Concentrated emulsion polymerisation

In the case of emulsion polymerisation, half the micelles will be reacting at any one time. The conversion rate is thus virtually independent of radical concentration (within limits) but dependent on the number of micelles (or swollen polymer particles). 

An increase in the rate of radical production in emulsion polymerisation will reduce the molecular weight since it will increase the frequency of termination. An increase in the number of particles will, however, reduce the rate of entry of radicals into a specific micelle and increase molecular weight. Thus at constant initiator concentration and temperature an increase in micelles (in effect in soap concentration) will lead to an increase in molecular weight and in rate of conversion. 

Poly(vinyl chloride) is commercially available in the form of aqueous colloidal dispersions (latices). They are the uncoagulated products of emulsion polymerisation process and are used to coat or impregnate textiles and paper. The individual particles are somewhat less than 1 p,m in diameter. The latex may be coagulated by concentrated acids, polyvalent cations and by dehydration with water-miscible liquids. 

Polymer materials can easily be prepared from HIPEs if one or the other (or both) phases of the emulsion contain monomeric species. This process yields a range of products with widely differing properties. Additionally, as the concentrated emulsion acts as a scaffold or template, the microstructure of the resultant material is determined by the emulsion structure immediately prior to polymerisation. 

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. 

Dispersed phase polymerisation of HIPEs has also been used to prepare polymer-supported quaternary onium phase transfer catalysts [162]. One strategy involved the polymerisation of a concentrated emulsion of vinyl benzyl chloride (VBC) in water and subsequent quaternisation of the polymer resin with tertiary amines and phosphines (Fig. 22). 

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. 

Gelatin-PMMA composite materials can also be prepared via the HIPE pathway [187]. Concentrated emulsions of methyl methacrylate in aqueous gelatin/surfactant solutions, upon polymerisation at 50°C, yielded composite membrane materials. The gelatin lends considerable stability to the emulsions, which will not form in its absence. The membranes swelled in water and certain... 

The conditions were determined for runaway/non-runaway polymerisation of styrene in an oil-heated batch reactor at 3 bar, using dibenzoyl peroxide as initiator at 3 concentrations. Results are presented diagrammatically [1], A calorimetric study of polymerisation initiated by benzoyl peroxide not surprisingly concluded that emulsion polymerisation in water is safer than bulk polymerisation [2]. 

The mechanism of emulsion polymerisation is complex. The basic theory is that originally proposed by Harkins21. Monomer is distributed throughout the emulsion system (a) as stabilised emulsion droplets, (b) dissolved to a small extent in the aqueous phase and (c) solubilised in soap micelles (see page 89). The micellar environment appears to be the most favourable for the initiation of polymerisation. The emulsion droplets of monomer appear to act mainly as reservoirs to supply material to the polymerisation sites by diffusion through the aqueous phase. As the micelles grow, they adsorb free emulsifier from solution, and eventually from the surface of the emulsion droplets. The emulsifier thus serves to stabilise the polymer particles. This theory accounts for the observation that the rate of polymerisation and the number of polymer particles finally produced depend largely on the emulsifier concentration, and that the number of polymer particles may far exceed the number of monomer droplets initially present. 

Monodispersed sols containing spherical polymer particles (e.g. polystyrene latexes22"24, 135) can be prepared by emulsion polymerisation, and are particularly useful as model systems for studying various aspects of colloidal behaviour. The seed sol is prepared with the emulsifier concentration well above the critical micelle concentration then, with the emulsifier concentration below the critical micelle concentration, subsequent growth of the seed particles is achieved without the formation of further new particles. 

Gordon, J.L. Emulsion polymerisation. 6. Concentration of monomers in latex particles. /. Polym. Sci. Al, 6(10PA), 2859. 

The particles are heterogeneous by definition. As with singlepolymer particles, heterogeneities in density can arise as a result of the mechanism of particle formation. Usually a polymer has a different density to the liquid monomer from which it is derived, and in most emulsion polymerisation processes to produce reasonably concentrated dispersions, propagation is dominated by arrival at the particle surface of oligomeric radicals which can lead to non-homogeneous shrinking within the particle. 

Effect of surfactant level. Much of the earlier work on this reaction system was carried out using sodium dodecylbenzene-sulphonate as the surfactant. In the course of experiments which were Intended to investigate the effect upon polymerisation rate of varying the surfactant level, it was discovered that purified sodium dodecylbenzenesulphonates are apparently able to act as initiators of free-radical emulsion polymerisation in the absence of other added Initiating substances. Furthermore, as the results summarised in Figure 5 show, the rate of polymerisation in the absence of added initiator is directly proportional to the concentration of sodium dodecylbenzenesulphonate In the aqueous phase. 

The emulsion polymerisation of styrene or MMA showed an optimum weight ratio of (INUTEC )/monomer of 0.0033 for PS particles, and 0.001 for PMMA particles. The (initiator)/(monomer) ratio was kept constant at 0.00125, and the monomer conversion was higher than 85% in all cases. Latex dispersions of PS reaching 50% and of PMMA reaching 40% could be obtained using such low concentrations of INUTEC SPl. Figure 17.1 shows the variation of particle diameter with monomer concentration. 

Reactions in micelle system are usually difficult for synthetic applications because of the problems in handling emulsions and the need for careful regulation of concentrations. However, there are many instances (e.g. emulsion polymerisation) where micelle systems are highly useful and are applied on a commercial basis. 

Micro-emulsion is another variant of emulsion polymerisation. Such emulsions are thermodynamically stable systems including swollen monomer micelles dispersed in a continuous phase. In general, they require fairly large concentrations of surfactants to be produced compared with the other dispersed polymerisation systems. Hence, the interfacial tension of the oil/water is generally close to zero. Polymers with ultra-high molecular weight, i.e. above 10 g/mol, can be obtained, as can copolymers with a very well-defined, homogenous composition. Whereas polymerisation can take 24-48 h in the normal emulsion process, it proceeds at a fast rate in micro-emulsion, as total conversion can be obtained in less than 30 min. Polymer particles of very small size (diameter < 100 nm) and narrow distribution can be obtained by this process. 

Emulsion droplets provide the large interfacial area necessary for efficient mass transfer during emulsion polymerisation. Most monomers have slight solubility in water so that they may be transported across the aqueous phase from the monomer droplets to the sites of polymerisation (i.e., the polymer particles). During polymerisation, the monomer concentration gradient will overcome Ostwald ripening forces, and diffusion of monomer from large drt lets to smaller monomer-swollen particles will occur. 

Inverse (or water-in-oil) emulsions (315, 401) are emulsions in which an aqueous phase is dispersed within a continuous organic phase. This system is essentially the inverse of a conventional emulsion, hence the name inverse emulsion. The organic phase is typically an inert hydrocarbon (such as mixed xylenes or low-odour kerosenes), and the aqueous phase contains a water-soluble monomer such as acrylamide (268). The aqueous phase may be dispersed as discrete droplets or as a bicontinuous phase (335), depending upon the formulation and conditions of the inverse emulsion. The hydrophilic-lipophilic balance (HLB) value of the stabiliser determines the form and stability of an inverse emulsion, with HLB values of less than 7 being appropriate for inverse emulsions. Steric stabilisers such as the Span , Tween , and Plutonic series of nonionic surfactants are usually used in preparing inverse emulsions. Inverse emulsions, suspensions, miniemulsions (199), and microemulsions have been prepared, primarily as a function of the stabiliser concentration. Commercial products produced by inverse emulsion polymerisation include polyacrylamide, a water-soluble polymer used extensively as a thickener. 

Gaseous vinyl chloride monomer is polymerised under high pressure conditions. Since polyvinyl chloride polymer is insoluble in its own monomer, the reaction kinetics do not follow the classical emulsion polymerisation kinetics. During polymerisation, chain transfer to monomer is extensive, and molecular weight development depends upon the reaction temperature rather than the initiator concentration. Consequently, lower reaction temperatures are needed to reach higher molecular weights. A typical formulation for the suspension polymerisation of polyvinyl chloride is given in Table 5. 

Details are given of a non-steady-state operation for controlling latex particle size distribution by using a continuous emulsion polymerisation of vinyl acetate. The experiment was conducted in a continuously stirred tank reactor under conditions below the critical micelle concentration of the emulsifier. The mean residence time was switched alternately between two values in the nonsteady-state operation to induce oscillations in monomer conversion in time. The effect of the switching operation on particle size distribution is discussed. 13 refs. 

PS/PU hybrid latex particles were synthesised using water soluble or dispersible PU resins as emulsifiers. Two kinds of PU resins were prepared from isophorone diisocyanate, poly(l,2-propylene glycol)s and 2,2-bis(hydroxymethyl)-propionic acid. Emulsion polymerisation of styrene with the PU resins showed similar kinetic dependence on stabiliser and initiator concentration as with conventional... 

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