Microemulsion and microsuspension polymerization of methyl methacrylate in surfactant-free microemulsions (SFME)

• Published in: Journal of colloid and interface science 2023-10, Vol.648, p.755-767
• Main Authors: Blahnik, Jonas, Krickl, Sebastian, Schmid, Klaus, Müller, Eva, Lupton, John, Kunz, Werner
• Format: Article
• Language: English
• Subjects: Free-radical polymerization; Mesostructuring; Microemulsion polymerization; Microsuspension polymerization; Polymerization; SFME; Surfactant-free microemulsion
• ISSN: 0021-9797; 1095-7103
• DOI: 10.1016/j.jcis.2023.06.025

Right side: Schematic drawing of a phase diagram expected for an SFME system and the region, which is suitable for polymerization in SFMEs. Left side: MD simulations of an SFME. Polymerization reactions take place in the oil-rich domains (marked orange). [Display omitted] This article presents a free-radical polymerization method in a mesostructured system – free of any surfactants, protective colloids, or other auxiliary agents. It is applicable for a large variety of industrially relevant vinylic monomers. The aim of this work is to study the impact of surfactant-free mesostructuring on the polymerization kinetics and the polymer derived. So-called surfactant-free microemulsions (SFME) were investigated as reaction media with a simple composition comprising water, a hydrotrope (ethanol, n-propanol, isopropanol, tert-butyl alcohol), and the monomer as the reactive oil phase (methyl methacrylate). Polymerization reactions were performed using oil-soluble, thermal- and UV-active initiators (surfactant-free microsuspension polymerization) and water-soluble, redox-active initiators (surfactant-free microemulsion polymerization). Structural analysis of the SFMEs used and the polymerization kinetics were followed by dynamic light scattering (DLS). Dried polymers were analyzed with regard to their conversion yield by mass balance, the corresponding molar masses were determined using gel permeation chromatography (GPC), and the morphology was investigated by light microscopy. All alcohols are suitable hydrotropes to form SFMEs, except for ethanol, which forms a molecularly disperse system. We observe significant differences in the polymerization kinetics and the molar masses of the polymers obtained. Ethanol leads to significantly higher molar masses. Within a system, higher concentrations of the other alcohols investigated give rise to less pronounced mesostructuring, lower conversions, and lower average molar masses. It could be demonstrated that the effective concentration of alcohol in the oil-rich pseudophases as well as the repulsive effect of the surfactant-free, alcohol-rich interphases constitute the relevant factors influencing polymerization. Concerning the morphology, the polymers derived range from powder-like polymers in the so-called “pre-Ouzo region” over porous-solid polymers in the bicontinuous region to dense, almost compacted, transparent polymers in unstructured regions, comparable to the findings for surfactant-based systems reported in the