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

Jonas Blahnik; Sebastian Krickl; Klaus Schmid; Eva Müller; John Lupton; Werner Kunz

doi:10.1016/j.jcis.2023.06.025

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

J Colloid Interface Sci. 2023 Oct 15:648:755-767. doi: 10.1016/j.jcis.2023.06.025. Epub 2023 Jun 10.

Authors

Jonas Blahnik¹, Sebastian Krickl¹, Klaus Schmid¹, Eva Müller¹, John Lupton², Werner Kunz³

Affiliations

¹ Institute of Physical and Theoretical Chemistry, University of Regensburg, 93040 Regensburg, Germany.
² Institute of Experimental and Applied Physics, University of Regensburg, 93040 Regensburg, Germany.
³ Institute of Physical and Theoretical Chemistry, University of Regensburg, 93040 Regensburg, Germany. Electronic address: werner.kunz@ur.de.

PMID: 37321095
DOI: 10.1016/j.jcis.2023.06.025

Abstract

Hypothesis: 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.

Experiments: 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.

Findings: 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 literature. Polymerizations in SFME comprise a new intermediate between well-known solution (i.e., molecularly dispersed) and microemulsion respectively microsuspension polymerization processes.

Keywords: Free-radical polymerization; Mesostructuring; Microemulsion polymerization; Microsuspension polymerization; Polymerization; SFME; Surfactant-free microemulsion.