Formation of Polymer Particles

Yeo, S.D. and Kiran, E. (2005) Formation of polymer particles with supercritical fluids a review. Journal of Supercritical Fluids, 34 (3), 287-308. 

All the described properties of such a s-fraction of poly(NVCl-co-NVIAz) synthesized at the temperature above the PST of the reacting system allowed us to draw the conclusion that the chains of this type had the comonomer sequence, which at the temperatures above the conformation transition facilitated the formation of polymer particles, where H-blocks are in the interior shielded by the P-blocks against additional intermolecular association. Such a behaviour of this copolymer in aqueous media is close to that of oligomeric proteins similar to casein [46] possessing a rather hydrophobic core surrounded by the polar segments. 

The predicted dependence of N on S and R,- for the formation of polymer particles by micellar and homogeneous nucleation followed by coagulative nucleation is given by Eq. 4-11 [Feeney et al., 1984] ... 

At low emulsifier concentrations near the CMC, an increase in the degree of agitation results in a reduction of the emulsifier used for the formation of polymer particles (like micelles). This is because the monomer droplets become smaller as the degree of agitation is increased, and so the amount of emulsifier adsorbed onto the surfaces of the monomer droplets increases in proportion to the increased surface area of the monomer droplets. This brings about a decrease in the number of polymer particles produced, and so a decrease in the rate of polymerization. 

Over the last two decades work on the formation and properties of polymer latices has developed extensively, and a very substantial amount of work has been devoted to the study of the processes of formation of polymer particles in a latex and to the characterization of the particles once formed (see for example, Fitch 1980). It is now generally recognized that in. 

Smith and Ewart (1948) proposed two idealized situations for the formation of polymer particles, assuming that (i) particle nucleation occurs in monomer-swollen emulsifier micelles, (ii) the volumetric growth rate of a particle is constant in the interval of particle formation, and (iii) radicals do not desoib from a panicle. 

Noting this, Nomura et oj. (1972,1976 Nomura, 1975) studied the effect of radical desorption on the formation of polymer particles from micelles for the case y = 0. They used the following non-steady-siate treatment under the assumption that the polymer particles contain at most one radical. 

The formation of polymer particles is perhaps the most difficult event to be investigated in emulsion polymerization, because its characteristic time and length scales are below the sensitivity of most experimental methods available (cf. discussion in Refs [54, 55]). 

Heterogeneous nucleation. The rate of formation of polymer particles by heterogeneous nucleation is... 

In this context, it is also instructive to define the critical point as the intersection of binodal and spinodal curves. When precipitation occurs above this point, the non-solvent diffuses like a droplet through a continuum of polymer. In contrast, higher dilutions of the casting solution which enter the unstable region below the critical point cause the formation of polymer particles in a continuous liquid phase, resulting in flocculation of the polymer or formation of very weak, powdery membranes. 

Continuous processing (e.g. encapsulation processes [3. 21. 26-28]) Controlled formation of polymer particles [38] and double emulsions [39]... 

The rate of formation of polymer particles by heterogeneous nucleation is expressed by Eq. (20), where is the nucleation rate, k m the rate coefficient for radical entry into the micelles, and the number of micelles in the reactor given by Eq. (21). 

Polymer particles can also be formed when the oligoradicals grow in the aqueous phase beyond the length at which they are still soluble in water and precipitate. The precipitated polymer chain is stabilized by the emulsifier present in the aqueous phase and monomer diffuses into the new organic phase, which allows a fast growing of the polymer chain. The process of formation of polymer particles by precipitation of oligoradicals is called homogeneous nucleation [5]. 

In the emulsion polymerization of VC, the formation of polymer particles may occur by a coagulative nucleation process [26,27]. In favor of this approach is the mutual insolubility of VCM and PVC Here, the primary particles formed undergo coalescence whereby large true polymer particles are formed. In addition, the formation of primary particles during polymerization favors the growth of particles by association of small with large particles. The latex particles were also formed after the disappearance of the micelles [92]. Thus, the polymerization in the monomer-saturated aqueous phase produces the new particles. The precipitation of oligomer radicals within the monomer droplets may increase the number of particles. In favor of this hypothesis is the formation of less reactive (occluded) radicals. 

In the emulsion polymerization of VC, the formation of polymer particles by the coagulative nucleation mechanism is expected to be operative. As polymerization proceeds, the formation of primary particles takes place. In this process, the primary particles undergo coalescence to form large polymer particles and/or they associate with large particles. 

Sedelmayer, R., Griesing, M., Halfar, A. H., Pauer, W., Moritz, H. U. (2013). Experimental investigation of the morphology formation of polymer particles in an acoustic levitator. Macromolecular Symposium, 333, 142-150. 

With the model presented in Eqs. (7.8.1)-(7.8.5) it is possible to model the transience of emulsion polymerization. As an example, let us derive the rate of formation of polymer particles as follows. If Nf represents the total number of particles in the reaction mass, then... 

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