Particle concentration, emulsions

Polystyrene latexes were similarly prepared by Ruckenstein and Kim [157]. Highly concentrated emulsions of styrene in aqueous solutions of sodium dodecylsulphate, on polymerisation, yielded uncrosslinked polystyrene particles, polyhedral in shape and of relative size monodispersity. Interestingly, Ruckenstein and coworker found that both conversions and molecular weights were higher compared to bulk polymerisation. This was attributed to a gel effect, where the mobility of the growing polymer chains inside the droplets is reduced, due to increased viscosity. Therefore, the termination rate decreases. 

Copolymer particles can also be prepared from HIPEs [159]. Thus, a HIPE dispersed phase consisting of styrene and methacrylic acid was polymerised to give copolymers. The surface concentration of carboxylic acid groups increased linearly with concentration of methacrylic acid in the feed. The small amount of water present in the concentrated emulsion, relative to conventional emulsion polymerisation, reduces the loss of methacrylic acid, which is highly water-soluble. 

The polystyrene latexes produced from concentrated emulsions have been used as carriers for the controlled release of herbicides [160], The release of 2-(2,4-dichlorophenoxy) propionic acid (2,4-DP) was found to depend on the water concentration, increasing with increasing dilution of the latex. High conversion to polymer was required to prevent a large initial release of herbicide on dilution however, a significant initial burst was still observed at almost complete conversion. This was reportedly due to dissolution of 2,4-DP at, or near, the surface of the latex particles. 

Water-in-oil concentrated emulsions have also been utilised in the preparation of polymer latexes, from hydrophilic, water-soluble monomers. Kim and Ruckenstein [178] reported the preparation of polyacrylamide particles from a HIPE of aqueous acrylamide solution in a non-polar organic solvent, such as decane, stabilised by sorbitan monooleate (Span 80). The stability of the emulsion decreased when the weight fraction of acrylamide in the aqueous phase exceeded 0.2, since acrylamide is more hydrophobic than water. Another point of note is that the molecular weights obtained were lower compared to solution polymerisation of acrylamide. This was probably due to a degree of termination by chain transfer from the tertiary hydroxyl groups on the surfactant head group. 

Problems with droplet concentration. It is necessary to use emulsion concentrations that are sufficiently dilute to prevent two or more droplets passing through the aperture simultaneously otherwise this will give particle sizes that are larger than expected. The upper particle concentration limit depends on the size of the aperture and the radius of the particles being examined (Lines, 1996). For example, the concentration should be less than 105 particles/ ml for a 100-pm aperture. 

The emulsion polymerization of vinyl hexanoate has been studied to determine the effect of chain transfer on the polymerization kinetics of a water-insoluble monomer. Both unseeded and seeded runs were made. For unseeded polymerizations, the dependence of particle concentration on soap is much higher than Smith-Ewart predictions, indicating multiple particle formation per radical because of chain transfer. Once the particles have formed, the kinetics are much like those of styrene. The lower water solubility of vinyl hexanoate when compared with styrene apparently negates its increased chain transfer, since the monomer radicals cannot diffuse out of the particles. 

The C12-(EO)9-MA macromonomer was found to be a very effective emulsifier for BzMA in water even at a concentration less than 5 wt%, to give a stable milky emulsion [42,96]. Table 3 shows that the rate of polymerization depends on the initiator type and polarity of continuous phase. In water solution, the rates are several times higher than in heptane. The rate of polymerization increases with increasing macromonomer concentration in systems with KPS and AIBN, and it is constant with AVA. The higher the macromonomer concentration, the higher the particle concentration and rate of polymerization. These results indicate that distribution of the initiator between the phases influences in complex way the polymerization and nucleation mechanism. 

The surfactant mixture used (CAV-CON Filmix 3) is identical to that used to form the artificial gas-in-water emulsions described in Chapter 9, where the concentrated emulsion particles observed were all above 0.3 pm in diameter (which is the lower detection limit of the laser-based flow cytometer instrument) and therefore did not include the co-existing micelles. 

Polymerizable surfactants capable of working as transfer agents include thiosulfonates, thioalkoxylates and methyl methacrylate dimer/trimer surfactants. Thioalkoxylates with 17-90 ethylene oxide units were produced from ethoxylated 11 bromo-undecanol by replacing the bromine with a thiol group via the thiazonium salt route [8]. In the presence of water-soluble azo initiator the thio ended Transurfs (used at a concentration above the CMC) gave monodispersed latex particles in emulsion polymerization of styrene. However, the incorporation of the Transurf remained low, irrespective of the process used for the polymerization (batch, semibatch, seeded). The stability of the lattices when the surfactant and the transfer function were incorporated in the same molecule was better than when they were decoupled. 

In the conventional emulsion polymerization, monomer droplets are dispersed ip an aqueous phase containing micellar aggregates of surfactant. In this case, the dispersed phase represents a relatively small volume fraction of the system and the micellar aggregates constitute the sites of the polymerization process. In the gel(paste)-like emulsions employed here, the volume fraction of the dispersed phase can be as high as 0.99, and the cells of the concentrated emulsion lead to the polymerized latex particles. 

In the methodology developed by us [24], the incompatibility of the two polymers was exploited in a positive way. The composites were obtained using a two-step method. In the first step, hydrophilic (hydrophobic) polymer latex particles were prepared using the concentrated emulsion method. The monomer-precursor of the continuous phase of the composite or water, when that monomer was hydrophilic, was selected as the continuous phase of the emulsion. In the second step, the emulsion whose dispersed phase was polymerized was dispersed in the continuous-phase monomer of the composite or its solution in water when the monomer was hydrophilic, after a suitable initiator was introduced in the continuous phase. The submicrometer size hydrophilic (hydrophobic) latexes were thus dispersed in the hydrophobic (hydrophilic) continuous phase without the addition of a dispersant. The experimental observations indicated that the above colloidal dispersions remained stable. The stability is due to both the dispersant introduced in the first step and the presence of the films of the continuous phase of the concentrated emulsion around the latex particles. These films consist of either the monomer-precursor of the continuous phase of the composite or water when the monomer-precursor is hydrophilic. This ensured the compatibility of the particles with the continuous phase. The preparation of poly(styrenesulfonic acid) salt latexes dispersed in cross-linked polystyrene matrices as well as of polystyrene latexes dispersed in crosslinked polyacrylamide matrices is described below. The two-step method is compared to the single-step ones based on concentrated emulsions or microemulsions. 

Encapsulation of Solid Particles by the Concentrated Emulsion Polymerization Method [35]... 

The role of the surfactant is to stabilize the gel-like concentrated emulsion. Upon heating at 40 °C, polymerization took place and the solid particles were encapsulated in the polymer. 

Very fine solid particles, namely fumed silica, were also encapsulated via the concentrated emulsion polymerization method. The amounts of the components involved are listed under PLS1 in Table 21. The PLS1 capsules range in size from 1.0 to 1.5 pm. 

Cross-linked polystyrene porous particles (with 21 mol% DVB) have been prepared by the concentrated emulsion polymerization method, using either toluene or decane as the porogen and an aqueous solution of SDS as the continuous phase. Since toluene is a good solvent for polystyrene while decane is a nonsolvent , the morphologies obtained in the two cases were different. The particles based on toluene (with a volume fraction of dispersed phase of 78%) have very small pores which could not be detected in the SEM pictures. The pore size distribution, which has sizes between 20 and 50 A and was determined with an adsorption analyzer, almost coincides with that in a previous study [49] in which porous polystyrene beads have been prepared by suspension polymerization. In contrast, the porous particles based on decane have pore sizes as large as 0.1-0.3 pm, which could be detected in the SEM pictures [44a], and also larger surface areas (47 m2 g ) than those based on toluene (25 m2 g ). The main difference between the concentrated emulsion polymerization and the suspension polymerization consists of the much smaller volume fraction of continuous phase used in the former procedure. The gel-like emulsion that constitutes the precursor in the former case contains polyhedral cells separated by thin films of continuous phase. The polymerization of the cells does not... 

Porous hydrophilic particles have previously been prepared [53] via suspension polymerization. A concentrated emulsion of water (containing a hydrophilic monomer (acrylamide or sodium acrylate) and a cross-linking agent) in oil containing Span-80 as emulsifier was employed by us to prepare hydrophilic particles. 

In summary, concentrated emulsions have been used to prepare latexes or composites, to encapsulate solid particles, for polymer blending and to generate molecular reservoirs. The mechanical properties could be controlled by combining suitable monomers in the latexes. The concentrated emulsion pathway was also employed in our laboratory in other directions than those presented in this review, and we would like to mention them in this final section without details, but with suitable references. 

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