Particle dispersion/redispersion

In these cases, the polymer remains processible in the gelled state, because it is in the form of discrete PSA particles dispersed in the reaction medium. However, once the particles are dried, redispersion may be difficult if strong interactions develop between the particle surfaces. Polymerization of the acrylic PSA directly on the substrate, as in the case of UV polymerization, can also yield a covalently crosslinked polymer that does not require any further coating steps [71]. 

PVA Particles. Dispersions were prepared in order to examine stabilization for a core polymer having a glass transition temperature below the dispersion polymerization temperature. PVA particles prepared with a block copolymer having M PS) x 10000 showed a tendency to flocculate at ambient temperature during redispersion cycles to remove excess block copolymer, particularly if the dispersion polymerization had not proceeded to 100 conversion of monomer. It is well documented that on mixing solutions of polystyrene and poly(vinyl acetate) homopolymers phase separation tends to occur (10,11), and solubility studies (12) of PS in n-heptane suggest that PS blocks with Mn(PS) 10000 will be close to dissolution when dispersion polymerizations are performed at 3 +3 K. Consequently, we may postulate that for soft polymer particles the block copolymer is rejected from the particle because of an incompatibility effect and is adsorbed at the particle surface. If the block copolymer desorbs from the particle surface, then particle agglomeration will occur unless rapid adsorption of other copolymer molecules occurs from a reservoir of excess block copolymer. 

If a polystyrene latex that is stabilized solely by an electrostatic mechanism is coagulated by the addition of electrolyte, that coagulation is usually irreversible to subsequent dilution. In contrast, sterically stabilized dispersions can usually be flocculated by the addition to the dispersion medium of a nonsolvent for the stabilizing moieties mere dilution of the concentration of the nonsolvent to a suitably low value is often sufficient to induce the particles to redisperse spontaneously. 

A helpful presentation which illustrates the effect of shear rate on dispersions is to use dimensionless quantities as the axes. In this way the colloidal forces can be represented as the ratio of electrostatic repulsive terms to the attractive term, i.e. ereo i R/A and the hydrodynamic terms as the ratio of the shear term to the attractive term, i.e. (mr)R y/A. Following Zeichner and Schowalter [90] this is illustrated in Figure 3.30. Thus as illustrated by the arrow the impact of a shear gradient can move particles out of the secondary minimum association into a region of stability. However, as the shear rate increases still further primary minimum coagulation can occur until at even higher shear rates the particles are redispersed again. 

Fig. 42 TEM images of (a) Ag nanoparticles adsorbed onto PS particles, and (b) Ag-PS core-shell particles, (c) Extinction spectra of as-prepared Ag-PS peirticles (curve a), Ag-PS particles dispersed in acetone (curve b), and redispersed back in water (curve c). Adapted from [346] with permission of the American Chemical Society...

In the example presented here the reverse micelle was prepared by dispersion of the surfactant, sodium bis(2-ethyl hexyl) sulfosuccinate (AOT) in isooctane. The hydrolysis and sol-gel processing of titanium isopropoxide was carried out in the 5 nm size cavity of the reverse micelle to produce highly uniform Ti02 nanoparticles. These particles were redispersed in a polymer (polyimide) solution and cast as a film of the polymer composite containing Ti02 nanoparticles. 

Fig. 13-2 Dispersion/redispersion - comparison of particle size distribution.

Stirred Vessels Gases may be dispersed in hquids by spargers or nozzles and redispersed by packing or trays. More intensive dispersion and redispersion is obtained by mechanical agitation. At the same time, the agitation will improve heat transfer and will keep catalyst particles in suspension if necessaiy. Power inputs of 0.6 to 2.0 kW/m (3.05 to 10.15 np/1,000 gal) are suitable. 

The zeta potential is a measurable indication of the apparent particle charge in the dispersion medium. When its value is relatively high, the repulsive forces usually exceed the attractive forces. Accordingly, the particles are individually dispersed and are said to be deflocculated. Thus, each particle settles separately, and the rate of sedimentation is relatively small. The settling particles have plenty of time to pack tightly by falling over one another to form an impacted bed. The sedimentation volume of such a system is low, and the sediment is difficult to redisperse. The supernatant remains cloudy even when settling is apparent. 

Flocculation studies (6) indicated that the mechanism of steric stabilization operates for the PMMA dispersions. The stability of PMMA dispersions was examined further by redispersion of the particles in cyclohexane at 333 K. Above 307 K, cyclohexane is a good solvent for PS and PDMS, and if the PS-PDMS block copolymer was not firmly anchored, desorption of stabilizer by dissolution should occur at 333 K followed by flocculation of the PMMA dispersion. However, little change in dispersion stability was observed over a period of 60 h. Consequently, we may conclude that the PS blocks are firmly anchored within the hard PMMA matrix. However, the indication from neutron scattering of aggregates of PS(D) blocks in PMMA particles may be explained by the observation that two different polymers are often not very compatible on mixing (10) so that the PS(D) blocks are tending to... 

With a careful redispersion technique stable dispersions free of excess block copolymer are produced for PVA particles with the anchoring PS block having Mn(PS) > 30000. This suggests that more effective anchoring occurs when the solubility of the block... 

Colloids are thermodynamically unstable conglomerates that form heterogeneous dispersions in aqueous systems. They tend to coagulate and precipitate, which means that the materials of which they are composed may be present both in the water column and in sediments. The coagulation and precipitation stages are generally considered irreversible, but forces in the environment can redisperse the particles. 

Both Vermeulen et al. (V3) and Calderbank (C3) conclude that the mean particle size is a function of impeller tip speed, whereas Rodriguez et al. (R7) find it to be a function of power input per unit volume. Jackson (J1) explains this apparent discrepancy on the basis that the System of Rodriguez et at. (R7) was more coalescing in nature than the systems studied by the others (C3, V3). In a coalescing dispersion there is frequent circulation and redispersion which requires impeller power. It is further pointed out (Jl) that, although the tip speed determines the mean particle size leaving the impeller, the particle size will also depend on the frequency of circulation which is a function of power input. 

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