Polymer-coated spherical particles

With the methods discussed in Chapter 6.5. we are able to produce small polymer particles from solutions of polymers in compressed gases [52,53], First results indicate that, depending on the process conditions and the nozzle design, a wide variety of spherical particles, small fibrils, or longer fibres can be obtained. Sometimes the particle-size distribution obtained is very uniform, in other cases, a wide variety of sizes is obtained. In some cases, the monomer was used as solvent for the equivalent polymer. The particles, fibrils, etc., obtained may be used for direct spray-coating of surfaces but they also offer new opportunities as raw materials in their own right. New products are underway that are based on such technologies. 

Emulsions and suspensions are colloidal dispersions of two or more immiscible phases in which one phase (disperse or internal phase) is dispersed as droplets or particles into another phase (continuous or dispersant phase). Therefore, various types of colloidal systems can be obtained. For example, oil/water and water /oil single emulsions can be prepared, as well as so-called multiple emulsions, which involve the preliminary emulsification of two phases (e.g., w/o or o/w), followed by secondary emulsification into a third phase leading to a three-phase mixture, such as w/o/w or o/w/o. Suspensions where a solid phase is dispersed into a liquid phase can also be obtained. In this case, solid particles can be (i) microspheres, for example, spherical particles composed of various natural and synthetic materials with diameters in the micrometer range solid lipid microspheres, albumin microspheres, polymer microspheres and (ii) capsules, for example, small, coated particles loaded with a solid, a liquid, a solid-liquid dispersion or solid-gas dispersion. Aerosols, where the internal phase is constituted by a solid or a liquid phase dispersed in air as a continuous phase, represent another type of colloidal system. 

Figure 10.5 Spherical ensembles of citrate-stabilized Au nanoparticles onto polymer-coated Si02 cores. Au/Si02 particle sizes (a) 40/330 nm (b) 60/460 nm (c) 80/550 nm. Scale = 200 nm.34 (Reprinted with permission from B. Sadtler and A. Wei, Chem. Commun. 2002, 1604-1605. Copyright the Royal Society of Chemistry.)...

The other kind of systems largely studied, consists of polymethylmethacrylate (PMMA) or silica spherical particles, suspended in organic solvents [23,24]. In these solvents Q 0 and uy(r) 0. The particles are coated by a layer of polymer adsorbed on their surface. This layer of polymer, usually of the order of 10-50 A, provides an entropic bumper that keeps the particles far from the van der Waals minimum, and therefore, from aggregating. Thus, for practical purposes uw(r) can be ignored. In this case the systems are said to be sterically stabilized and they are properly considered as suspensions of colloidal particles with hard-sphere interaction [the pair potential is of the form given by Eq. (5)]. 

Consider a dilute suspension of spherical soft particles in an electrolyte solution of volume V in an applied shear field. We assume that the uncharged particle core of radius a is coated with an ion-penetrable layer of polyelectrolytes of thickness d. The polymer-coated particle has thus an inner radius a and an outer radius b = a + d. The origin of the spherical polar coordinate system (r, 0, (p) is held fixed at the center of one particle. We consider the case where dissociated groups of... 

Coating PANI onto various matrixes has been studied for a long time [38,39]. In particular, monodispersed particles with a perfect spherical shape are generally preferred as model ER materials to investigate ER effect since the morphology of the dispersed phase is one of the critical parameters. However, PANI particles synthesized by conventional oxidization polymerization are often of irregular shape. Therefore, to obtain monodispersed PANI ER particles, researchers have used monodispersed polymer spheres as core to develop various PANI-coated ER particles. For example, Jun et al. [40,41] have used monodisperse micron-sized porous... 

The particle morphology can have important ramifications for the latex product performance. Because multi-lobed particles have a larger hydrodynamic volume than a spherical particle of equal polymer mass, such types of latexes have been used to raise the viscosity in coatings applications. Hollow particles are used in paper coatings to improve the optical properties and surface smoothness. Particles with core-shell morphologies or with domains have been developed for impact modification. In addition, various microencapsulation techniques have been employed to enclose a wide variety of materials (47, 97,239) for pharmaceutical, agricultural and cosmetic applications. 

These microspheres are precisely calibrated, spherical, hydrophilic, microporous beads made of tris-acryl co-polymer coated with gelatin. They come in defined range of sizes, ranging from 40 to 1200 pm in diameter. Their smooth hydrophilic surface, deformability and minimal aggregation tendency have been shown to result in a lower rate of catheter occlusion and more distal penetration into the small vessels [32]. Their efficacy has been evaluated in several conditions, and vdien compared to the standard polyvinyl alcohol particles (PVA) particles, a deeper penetration and embolization of smaller and more peripheral vessels may be achieved. This distal embolization may limit the development of any collateral blood supply. Also, in a study where PVA particles and tris-acryl microspheres of similar size were compared, the level of vascular occlusion with calibrated tris-acryl microspheres precisely correlated with particle size whereas the level of vascular occlusion with PVA particles did not. Another study has demonstrated that in embolized tumors. 

For the entropic repulsion calculations, the model system considered is that of a spherical particle, radius a, separated by a distance H from a plate, with both surfaces coated with adsorbed polymer chains of root-mean-square (rms) height /r at a surface coverage 0. If pairs of chains of height /i on opposing surfaces just touch when those on the sphere lie on a circle which subtends, at the centre of the sphere, a semi-angle

entropic repulsion Kr for the system is given in kT units by Equation 9.9, where is the... 

Electropolymerization can be used to coat other substrates, not just planar electrodes, resulting in the production of conducting polymer "composites." For example, electrochemical methods have been used to coat spherical graphite particles in a pulsed bed reactor. In fact the substrate to be coated need not be conductive. Electropolymerization can be used to coat other structures by producing polymer in solution that is subsequently deposited onto other surfaces. This is possible because conducting polymers are formed... 

Consider a bare surface interacting with a surface, which is coated with a polymer brush ina good solvent. In analogy to Eq. (11.18), derive an expression for the interaction energy. Plot the force versus distance for a spherical particle of 3 [xm radius and a planar surface for the symmetric (both surfaces bear a polymer brush) and the asymmetric (only the planar surface is coated) case for N = 50, k = 1.8 nm, f = IQ- m and v = O.IJ ... 

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