Unraveling the Complexities of Silica Nanoparticle\nAdsorption onto Polymer Latexes in Pickering Emulsion Polymerization

Incorporating unmodified\nsilica nanoparticles onto polymer latexes\nto fabricate aqueous polymer dispersions without relying on electrostatic\nattraction during the Pickering emulsion polymerization process still\nfaces challenges. For negatively charged silica nanoparticles to successfully\nadsorb onto polymer latexes, particularly in an anionic initiator\nemulsion polymerization system, they have remained elusive without\nthe use of auxiliary monomers and cationic initiators. This study\ninvestigates various experimental parameters, such as emulsion polymerization\ntemperature, monomer solubility, salt concentration, and cation type,\nto elucidate the factors influencing the adsorption of unmodified\nsilica nanoparticles in Pickering emulsion polymerization. While poly(methyl\nmethacrylate) (PMMA)/SiO<sub>2</sub> hybrid latexes can be obtained\nunder pH conditions of 5–6 and at temperatures of 65 °C\nor below, the loading rate of silica nanoparticles decreases as the\nreaction temperature increases, resulting in bare PMMA latexes without\nsilica nanoparticle adsorption at temperatures exceeding 70 °C.\nIntroducing styrene (St) into the monomer mixture with methyl methacrylate\nin a ratio of up to 10 wt % leads to a gradual decrease in silica\nnanoparticle loading rate, from 27.3 to 8.2 wt %, attributed to the\nlow solubility of St in water. Furthermore, the presence of sodium\nions (Na<sup>+</sup>) is found to be crucial for silica nanoparticle\nadsorption onto PMMA latexes, as the sodium ions have a stabilizing\neffect on both the silica nanoparticles and the silica nanoparticle-armored\nlatexes. These findings highlight the complex nature of Pickering\nemulsion polymerization in the presence of unmodified silica nanoparticles,\ndemonstrating that the loading rate of silica nanoparticles onto polymer\nlatexes is influenced by various factors. These insights pave the\nway for developing aqueous polymer dispersions with high silica nanoparticle\nloading rates onto polymer latexes, which is a desirable trait in\nthe coating industry.