Polymer-covered particles

Addition of different kinds of charged polymers (polyelectrolytes) offers one effective way to control the stability of a colloidal solution. When charged polymers adsorb on neutral colloids, the colloids repel each other for electrostatic reasons. This behavior is called electrostatic stabilization and is responsible for the long shelf-life of certain latex paints. Polymers can also stabilize a dispersion for steric reasons when they are grafted or adsorbed to the particles. If two polymer covered particles approach it will lead to a restriction on the configurational freedom for the polymers giving rise to a repulsive force. 

Figure 5. Excess polymer can be used to bridge polymer-covered particles - but Its concentration must be high enough to allow It to penetrate the adsorbed layers.

Polymer covered particles are vtnusual objects, and they can Interact to form a variety of original structures. The examples which have... 

The interfacial electric polarizability y, being an important dynamic characteristic of the particle surface charge, can be easily determined from the electro-optical effect dependence on the square of the electric field strength (Eq. 6). A significant increase in the particle dimensions as well as the low surface charge of the colloid-polymer complex complicate the electric polarizability determination near to the system s isoelectric point (Figure 2). The electric polarizabilities are calculated in this review only for polymer covered particles in stabilized suspensions. One way to obtain correct values... 

This volume consists of four parts. The first part is devoted to theoretical studies and computer simulations. These studies deal with the structure and dynamics of polymers adsorbed at interfaces, equations of state for particles in polymer solutions, interactions in diblock copolymer micelles, and partitioning of biocolloidal particles in biphasic polymer solutions. The second part discusses experimental studies of polymers adsorbed at colloidal surfaces. These studies serve to elucidate the kinetics of polymer adsorption, the hydrodynamic properties of polymer-covered particles, and the configuration of the adsorbed chains. The third part deals with flocculation and stabilization of particles in adsorbing and nonadsorbing polymer solutions. Particular focus is placed on polyelectrolytes in adsorbing solutions, and on nonionic polymers in nonadsorbing solutions. In the final section of the book, the interactions of macromolecules with complex colloidal particles such as micelles, liposomes, and proteins are considered. 

PVA and TaM -for the 88%-hydrolyzed PVA. The same dependence was found for the adsorbed layer thickness measured by viscosity and photon correlation spectroscopy. Extension of the adsorption isotherms to higher concentrations gave a second rise in surface concentration, which was attributed to multilayer adsorption and incipient phase separation at the interface. The latex particle size had no effect on the adsorption density however, the thickness of the adsorbed layer increased with increasing particle size, which was attributed to changes in the configuration of the adsorbed polymer molecules. The electrolyte stability of the bare and PVA-covered particles showed that the bare particles coagulated in the primary minimum and the PVA-covered particles flocculated in the secondary minimum and the larger particles were less stable than the smaller particles. 

Effect of PVA Molecular Weight on Adsorbed Layer Thickness. Figure 4 shows the variation of reduced viscosity with volume fraction for the bare and PVA-covered 190nm-size PS latex particles. For the bare particles, nre(j/ is independent of and the value of the Einstein coefficient is ca. 3.0. For the covered particles, rired/ t increases linearly with tp. Table IV gives the adsorbed layer thicknesses calculated from the differences in the intercepts for the bare and covered particles and determined by photon correlation spectroscopy, as well as the root-mean-square radii of gyration of the free polymer coil in solution. The agreement of the adsorbed layer thicknesses determined by two independent methods is remarkable. The increase in adsorbed layer thickness follows the same dependence on molecular weight as the adsorption density, i.e., for the fully hydrolyzed PVA s and... 

Various novel imprinting techniques have also been presented recently. For instance, latex particles surfaces were imprinted with a cholesterol derivative in a core-shell emulsion polymerization. This was performed in a two-step procedure starting with polymerizing DVB over a polystyrene core followed by a second polymerization with a vinyl surfactant and a surfactant/cholesterol-hybrid molecule as monomer and template, respectively. The submicrometer particles did bind cholesterol in a mixture of 2-propanol (60%) and water [134]. Also new is a technique for the orientated immobilization of templates on silica surfaces [ 135]. Molecular imprinting was performed in this case by generating a polymer covering the silica as well as templates. This step was followed by the dissolution of the silica support with hydrofluoric acid. Theophylline selective MIP were obtained. 

Total partial pressure range (sum of the partial pressures of aU three MoOs vapor polymers) covered in these measurements is from about 9 X 10 to 4 X 10 s atm. Average particle diameters were 0.22 0.02 cm. Rates of uptake on the clay loam particles are shown at 0,10, and SO minutes. Dashed line indicates the rate of uptake calculated by Maxwell s equation... 

The stability of electrostatically charged sols has been studied extensively and is now reasonably well understood. More recently the stabilising action of adsorbed or chemically anchored non-ionic polymers has received much attention. There has been however little systematic work on polyelectrolyte stabilisers apart from a number of investigations of the flocculation of particles bearing adsorbed biopolymers, usually proteins, by simple salts ( 2). These have shown that polyelectrolyte covered particles can be more stable with respect to the addition of salt than simple charged systems, and the extra stability has been ascribed to the polymeric nature of the surface layer. The precise mechanism by which polyelectrolytes stabilise dispersions in the presence of high concentrations of salt has however remained unclear. 

Constant composition copolymer may be produced in this process, but particle size changes during the process. Particle number increases continuously, again following polymer yield. Although emulsifier tends to be more completely used for covering particle, coverage itself remains weak. 

Polymer-protected bimetallic clusters were also formed using a modified polyol process. The modification included addition of other solvents and sodium hydroxide. In the synthesis of Co-Ni with average diameters between 150 and 500 nm, PVP and ethylene glycol were mixed with either cobalt or nickel acetate with PVP. The glycol and organic solvents were removed from solution by acetone or filtration. The PVP-covered particles were stable in air for extended periods of time (months). 

Electron microscopy shows that polymerization starts at active centres on the surface of the particle. During this initial stage, a thin polymer cover is formed on and just below the outer surface of the silica support. This thin cover consists of highly crystalline polypropylene, which acts as a diffusion barrier for the monomer. Diffusion of propylene through this layer thus becomes rate-limiting for polymer formation consequently the high initial polymerization activity decreases sharply after a few minutes and a period of relatively low activity is reached. 

Consider a cylindrical soft particle, that is, an infinitely long cylindrical hard particle of core radius a covered with an ion-penetrable layer of polyelectrolytes of thickness d in a symmetrical electrolyte solution of valence z and bulk concentration (number density) n. The polymer-coated particle has thus an inner radius a and an outer radius b = a + d. The origin of the cylindrical coordinate system (r, z, cp) is held fixed on the cylinder axis. We consider the case where dissociated groups of valence Z are distributed with a uniform density N in the polyelectrolyte layer so that the density of the fixed charges in the surface layer is given by pgx = ZeN. We assume that the potential i/ (r) satisfies the following cylindrical Poisson-Boltz-mann equations ... 

It should be realized that hydrodynamic techniques measure an effective thickness obtained by comparing the tangential flow along a polymer-covered surface with that along a bare surface. The effective thickness thus found is usually called the hydrodynamic layer thickness. The exact shape of the flow velocity profile is Important, and this depends on the shape of the surface in question, and on its orientation with respect to the flow field. Detailed data Interpretation is therefore only possible if simple geometries are chosen such as smooth cylindrical channels or spherical colloidal particles. 

Continuons emulsion polymerization is one of the few chemical processes in which major design considerations require the use of dynamic or unsteady-state models of the process. This need arises because of important problems associated with sustained oscillations or limit cycles in conversion, particle number and size, and molecular weight. These oscillations can occur in almost all commercial continuous emulsion polymerization processes such as styrene (Brooks et cl., 1978), styrene-butadiene and vinyl acetate (Greene et cl., 1976 Kiparissides et cl., 1980a), methyl methacrylate, and chloropene. In addition to the undesirable variations in the polymer and particle properties that will occur, these oscillations can lead to emulsifier concentrations too low to cover adequately the polymer particles, with the result that excessive agglomeration and fouling can occur. Furthermore, excursions to high conversions in polymer like vinyl acetate... 

For the purpose of molecular weight determination 10 g titanium dioxide (rutile) was grafted with ACTU in the same way as were the plates. Polymerization of styrene was carried out as previously the polymer-covered rutile was thoroughly extracted with toluene in a Soxhlet apparatus. Polystyrene was then detached fi jm the Ti02 particles as shown in Scheme 3 and subjected to molecular weight measurements by size exclusion chromatography (SEC), which gave... 

Covering particles of ammonium nitrate with non-hygroscopic layers of polymers, such as polyvinyl compounds. This however should be Uiniied to substances which could not produce harmful influence on the explosive properties of ammonium nitrate and (for underground work) would not give harmful products after detonation, such as chlorine compounds from polyvinyl chloride. 

Steric stabilization is another well-established method of stabilizing colloidal suspensions of submicron to micron size [23]. The particles are coated with a layer of adsorbed or grafted polymer chains that provides a steric repulsion of entropie origin and helps disperse the particles by counterbalancing van der Waals attraction (Fig. la). The polymeric nature of the adsorbed or grafted layer softens the interparticle interactions and makes the particles intrinsically deformable. Many polymer chain/particle combinations have been synthesized and studied, and are described in the literature. Several popular colloidal systems consist of silica particles covered with various polymers such as polydimethylsiloxane [24], stearyl alcohol [25], alkyl chains [26], and polyethylene oxide [27]. Polymethylmethacrylate and polystyrene particles grafted with polymer chains have also been used extensively. For a review on the impressive literature on the subject we refer the interested reader to Vlassopoulos and Fytas [2]. 

The pore shape influences the mass transfer rate and thus the efficiency of separation. The effective diameter of pores determines the range of separated molar masses. The pore size distribution and the pore volume are decisive for selectivity of separation (section 4.6.2.3). The pore sizes of commercially available gels cover the region necessary for separation of the wide spectrum of substances — from low molecular samples to very high polymers, colloidal particles and viruses. The mean values of pore diameters range from few nanometers to about 2.5 /xm. Gels with various pore sizes, but of the same type, can be combined within the same column. 

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