Cold and pressure: a successful duo for processing of sustainable porous materials
- Jun.-Prof. Dr. Pavel Gurikov, Laboratory for Development and Modelling of Novel Nanoporous Materials -
In an effort to tackle pollution in seas, fields and waterways, the European parliament has overwhelmingly supported a wide-ranging ban on single-use plastics. This trend on minimizing the dependency on synthetic crude-oil derived plastics is also echoed by chemical industry, both in Europe and worldwide. One research line of our laboratory is to develop high performance porous materials, in particular, for thermal and sound insulation, for example, for buildings, clothing, aircrafts and packaging. The focus here is on the use of natural and renewable sources of biopolymers. Biopolymers are traditionally thought to be tricky due to impurities, lability against microorganisms and variable properties like molecular weight. Conventional processing of biopolymers into highly porous solids materials begins with dissolution of the biopolymer in a proper solvent. The solution is then gelled: biopolymer molecules become linked to each other so they cannot freely move anymore. Everybody knows gels pectin and gelatin gels in home-made jam and marmalade. If a gels can be formed, next steps are known: the solvent from the gel is extracted under pressure with green recyclable supercritical solvent, carbon dioxide. The resulting product – aerogel – is light, highly porous material with a giant internal surface, Figure 1. The described process is the golden standard for processing of gels into aerogels and works perfectly for many biopolymers which can be converted into gels.
How about other starting polymers? In fact, not all biopolymers have the ability to form gels, such as cellulosic polymers, amylose, lignin, poly(lactic) acid and many others. So, the state-of-the-art pathway is fundamentally limited to the starting material with gelation ability.
To expand the scope of accessible porous materials to any kind of polymers, we have developed a workaround. The core idea of the novel approach is to dissolve or disperse the starting polymer in a solvent with a melting point just below room temperature. After freezing the polymer molecules are immobilized like in a gel. To stabilize this “gel-like” state, the solvent is extracted with ethanol and supercritical carbon dioxide.
Obtained materials demonstrate an interconnected porous architecture and large internal surface area, Figure 2. We currently test the novel approach on various bio- and industrial polymers like agarose, starch and polyacrylonitrile. The use of green solvents, ethanol and carbon dioxide, and also a straightforward downstream processing make the new process especially attractive for further exploration in industrial environment. Our ultimate goal of this ongoing project is to offer a wide palette of sustainable functional materials alternative to plastics.