When we synthesize polymer colloids by emulsion polymerization, molecular surfactants are often employed. These are required to keep the polymer latex particles dispersed in the water phase, so that they do not clump together, a phenomenon known as coagulation. Keeping polymer dispersions stable is especially important in end applications, such as waterborne coatings and adhesives.
A downside of the use of surfactant molecules is that they can desorb from the surface of the latex particles. This makes the particles colloidally unstable, and they coagulate. This can be disastrous in product formulations, such as water-based paints which have many components. Another downside of this mobility of the surfactant molecules is that they can migrate in the final coating, once applied on a substrate. This leads to deterioration of the properties of the coated film.
A solution to these problems is to use reactive surfactants. These have the ability to attach themselves to the latex particles irreversibly, through a covalent chemical bond.
Catalytic chain transfer polymerization (CCTP), a free radical polymerization processs that employs a cobalt catalyst in tiny amounts, is able to make polymer molecules with a reactive unsaturated end group. These macromolecules, also called macromonomers, can be chain extended using a process referred to as RAFT (Reversible Addition Fragmentation chain Transfer), into longer polymer molecules that can behave like surfactant molecules. The reactive end group is preserved, which means that if our polymer reactive surfactant is used in an emulsion polymerization to make latex particles, it will be attached permanently to the latex particles.
The technology to fabricate blockcopolymers under emulsion polymerization conditions using CCTP derived macromonomers, was originally reported in the 1990s by Moad and coworkers. Emphasis has been lying on polymer chain growth and hence the molecular weight distribution.
In our paper published in a special polymer colloids issue of the ACS journal Biomacromolecules PhD students Wai Hin Lee and Joshua Booth investigated the emulsion polymerization processes from a different angle. We investigated the particle size distributions and studied how these developed throughout a three-stage emulsion polymerization process (see the image below).
Three stages of emulsion polymerizations: Catalytic chain transfer emulsion polymerization to make an alkali soluble macromonomer, chain extension to make a reactive block copolymer surfactant, emulsion polymerization to prepare soft polymer colloids using the reactive surfactant.
Firstly, we looked at the catalytic chain transfer emulsion polymerization to make the hydrophilic block of the reactive polymer surfactants. Next we looked at how the particle size distributions changed upon chain extension with a hydrophobic block. Finally, we disintegrated the latex particles into a reactive polymer surfactant micellar dispersion. This was then used to carry out emulsion polymerizations to produce polymer latexes in absence of conventional surfactants.
Prof. dr. ir. Stefan Bon says: “I hope that people like the angle we took in our work, to shine a light on how the latex particles are formed and how the particle size distributions develop. We showed that it was possible to make soft polymer latexes with average particle diameters below 100 nm using our reactive surfactants, easily up to overall solids content of 30wt%. This is an excellent achievement. We believe that the reported emulsion polymerization processes are robust and versatile, and are delighted that we can produce polymer latexes that do not suffer from surfactant migration upon application.”
Details on the paper : https://pubs.acs.org/doi/10.1021/acs.biomac.0c00766