Charge-stabilized particles

Figure 7.13. Sequence showing schematically (a) charge-stabilized particles, (b) particles flocculated by the polymer bridging mechanism, and (c) particles restabilized by steric repulsive (chain overlap) interactions at higher levels of polymer surface...

The observable properties of a latex, ie, stability, rheology, film properties, interfacial reactivity, and substrate adhesion, are determined by the colloidal and polymeric properties of the latex particles. Important colloidal properties include ionic charge, stability, particle size and morphology distribution, viscosity, solids. 

Two major types of stabilization mechanisms are described for submicron particles (1) charge stabilization, where surface charge forms a repulsive screen that prevents the particles from flocculation, and (2) steric stabilization, where a surface repulsive screen is formed by solvent-compatible flexible polymeric chains attached to the particle s surface. 

In a qualitative way, colloids are stable when they are electrically charged (we will not consider here the stability of hydrophilic colloids - gelatine, starch, proteins, macromolecules, biocolloids - where stability may be enhanced by steric arrangements and the affinity of organic functional groups to water). In a physical model of colloid stability particle repulsion due to electrostatic interaction is counteracted by attraction due to van der Waal interaction. The repulsion energy depends on the surface potential and its decrease in the diffuse part of the double layer the decay of the potential with distance is a function of the ionic strength (Fig. 3.2c and Fig. 

The choice of carrier liquid is primarily based on the suspension stability for colloids or solvent goodness for macromolecule solutions. Moreover, surface-charged colloidal particles are also sensitive to ionic strength and addition of surfactants. 

Similarly, charged solid particles (such as latex spheres) —kinetically stable lyophobic colloids —may exist in colloidal crystalline phases (with body-centered or face-centered cubic structures) as a consequence of thermodynamically favored reduction in free energies (see Chapter 13). Even neutrally charged spherical particles ( hard spheres ) undergo a phase transition from a liquidlike isotropic structure to face-centered cubic crystalline structures due to entropic reasons. In this sense, the stability or instability is of thermodynamic origin. 

ZETA POTENTIAL. The potential across the interface of all solids and liquids. Specifically, the potential across the diffuse layer of ions surrounding a charged colloidal particle, which is largely responsible for colloidal stability. Discharge of the zeta potential, accompanied by precipitation of the colloid, occurs by addition of polyvalent ions of sign opposite to that of the colloidal particles. Zeta potentials can be calculated from electrophoretic mobilities, i.e., the rates at which colloidal particles travel between charged electrodes placed in the solution. 

Thus the use amphiphilic macromonomers is another method to achieve the particle formation and their subsequent stabilization. Macromonomers can be pre-reacted to form graft copolymers, which are be introduced into the reaction medium afterwards. Macromonomers can also be copolymerized with classical monomers in situ to form graft copolymers. This is a simple and flexible method for producing monodisperse micron-sized polymer particles. Macromonomers can produce ion-free acrylic lattices with superior stability and film forming properties compared to conventional charge stabilized lattices. These non-con-... 

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