A Paradigm Change: Producing High Performance Macroporous Polymers

High porosity, low density, closed cell and interconnected porous polymers are attractive materials for numerous applications. Macroporous polymers can be prepared by various methods, such as chemical and physical blowing, salt leaching, but also by emulsion templating first reported in 1962. Emulsion templating has now emerged as an effective method to synthesise macroporous polymers. Typically, water-in-oil High or Medium Internal Phase Emulsions (HIPEs or MIPEs) with a continuous oil phase consisting of or containing monomers are used as templates stabilised by surfactants and/or particles for the preparation of interconnected macroporous polymers, called poly(merized)H/MIPEs. The resulting polymers can be tailored to different degrees of interconnected pore morphology, physical and mechanical properties, and can have porosities of up to 99%. However, applications of polyH/MIPEs remain limited mainly because of their inherent brittleness. In the last decade, much research focused on improving the mechanical properties, which was achieved by incorporating particle reinforcements, increasing the foam density, or by using different monomers and polymerisation routes. However, so far the polymerisation within emulsion templates is mainly limited to radical polymerisations apart from very few exceptions. This means that the choice of monomers to be used for the synthesis of macroporous polymers in emulsion templates is rather restricted mainly to styrene and its derivatives and acrylates. It is our hypothesis that by creating non-aqueous emulsion templates with a temperature stable and chemically inert internal phase it should be possible to utilize other monomers than the typical vinyl monomers and, therefore, different polymerisation routes. This would allow to produce various new macroporous polymers that will have the benefits of the properties of their high performance or specialty bulk polymer counterparts. After 50 years of research, we will finally be able to produce macroporous polymers with tailored chemical, physical and mechanical properties that not only have the potential to be used, but will find real life applications. Based on this hypothesis, our aim is to produce high porosity, closed and interconnected, specialty and high performance, polymers focusing on typical examples: thermoset polymers, such as epoxy resins, thermoplastic poly acryl esters, polyimides and an engineering polymer, polyethylene terephthalate.

Porous polymers have a huge range of applications as filters, separators in batteries, scaffolds for tissue engineering, substitutes for metal parts in planes and cars. The main feature of a porous material is that it consists of many voids, also called pores. It is possible to significantly influence the properties of porous materials by changing the overall ratio of void space to bulk material and by controlling pore size and structure. Each polymer system has its characteristics, advantageous as well as disadvantageous, in terms of mechanical, thermal and chemical stability, or conductivity for instance, and thus behaves differently under certain conditions. This implicates that processing the preparation of porous materials is different for each system and cannot be readily generalised. It is therefore crucial to explore the restrictions of well-established preparation methods and push their boundaries to fully exploit the potential of porous polymers. Poly ether ether ketone (PEEK) and poly ether ketone ketone (PEKK) are known for their excellent mechanical properties combined with very high operating temperatures. However, they cannot be easily processed into polymer foams. Utilising thermally induced phase separation, which is based on the temperature dependent solubility of polymers in suitable solvents, it was possible to produce PEEK and PEKK foams with porosities up to 90% and well-defined pore structures. Another method to produce porous polymers is emulsion templating, which allows to prepare porous polymers with accurate shape and tailored properties. In the past emulsion templating was extensively but also predominantly to synthesise porous polymers by polymerisation of the continuous phase of water-in-divinylbenzene/styrene or acrylates. By preparing porous epoxies, representing a class of polymers which undergo hardening through polyaddition, with up to 60% void content by this route, we succeeded to show that this method is applicable for a wider range of polymers. Additionally, we developed a quick, clean and straight forward method for preparing porous epoxy foams solid epoxy foaming starting from only solid compounds.