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Efficient Synthesis of Sterically Stabilized pH-Responsive Microgels of Controllable Particle Diameter by Emulsion Polymerization
journal contribution
posted on 2006-03-28, 00:00 authored by Damien Dupin, Syuji Fujii, Steven P. Armes, Paul Reeve, Steven M. BaxterEmulsion polymerization of 2-vinylpyridine (2VP) in the presence of divinylbenzene (DVB) cross-linker, a cationic
surfactant, and a hydrophilic macromonomer, monomethoxy-capped poly(ethylene glycol) methacrylate (PEGMA),
at around neutral pH and 60 °C afforded near-monodisperse, sterically stabilized latexes at approximately 10% solids.
Judicious selection of the synthesis parameters enabled the mean latex diameter to be varied over an unusually wide
range for one-shot batch syntheses. Scanning electron microscopy studies confirmed near-monodisperse spherical
morphologies, with mean weight-average particle diameters ranging from 370 to 970 nm depending on the initiator,
polymeric stabilizer, and surfactant concentrations. Particle sizing studies were also conducted using disk centrifuge
photosedimentometry and dynamic light scattering and gave similar data. These lightly cross-linked latexes acquired
cationic microgel character at low pH, as expected. The critical pH for this latex-to-microgel transition was around
pH 4.1 at 1.0 wt % DVB, which is significantly lower than the pKa of 4.92 estimated for linear P2VP homopolymer
by acid titration. 1H NMR and aqueous electrophoresis studies indicated that substantial swelling occurred at low pH
due to protonation of the 2VP groups, while dynamic light scattering (DLS) studies indicated volumetric swelling
ratios of up to 3 orders of magnitude, depending on the initial latex diameter. Systematic variation of the degree of
cross-linking led to a monotonic decrease in the pKa values of the P2VP latexes (as judged by acid titration) and also
the critical swelling pH (as judged by visual inspection). This was attributed to the increasingly branched nature of
the P2VP chains in their swollen microgel form. Preliminary studies of the kinetics of acid-induced swelling were
also conducted using the pH jump method in conjunction with a stopped-flow apparatus. These P2VP latexes swell
significantly faster than P2VP latexes described in the literature and the characteristic time scales observed in the
present study are much closer to those predicted by the Tanaka equation.
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PEGMAcationic microgel characterpH jump methodP 2VP homopolymer1 H NMRDLSEmulsion Polymerization Emulsion polymerizationlatex diameterDVBdisk centrifuge photosedimentometryScanning electron microscopy studiesParticle sizing studiesP 2VP chainsP 2VP latexes2 VP groupsControllable Particle Diameteracid titration
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