Surfactant-free synthesis of layered double hydroxide-armored latex particles

MgAl-layered double hydroxide (LDH)-armored latexes were produced by Pickering emulsion polymerization of styrene (St) using 2-hydroxyethyl methacrylate (HEMA) and methyl methacrylate (MMA) as auxiliary comonomers. While St led to bare polymer latex particles, St/HEMA and St/MMA mixtures produced nanocomposite latexes. Clearly, the use of hydrophilic comonomers such as HEMA or MMA is key to promoting adhesion of the LDH nanoplatelets onto the polymer particle surface and latex stabilization. Several parameters such as the nature and amount of auxiliary comonomer, the ionic strength and the LDH percentage were shown to play a crucial role in the formation and stability of the resulting MgAl-LDH-armored particles. Increasing the HEMA content above 8 wt% (based on total monomer) induced aggregation of both the LDH and latex particles, which was tentatively attributed to HEMA hydrolysis under basic conditions. Similar results were observed for MMA although destabilization occurred for higher concentrations (i.e., >30 wt%). Transmission electron microscopy confirmed the armored morphology with the concomitant presence of excess of free-standing platelets for high LDH contents. The average particle diameter was strongly dependent on the synthesis conditions and decreased with increasing the amount of MMA and/or the LDH content, indicating that the inorganic particles effectively played the role of a Pickering stabilizer. The addition of salt screened the positive charges between adjacent LDH sheets allowing closer packing of the LDH platelets onto the latex particle surface. The higher the salt content in the polymerization medium, the larger was the size of the nanocomposite particles.