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

• Published in: Journal of colloid and interface science 2024-02, Vol.655, p.371-382
• Main Authors: Blahnik, Jonas, Schuster, Jennifer, Müller, Rainer, Müller, Eva, Kunz, Werner
• Format: Article
• Language: English
• Subjects: adsorption; electron microscopy; emulsions; freeze drying; isopropyl alcohol; light microscopy; Ouzo effect; Polymerization; polymers; Porous materials; Porous polymers; pre-Ouzo; SFME; Surfactant-free microemulsion; Template synthesis; Ultra-flexible microemulsions
• ISSN: 0021-9797; 1095-7103
• DOI: 10.1016/j.jcis.2023.10.162

The graphical abstract shows a typical phase diagram for a polymerizable SFME system. The area in which SFMEs can be expected is marked in blue. Three SEM images of polymers and their area of origin are given: droplet-like polymers from o/w-like SFME, sponge-like polymers from w/o-like SFME, and a polymer without observable nanostructures from the region where no mesostructures were found before polymerization. [Display omitted] 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. 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. 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 th