Preparation of CO2-Switchable Poly(Methyl Methacrylate) Latexes by Surfactant-Free Emulsion Polymerization
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The work presented herein focused on the preparation of CO2 switchable poly(methyl methacrylate) latexes by surfactant-free emulsion polymerization. N,N-Dimethylaminoethyl Methacrylate (DMAEMA) was chosen as the functional comonomer to act as an in-situ generated surface active agent as well as the CO2 responsive agent. The azo initiator 2,2'-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride (VA-044) also partially contributed to both the stabilization and the CO2 switchability. DMAEMA was first protonated by HCl during the emulsion polymerization to prevent hydrolysis and enhance DMAEMA partitioning into the water phase. Two switching on/off cycles were conducted to the latex obtained from the surfactant-free emulsion polymerization. The first switching-off cycle was carried out by NaOH neutralization, followed by simple CO2 purging for redispersion. Full recovery of particle size and distribution, solids content and latex stability was achieved within 5 min of applying the triggers. The second switching cycle was carried out by alternating N2/CO2 purging. Sonication was not needed in each switching-on process (a significant and important improvement over previous work in the Cunningham and Jessop groups). A ratio of VA-044:DMAEMA within the range of 1:8~1:2, a minimum overall N% (NDMAEMA+NVA-044) at 1% and a maximum solids content at 20% were the required conditions in order to make latex successfully recover from two switching cycles. Systematic particle size manipulation was carried out by varying temperature, VA-044:DMAEMA ratio and solids content. A wide size range 170~500 nm was achieved, allowing for multiple potential applications.