Efficient preparation of macroporous poly (methyl methacrylate) materials using high internal phase emulsions

A variety of approaches can be used to generate porous polymers. All factors considered, emulsion templating stands out and has as such been established as a versatile and reliable method for making macroporous polymeric materials. Materials resulting from this approach possess tunable porosity, pore sizes and connectivity. Consequently, emulsion templating-synthesized materials have found a slew of applications in tissue engineering, gas storage, catalysis, chromatography, filtration and separation technology.

Technically, emulsion templating extends far beyond the original hydrophobic porous polymers that were synthesized within surfactant-stabilized water-in-oil high internal phase emulsions (HIPEs) by using free radical polymerization. The polymerization of the continuous phase of water-in-oil (w/o) emulsion templates gives polymerized HIPEs (polyHIPEs) which upon evaporation of the water droplets produces macroporous polymers. However, the polyHIPE preparation using methyl methacrylate (MMA) has been found challenging. Purified monomers and inert atmosphere have been used to enhance the polymerization of w/o HIPE templates stabilized either by solid particles or by 35% surfactant and up to 20% co-stabilizer in the oil phase. As of now, approaches in use for the thermo- and photo-initiated polymerization of emulsion templates for making macroporous poly (methyl methacrylate) materials both suffer from low energy efficiency and require tedious sample preparation (removal of inhibitors, inert atmosphere) or specialized equipment to be used.

To address this, Tommy S. Horozov together with his PhD student Khaled M. Althubeiti at the University of Hull proposed an easy and efficient technique for the preparation of open pore materials with up to 93% porosity by using redox-initiated polymerization in the oil phase of high internal phase emulsion templates. To be specific, the authors focused on the preparation of porous PMMA materials due to the fact that the MMA is an inexpensive widely used monomer, while as PMMA is a polymer with excellent mechanical properties, thus suitable for a variety of engineering applications. Their work is currently published in the research journal, Reactive and Functional Polymers.

In brief, their approach commenced with sample preparation where emulsion templates were prepared and afterwards subjected to polymerization. Subsequently, the samples were characterized using various techniques with the aim being to reveal various morphologies and mechanical aspects of the samples.

The authors demonstrated that their approach eliminated the need of heating/irradiation and monomer purification, consequently reducing fabrication costs without sacrificing the mechanical properties of the porous materials. In addition, they showed that their approach allowed the two component HIPE curing systems to be made, stored and used in the field for rapid preparation of open pore monoliths in spaces with complex geometry. They illustrated this by making polymeric microfilters or static mixers which were ready to use in less than 20 minutes.

In summary, their study demonstrated an efficient method for the preparation of open pore polyHIPE materials by using benzoyl peroxide (BPO) – tertiary amine (DMPT) redox-initiated polymerization of w/o HIPE templates formulated with MMA or other methacrylates (such as isobutyl or benzyl methacrylate). Overall, they demonstrated for the first time that surfactant stabilized w/o HIPEs with a continuous oil phase of methyl methacrylate and 10–30 vol% ethylene glycol dimethacrylate crosslinker could be easily polymerized via free radical initiation in the oil phase using a BPO – amine redox couple. In an interview with Advances in Engineering, Professor Tommy Horozov further illuminated that their approach has a considerable potential for further practical exploitation in filtration, separation, microfluidics, chromatography and other fields where porous materials are used.