Unraveling Mechanistic Events in Solids-Stabilized Emulsion Polymerization by Monitoring the Concentration of Nanoparticles in the Water Phase

Abstract

The fate of nanoparticles used as stabilizers in solids-stabilized, or Pickering, emulsion polymerization for the formation of armored hybrid polymer latexes was studied. We showed that disk centrifugation can be used as a powerful quantitative tool to analyze and determine the concentration of nanoparticles in the water phase throughout solids-stabilized emulsion polymerizations. We performed a series of emulsion polymerizations using vinyl acetate, vinyl pivalate, methyl methacrylate, or butyl acrylate in presence of silica nanoparticles (Ludox TM-40, ca. 25 nm in diameter). The developed method to quantify the number of silica nanoparticles in the water phase proved to be an invaluable tool for determining key features of the polymerization process. The obtained concentration profiles versus monomer conversion explained the existence of limited coalescence of armored particles in the later stages of the solids-stabilized emulsion polymerization process of vinyl acetate, leading to nonspherical structures. Moreover, we demonstrated that the correlation of the measured number of silica nanoparticles present in the water phase with the average particle sizes of the latex particles provided excellent predictions for the coverage of the armored layer of nanoparticles on the surfaces of the polymer particles, corresponding to observed packing patterns.

A mechanistic investigation of Pickering emulsion polymerization

“…MMA was chosen because with this monomer, armoured latex particles can be formed without the need of an auxiliary comonomer. 24 The polymerizations were performed at different silica-tomonomer ratios (SiO2/M). The water-to-monomer ratio and initiator concentration were kept constant (See Table 2).…”

“…At the end of Pickering emulsion polymerizations of methacrylates there is an excess of stabilizer left in the water phase. 24 One could argue then that the presence of substantial amounts of Pickering stabilizer would lead to prolonged nucleation periods. However, latexes with a relatively low dispersity in size are obtained.…”

“…15 In their work, the authors used talc, barium sulphate, kaolin clay and aluminium oxide in the suspension polymerization of a variety of monomers. Since then Pickering hetero-phase polymerization has been developed further and now includes mini-emulsion, [16][17][18] dispersion 19,20 and emulsion polymerization processes, [21][22][23][24][25][26][27] using a variety of monomers and with a range of fillers, such as silica, laponite clay, magnetite, titanium dioxide, graphene oxide. 28 One driver for the development of heterogeneous Pickering polymerization processes is that they do not require molecular surfactants, offering a big advantage to the paint industry as surfactant migration often deteriorates the performance of coatings.…”

All-Acrylic Film-Forming Colloidal Polymer/Silica Nanocomposite Particles Prepared by Aqueous Emulsion Polymerization

“…DCP is a high resolution particle sizing technique that has been widely used in colloid science to size a range of lyophobic colloids including silica, 53 carbon nanotubes, 54 a range of copolymer latexes, 55,56 gold sols 57,58 and various nanocomposite particles. 10,14,16,18,29,32,39,59,60 In addition, DCP can be used to probe the colloidal stability of particulate dispersions.…”

“…[61][62][63] For example, Colard et al monitored the depletion of an ultrafine silica sol from the reaction solution during the in situ formation of polymer/silica colloidal nanocomposite particles. 53 However, one drawback associated with DCP is that a single value for particle density must be determined and input into the software to obtain meaningful size distributions. This is usually overcome by measuring the bulk density of the dried particles using helium pycnometry.…”