Surfactant-free microemulsions (SFMEs) as a template for porous polymer synthesis

Jonas Blahnik; Jennifer Schuster; Rainer Müller; Eva Müller; Werner Kunz

doi:10.1016/j.jcis.2023.10.162

Surfactant-free microemulsions (SFMEs) as a template for porous polymer synthesis

J Colloid Interface Sci. 2024 Feb:655:371-382. doi: 10.1016/j.jcis.2023.10.162. Epub 2023 Nov 6.

Authors

Jonas Blahnik¹, Jennifer Schuster¹, Rainer Müller¹, Eva Müller¹, Werner Kunz²

Affiliations

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

PMID: 37948811
DOI: 10.1016/j.jcis.2023.10.162

Abstract

Hypothesis: Surfactant-free microemulsions (SFMEs) were recently reported to be an interesting medium for free-radical polymerizations. The aim of this study is to investigate the link between the morphology of PMMA monopolymers as well as PMMA-PHEMA-copolymers with the expected nature of the SFME before polymerization. A surfactant-based microemulsion with nonionic surfactants was investigated as a reference system. It is expected that the kind of mesostructuring of the SFME (oil-in-water-like, bicontinuous, inverse) corresponds with the latter polymer morphology, just like it is the case in surfactant-based systems.

Experiments: Simple SFME systems composed of water, a hydrotrope (isopropanol or tert-butyl alcohol), and methyl methacrylate (MMA) as polymerizable oil as well as the more complex system comprising 2-hydroxyethyl methacrylate (HEMA) as an additional amphiphilic co-monomer, were investigated. A surfactant-based system using a mixture of Tergitol 15-S-12 and Synperonic A11-LQ-(TH) as surfactants, water, and MMA in the presence and absence of HEMA as polymerizable co-surfactant was investigated as a reference system. Structural analysis was done by recording (pseudo-)ternary phase diagrams, dynamic light scattering (DLS), and conductivity measurements. Polymerizations were performed using the oil-soluble initiator PEROXAN BCC at 318 K for 24 h with adjacent lyophilization. The morphology of dried polymers was determined by light microscopy, scanning electron microscopy, and BET adsorption isotherms.

Findings: Porous polymers of different morphologies (from coagulations of droplet-like aggregates to sponge-like ones) in the size range of 200 nm up to some µm can be derived from previously mesostructured, surfactant-free mixtures. Previously unstructured, oil-rich regions lead to solid, transparent polymers without nanostructured morphologies. The surfactant-based reference system comprises remarkably similar phase behavior before polymerizations and similar polymer morphologies as the comparable surfactant-free system. This leads to the assumption that the hydrotropic behavior of HEMA and its interplay with MMA and water is the structure-giving factor in this system.

Keywords: Ouzo effect; Polymerization; Porous materials; Porous polymers; SFME; Surfactant-free microemulsion; Template synthesis; Ultra-flexible microemulsions; pre-Ouzo.