Microgel Colloidosomes Based on pH-Responsive Poly(tert-butylaminoethyl methacrylate) Latexes

Abstract

Emulsion copolymerization of 2-(tert-butylamino)ethyl methacrylate (TBAEMA) with divinylbenzene (DVB) cross-linker in the presence of monomethoxy-capped poly(ethylene glycol) methacrylate (PEGMA) at 70 °C afforded sterically stabilized poly[2-(tert-butylamino)ethyl methacrylate] (PTBAEMA) latexes at 10% solids at pH 9. Such particles proved to be an effective Pickering emulsifier at pH 10 for both n-dodecane and n-hexane. (1)H NMR spectroscopy was used to follow the model reaction between the secondary amine of the TBAEMA monomer and the isocyanate groups of tolylene 2,4-diisocyanate-terminated poly(propylene glycol) (PPG-TDI). Cross-linking the PTBAEMA latex particles adsorbed at the n-dodecane/water interface using this oil-soluble PPG-TDI cross-linker at around 0 (o)C led to robust colloidosomes that survived an acid challenge. This resistance to demulsification was confirmed via laser diffraction studies following an in situ switch from pH 10 to 3, since no change was observed in either the oil droplet size or concentration (compared to non-cross-linked PTBAEMA-stabilized Pickering emulsions). Such PTBAEMA colloidosomes survived removal of the internal oil phase on washing with excess ethanol. However, because ethanol is a good solvent for the PTBAEMA chains, imaging the ethanol-treated colloidosomes via electron microscopy proved rather problematic due to partial film formation. Therefore, a series of TBAEMA/styrene copolymer latexes (comprising 10, 30, 50, or 60 mol % styrene) were prepared via emulsion copolymerization at 70 °C in the presence of DVB and PEGMA. The higher glass transition temperatures exhibited by these copolymer particles (and their greater resistance to ethanol swelling) enabled better-quality electron microscopy images to be obtained. The presence of nitrogen atoms at the surface of these copolymer latex particles was confirmed via X-ray photoelectron spectroscopy studies; these secondary amine groups allow covalent cross-linking via PPG-TDI when adsorbed at the surface of n-dodecane droplets at TBAEMA comonomer contents as low as 40 mol %. After removal of the n-hexane oil phase by evaporation, fluorescence microscopy studies indicate that these colloidosomes undergo collapse in their latex form at pH 10 but regain their original spherical morphology in their cationic microgel form at pH 3.5.

Doubly crosslinked microgel-colloidosomes: a versatile method for pH-responsive capsule assembly using microgels as macro-crosslinkers

“…To overcome this performance constraint microgel particles have been employed to prepare stimulus-responsive microgelcolloidosomes. 10,11 Microgels are crosslinked polymer particles that swell in a good solvent or when the pH approaches the pK a of the constituent polymer. 12 One of the inherent advantages offered by microgel-colloidosomes is that the pore size within the shells can be tuned using environmental response (e.g., pH 11 ) which enables triggered release for encapsulated actives.…”

“…10,11 Microgels are crosslinked polymer particles that swell in a good solvent or when the pH approaches the pK a of the constituent polymer. 12 One of the inherent advantages offered by microgel-colloidosomes is that the pore size within the shells can be tuned using environmental response (e.g., pH 11 ) which enables triggered release for encapsulated actives. Furthermore, because microgel particles can themselves exhibit triggered release of actives, 13 microgel-colloidosomes could provide opportunity for a hierarchical release system.…”

“…15,16 This approach benefits from the very high stabilisation afforded to emulsion droplets by adsorbed microgel particles. However, a limitation of previous methods used to prepare microgelcolloidosomes, 11,15,17 and indeed more generally of conventional polymer particle based colloidosomes, 9 is that construction requires two (or more) structure-forming components. Typically a small molecule or polymer-based cross-linking agent is used to link adsorbed particles together at the droplet interface.…”

“…Typically a small molecule or polymer-based cross-linking agent is used to link adsorbed particles together at the droplet interface. 11 Approaches using added cross-linkers have the disadvantages of diluting colloidosome stimulus responsiveness and may introduce associated toxicity concerns. We address these problems by using a new assembly approach here.…”

Tuning Amphiphilicity of Particles for Controllable Pickering Emulsion

“…Many examples involving microgels in emulsion systems have been reported in the past few years [106,107,108,109,110,111,112]. Fujii and coworkers [113] synthesized poly(4-vinylpyridine)/silica (P4VP/SiO 2 ) microgels for Pickering emulsion.…”