Diffusional Degradation

Besides coalescence, there is another mechanism by which emulsions degrade (or coarsen) into fewer, larger-sized droplets diffusional degradation. Monomer from smaller droplets diffuses to larger ones as the result of the process of interfacial free energy minimisation. This phenomenon is called Ostwald ripening (224). 

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. 

The extent of monomer diffusion can be reduced by the incorporation of a costabiUser. Costabiliser (104, 314, 354) must have two properties low molecular weight (to maximise the mixing free energy contribution to the droplet diffusional stability) and low 

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Steady-state dispersed miniemulsions are stable against diffusional degradation, but critically stabilized with respect to colloidal stability. 

The stability of miniemulsion droplets against diffusional degradation results from an osmotic pressure in the droplets, which controls the solvent or monomer evaporation. The osmotic pressure is created by the addition of a substance, which has extremely low water solubility, the so-called hydrophobe. This crucial prerequisite is usually not present in microemulsions, but... 

Reimers and Schork [94, 95] report the use of PMMA to stabihze MM A miniemulsions enough to effect predominant droplet nucleation. Emulsions stabilized against diffusional degradation by incorporating a polymeric costabilizer were produced and polymerized. The presence of large numbers of small droplets shifted the nucleation mechanism from micellar or homogeneous nucleation, to droplet nucleation. Droplet diameters were in the miniemulsion range and reasonably narrowly distributed. On-hne conductance measurements were used to confirm predominant droplet nucleation. The observed reaction rates were dependent on the amount of polymeric costabilizer present. The latexes prepared with polymeric costabilizer had lower polydispersities (1.006) than either latexes prepared from macroemulsions (1.049) or from alkane-stabilized miniemulsions (1.037). 

The colloidal stability (or diffusional degradation) of monomer droplets can be further understood in terms of the Morton equation [48], in which the partial molar free energy of mixing of polymer in the monomer phase (spherical droplets) is expressed as ... 

In the presence of a hydrophobe, characterized by a relatively low molecular weight and water-solubility, monomer droplets are stabilized against diffusional degradation. The volume fraction of monomer in the hydrophobe-containing monomer droplets may be evaluated by the following equation ... 

For a mini-emulsion prepared with a long-chain alcohol, the enhanced colloidal stability is attributed to both the retardation of diffusional degradation of mon-... 

The smallest droplets (i.e., the slowest creaming rate) are produced for the mini-emulsion in the absence of polymer, and the droplet size increases slightly with increasing polymer content (in the range of 0-1 wt %). In addition, the mini-emulsion prepared by 1 % PSt and without CA phase separated very quickly, indicating that such a product is probably not stable toward diffusional degradation. 

Refiners and Schork used lauroyl peroxide as the initiator and also as the costabUizer, and found that diffusion instability was reduced to the point where nucleation in the monomer droplets and polymerization conld be carried ont before significant diffusional degradation of the droplets took place. Alduncin et al. compared different oil-solnble initiators in conjunction with hexadecane as the costabUizer, and fonnd that only lanroyl peroxide was water-insolnble enongh to stabilize the monomer droplets against degradation by molecnlar diffusion. ... 

Finally, miniemulsion and microemulsion polymerization are two subcategories of emulsion polymerization with monomer droplets in water with much smaller droplets than in emulsion polymerization (about 50-1,000 nm in miniemulsion polymerization and 10-100 nm in microemulsion process, compared to 1-100 mm in diameter) [19, 20]. Water-insoluble costabilizers such as hexadecane and cetyl alcohol are present along with the surfactant to stabilize the monomer droplets against diffusional degradation [16]. 

Chern et al. [33-39] used stearyl methacrylate or lauryl methacrylate as the reactive costabilizer to stabilize styrene miniemulsion polymerizations. Just like conventional costabilizers (e.g., hexadecane), long-chain alkyl methacrylates act as costabilizers in stabilizing the homogenized submicron monomer droplets. Furthermore, the methacrylate group (—C=C(CH3)COO—) of the polymerizable costabilizer can be chemically incorporated into latex particles in the subsequent free radical polymerization and thereby reduce the level of volatile organic compounds (VOC). As the polymerization proceeds, the reactive costabilizer concentration in the nucleated monomer droplets will decrease. The initial decrease of the costabilizer concentration should not cause any diffusional degradation because the hydrophobic polymer formed inside the nucleated monomer droplets can help stabilize the polymerizing miniemulsion. 

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