Equilibrium swelling of latex particles

Modeling the Equilibrium Swelling of Latex Particles with Monomers... 

We now report on some experiments using seeded emulsion polymerization of styrene in which conditions were carefully chosen to ensure that Smith-Ewart Case 2 kinetics (6) would obtain throughout, in the absence of chain transfer/radical desorption effects. Various hydrocarbons were investigated for their effects on kinetics of polymerization and equilibrium swelling of the latex particles. 

The ethyl acrylate polymerization rate remains constant up to 38-40% conversion (Fig. 17), whereas according to data on equilibrium swelling of etbyl acrylate latex particles (Mamadaliev, 1978) the monomer phase disappears at 20% conversion. Hence, the polymerization rate remains constant after the disappearance of the monomer drops. Since the monomer drops... 

When equilibrium swelling of the latex particles is attained (which, of course, is saturation swelling because excess monomer is present in the form of droplets)... 

The expoimaital observation [12,20-28] which led German, Maxwell and co-workers directly to simplification of the theory for partitioning, was that for equilibrium swelling of sevoal Qqres of latex particles by specific monomer mixtures, the mole fraction of each particular monomer in the latex particles and in the droplets was the same, i.e. 

Table IX gives the recipe used for these pol3nnerizations. The polybutyl acrylate seed latex was prepared by heating the ingredients for 24 hours at 70° the styrene, water, and potassium persulfate were then added and polymerized for another 8 hours at 70°. Three methods of adding the styrene monomer were used in the second-stage polymerization (i) batch polymerization (ii) equilibrium swelling of the seed latex particles followed by batch pol3nneriza-tion (iii) starved semi-continuous pol3nnerization. The particle growth was essentially stoichiometric, i.e., no new particles were initiated. All three latexes formed transparent continuous films upon drying, whereas a 50 50 mixture of polybutyl acrylate and...

It is usual to consider the course of emulsion polymerization to proceed through three intervals [16,17]. The particle number increases with time in Interval I, where latex particles are being formed, and then remains constant during Intervals II and II. The monomer concentration in particles is in equilibrium with a monomer saturated aqueous solution. Swelling is limited only by the opposite force of the particle surface/water tension. Hence, the concentration of monomer in the particles is usually taken as constant up to the point where free monomer droplets disappear. In Intervals I and II, the monomer concentration... 

A. Loxley and B. Vincent, Equilibrium and kinetic aspects of the pH swelling of poly(vinylpyridine) latex particles, Colloid Polymer Sci. 275, 1108-1114 (1997). 

If the monomer is a good solvent for the polymer, the latex particles might be assumed to expand indefinitely beeause of inhibition of monomer. An equilibrium monomer concentration and swelling equilibrium is reached, however, because the free energy decrease due to mixing of polymer and monomer is eventually balanced by the increase in surface free energy which accompanies expansion of the particle volume. 

This relation has been tested by Morton, who determined values of and y from equilibrium swelling measurements of polystyrene latices. The results obtained were in good agreement with values of and y (54) determined by other methods. Allen (J) measured the swelling of natural rubber latex with methyl methacrylate and found the dependence of swelling on particle size to agree well with the above equation. 

Values for the propagation rate constant can be determined from bulk or solution experiments. Values of k have been published for a wide variety of monomers as a function of temperature. With standard emulsion polymerization recipes the value of [M]p is determined from equilibrium swelling measurements if a free monomer phase is present and by a mass balance if all the monomer is in the polymer particles. One normally assumes that [M] is not dependent on particle size in latexes comprised of different-sized particles. This assumption will be questionable in some systems, especially those involving high-swelling particles. 

The monomer concentration can remain constant for two reasons. First, a steady state can exist in which consumption of monomer by polymerization is directly compensated by subsequent diffusion of free monomer into the latex particle. The monomer concentration is then always lower than the saturation concentration at equilibrium. Second, the monomer concentration in the latex particles will be constant when the Gibbs interfacial energy A Gy and swelling A Gq cancel, so that the Gibbs energy of the monomer in equilibrium will be zero ... 

Electron microscopy of the final latex of the experiments given in Table I showed almost no new nucleation. The particle size distributions were narrow and indicated no noticeable coagulation as well. New nucleation would lead to increased rates whereas coagulation would have the opposite effect. Any decrease in the rate therefore must be due to a decrease in [m], if we assume n to be constant. We therefore determined the tofuene/polymer ratio in the seed latex in the absence and presence of the various additives. Toluene was chosen as the solvent, because it is similar to styrene and allows the measurement of equilibrium solubilities without the risk of polymerization. Table II gives the experimental values of the toluene solubility in the seed as a function of time. The results indicate that the swelling is nearly complete within 5 to 10 min. 

Chen et al. [39] and Jonsson et al. 140,41] independently proved that the composite particle morphology could be brought towards the equilibrium morphology predicted by the thermodynamic model by a post-polymerization swelling treatment of the composite latexes with solvents. 

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