethylammonium chloride), poly(N-vinylbenzyl-N,N-dimethyl-N-butylammonium chloride), and poly(N-vinylbenzylpyridinium chloride)] were investigated in dispersions of monodisperse polystyrene latexes and kaolin. Apparently, the charge density of the polycations decreases with increasing substituent hydrophobicity and increasing molecular weight of the polyelectrolytes. The necessary amount of flocculant for phase separation in dispersions with high substrate surface charge densities increases with increasing hydrophobicity of the polyelectrolyte. Nevertheless, the introduction of hydrophobic functionalities is beneficial, resulting in a substantial broadening of the range between the minimum and maximum amounts of flocculant necessary for efficient flocculation (flocculation window). An increase in ionic strength supports this effect. When the substrate has a low charge density, the hydrophobic interactions play a much more significant role in the flocculation process. Here, the minimum efficient doses remained the same for all three polyelectrolytes investigated, but the width of the flocculation window increased as the polycation hydrophobicity and the molecular weight increased. The necessary amount of flocculant increased with an increase in particle size at constant solid content of the dispersion, as well as with a decreasing number of particles at a constant particle size.

Metastability and Multistability: Gelation and Liquid Crystal Formation in Suspensions of Colloidal Rods

The competition between gelation and isotropic−nematic (I−N) phase separation in dispersions of moderately attractive sterically stabilized boehmite (AlOOH) rods is studied. Two different scenarios...

Morphology and Kinetics of the Isotropic−Nematic Phase Transition in Dispersions of Hard Rods

The time evolution of the isotropic−nematic phase separation in dispersions of sterically stabilized colloidal rods was studied with polarization microscopy and static small angle light scattering (SALS). The rods aspect ratio is 14. In the biphasic region (between the isotropic−nematic transition volume fraction φI =12.1% and the nematic−melting volume fraction φN = 35.1%) a SALS ring develops at a wavevector Kmax which shifts to smaller K values in time. Increasing the concentration from φI to φN, polarization microscopy indicates a crossover from nucleation-and-growth to spinodal decomposition. Nucleation is accompanied by a decrease of the intensity at large K as ∼K-4, which is typically found when sharp interfaces have developed in the system. For spinodal decomposition a much less pronounced decrease is observed within the experimental time window. The rate of phase separation was studied by monitoring both the change of the turbidity and the shift of Kmax in time. A maximum rate is found just withi...

Phase separation in dispersions of weakly interacting particles induced by non-adsorbing polymer

Abstract The addition of a non-adsorbing polymer (polydimethylsiloxane) to dispersions of hydrophobic silica particles in cyclohexane induces phase separation by means of the depletion interaction. The strength and range of the weak, attractive potential, in this otherwise hard sphere system, may be independently controlled by means of the free polymer concentration and molecular weight, respectively. Depending on the particle size and molecular weight of the polymer, as well as the particle number concentration, analogues of the molecular gas/liquid, gas/solid and liquid/solid phase transitions may each be observed, in accordance with the predictions of recent statistical mechanical theories.

Phase separation in dispersions of weakly-interacting particles in solutions of non-adsorbing polymer

Phase separation may be induced in dispersions of hydrophobic silica particles in cyclohexane upon the addition of non-adsorbing polymer (polydimethylsiloxane). The origin of this effect is ascribed to the depletion interaction. Depending on the particle size and molecular weight of the polymer, the analogues of the molecular gas/solid, gas/liquid and liquid/solid co-existing phase regions may each be observed. It is demonstrated that using relatively simple, mean-field theories for both the fluid state (random phase approximation or adhesive sphere model), and also the solid state (cell model), all these transitions may be predicted. Although the quantitative agreement between experiment and theory is not exact, qualitatively all the observed trends in behaviour are accounted for.

Phase separation in dispersions of weakly interacting particles

The close analogy is explored between the phase behaviour of colloidal dispersions of weakly interacting particles and that of conventional molecular systems. The analogues of all three states of matter and phase equilibria may be observed. Different behaviour is predicted depending on the range of attractive forces.

Coalescence phenomena in liquid‐liquid systems

AbstractThe factors affecting phase separation of liquid dispersion are discussed in order to identify the parameters controlling the coalescence processes occurring in primary and secondary dispersions. Models for the estimation of wedge length and band thickness in gravity settlers are reviewed, and the trends in future studies in the design and analysis of this type of equipment introduced. The phase separation of dispersions in packed beds is also considered, and the effects of the properties of the packing and the system on the coalescence processes occurring in the packing analysed. The importance of the system properties, packing voidage and the exit drop mechanism on the design of a packing for a phase separation operation are emphasised. Some recent results explaining the increased coalescence rate at the junction between a high energy and low energy surface are presented. In addition, some recent work in these laboratories on the separation of secondary dispersions are also presented.