Synthesis of Novel pH-Responsive Latexes via Emulsion Polymerisation

Emulsion copolymerisation of 2-(tert-butylamino)ethyl methacrylate (TBAEMA) at 70 °C afforded sterically-stabilised latexes at approximately 10% solids. Dynamic light scattering (DLS) and scanning electron microscopy (SEM) studies confirmed that relatively narrow size distributions were obtained. Lightly cross-linked latexes acquired cationic microgel character upon lowering the solution pH, as expected. Poly(2-(tert-butylamino)ethyl methacrylate) (PTBAEMA) latex proved to be an effective Pickering emulsifier at pH 10, forming stable oil-in-water emulsions when homogenised with either n-dodecane or sunflower oil. These Pickering emulsions exhibited pH-responsive behavior: lowering the solution pH to 3 resulted in immediate demulsification due to spontaneous desorption of the cationic microgels from the oil/water interface. The secondary amine groups present on TBAEMA residues can react with isocyanates forming a urea linkage. Thus PTBAEMA latex-stabilised Pickering emulsions were readily converted into covalently cross-linked colloidosomes following reaction with a polymeric diisocyanate (which was dissolved in the oil phase prior to homogenisation). Such colloidosomes survived both an acid and ethanol challenge, confirming their robust latex super-structure. Copolymerisation of TBAEMA with styrene (S) afforded copolymer latexes with higher glass transition temperatures, which facilitated imaging of colloidosomes via SEM. However, the shell of these colloidosomes was found to be highly permeable, with the rapid release of a small molecule dye being observed by UV visible adsorption spectroscopy.

Lightly cross-linked poly(2-(diethylamino)ethyl methacrylate) (PDEA) latex particles of 190 to 240 nm were prepared via emulsion copolymerisation at 10% solids in the presence of a hydrophilic poly(ethylene glycol)-based macromonomer. A latex-to-microgel transition occurred on lowering the solution pH below the latex pKa of 6.9. When using dilute HCl/KOH to adjust the aqueous pH, a systematic reduction in the cationic microgel hydrodynamic diameter of 80 nm was observed over ten pH cycles. No such size reduction was observed when using CO2/N2 gases to regulate the aqueous pH. PDEA microgels do not stabilise Pickering emulsions when homogenised at pH 3 with n-dodecane, sunflower oil, isononyl isononanoate or isopropyl myristate. In contrast, PDEA latexes proved to be a ubiquitous Pickering emulsifier at pH 10, forming stable oil-in-water emulsions with each of these four model oils. Lowering the solution pH from 10 to 3 resulted in demulsification within seconds due to spontaneous desorption of the swollen cationic microgels. Six successive demulsification/emulsification cycles were performed on these Pickering emulsions using HCl/KOH to adjust the solution pH. Demulsification could also be achieved by purging with CO2 gas to lower the aqueous pH to pH 4.8. However, this required prolonged purging for 2 h.

Finally, the kinetics of swelling of near-monodisperse, lightly cross-linked 200 nm PTBAEMA, PDEA, poly(2-vinylpyridine) (P2VP) and poly(2-(diisopropylamino)ethyl methacrylate) (PDPA) latexes was investigated by the pH-jump method using a commercial stopped-flow instrument. The kinetics of swelling of each latex-to-microgel transition for sub-stoichiometric acid/amine molar ratios (at the particle pKa), stoichiometric and excess acid was examined. Fastest swelling times (tens of milliseconds) were observed for P2VP particles, followed by PTBAEMA and PDEA, (for which swelling times were comparable), with PDPA latexes swelling the slowest. This rank order correlates with the monomer repeat unit mass, which suggests that the cationic charge density plays an important role in determining the swelling kinetics Kinetics of deswelling for P2VP and PTBAEMA were also examined. Slower deswelling time scales (tens of seconds) were observed which we attribute to the formation of a latex-type skin upon deprotonation.