A mini-emulsion polymerization is a variation on the more conventional emulsion polymerization process in that in the ideal scenario latex particles are formed by monomer droplet nucleation. The monomer droplets are turned into polymer particles. The trick to achieve this is to shrink monomer emulsion droplets to sub-micrometer diameters. For this two ingredients are key, one is a lyophobe, a compound that dissolves in the monomer droplet but does not like to partition into the continuous phase, here water. Typically n-hexadecane is used. This compound suppresses coarsening, also called Ostwald ripening, of the droplets by providing an Osmotic counter pressure. The other essential ingredient is a surfactant which aids to stabilize the large combined surface area of the droplets and keeps then from colliding and fusing (colloidal stability).

The use of molecular surfactants, however, can have negative impacts when the polymer latex is used in formulations and applications as the surfactant can migrate. For example in a clear coating it could lead to uptake of water, causing the transparent coating to become opaque, a phenomenon known as water whitening.

Already in the 1990s one came up with the solution to use surfactants that could become part of the polymer. Buzzwords were inisurfs, surfmers, and transsurfs.

In our paper we look at macromonomers that are surfactants, which have been made by a technique called catalytic chain transfer emulsion polymerization (see image for chemical structure), and use these as reactive stabilizers in mini-emulsion polymerization.

These macromonomers are of renewed interest, and indeed we have used them in bonlab before to demonstrate polymerization induced self-assembly (PISA), their modification into reactive nanogels as Pickering stabilizers to make Janus and patchy polymer colloids, and as stabilizers in conventional emulsion polymerization.

The paper focusses on a mechanistic study on how they operate as colloidal stabilizers and what the impact of their use is on the mini-emulsion polymerization process with a focus on particle size distribution and morphology. PhD researchers Josh Booth and Josh Davies (an undergraduate student at the time) carried out the experimental work which formed the basis of this study.

We believe there is evidence that polymerization triggers a destabilization of the balance between Ostwald ripening and the added hexadecane hydrophobe, evident from particle shape analysis. Also we show that too much surfactant is bad, as it can trigger depletion flocculation on the mini-emulsion droplets during high-shear emulsification.

The paper is published in the scientific journal Polymer Chemistry from the Royal Society of Chemistry.

More on the paper:

https://doi.org/10.1039/D1PY01664D