Mild oxidation and functionalisation of synthetic polymer surfaces

How to extend the lifetime of plastics? This might sound as a somewhat odd question in the age of bio-degradable plastics, but plastics that can withstand extreme conditions can be (re)used more often and thereby thus contribute to an eco-friendly economy. In order to improve these plastics, we need to adjust them and that is not easy! Traditional plastic modification technologies often require a lot of energy or dangerous chemicals. To by-pass these traditional measures, we explored, and pushed, the boundaries of novel eco-friendly technologies. We studied how enzymes, nature’s architects, modified plastic drinking water filtration membranes, which proved to occur in in a completely unprecedented manner. Additionally, a novel eco-friendly chemical tool for modifying plastics opened up a whole new route towards water-repellent materials. We hope that, through our research, we get yet a little closer to a sustainable future.

Chapter 1 provides the required background knowledge for any chemically oriented scholar to comprehend the interdisciplinary work presented herein. Crucial topics, such as polymer surface modification and analysis, wetting behaviour and adhesion prevention were introduced. Current methodologies for modifying polymer surfaces typically require harsh chemicals and conditions. The research described herein has therefore been focussed on acquiring a better understanding and increasing the scope of novel tools for mildly modifying polymers.

One of these novel tools is the laccase-mediated surface functionalisation of poly(ethersulfone) membranes using 4-hydroxybenzoic acid (4-HBA). The resulting overlayer minimised membrane fouling by several biofoulants. In order to comprehend the underlying functionalisation mechanism, the solution-phase oligomerisation of 4-HBA had to be studied first, which is described in Chapter 2. Initial conversion of 4-HBA proved to occur only slowly and resulted in two main products: a C3-C3’-bound and a C3-O-bound dimer. A plurality of other products were found after 24 h of incubation, which included a C1-C3’-bound and possibly a C1-O-bound dimer. Furthermore, laccase-mediated conversion of these dimers proved to be far more rapid than conversion of 4-HBA itself, and correlated strongly with the abundance of the individual dimers. The influence of dimer reactivity on their abundance was confirmed by quantum chemical calculations. These findings provided us with handles for designing phenols with enhanced reactivity and controlled binding profiles.

We used the gained knowledge to synthesise novel positively charged phenolic monomers that were anticipated to, upon laccase-mediated surface functionalisation, introduce anti-bacterial properties to the membrane while allowing it to be used as support membrane for layer-by-layer deposition. As is described in Chapter 3, however, in-situ laccase-mediated conversion of these phenolics did not lead to significant surface functionalisation. In order to understand why functionalisation was achieved for other monomers (i.e. 4-HBA), 4-HBA, laccase and any of several PES model compounds were incubated together and the resultant mixture was studied using LC-MS. However, no covalent bond formation between (oligomeric) 4-HBA and either of the soluble, insoluble or resin-bound PES model compounds could be observed. The use of phenols bearing negatively charged substituents did also not lead to membrane surface modification. Finally, membranes having an overlayer of oligomeric 4-HBA proved to be extensively decolourised upon washing with a detergent solution. Considering all of the above, it was concluded that laccase-mediated surface modification resulted from strong physisorption, rather than from covalent grafting of oligomeric 4-HBA.

As it was challenging to reveal the mechanisms underlying our functionalisation strategy, we anticipated that other researchers might also have encountered similar challenges. It is therefore that in Chapter 4 recently published laccase-mediated surface modification strategies are discussed and assessed on whether grafting is likely to have occurred. This assessment was based on five factors: mechanistic rationale, pre-treatment, control experiments, washing/cleaning and the used analytical tools. Generally speaking, laccase-mediated grafting on lignocelluloses proved to be likely. Quite commonly, however, grafting coincided with physical adsorption due to insufficient washing. We concluded that a lack of proper surface analyses and studies towards the mechanisms underlying grafting on polysaccharides, proteins and synthetic polymers regularly hampered achieving covalent grafting on these materials.

Apart from enzymatic surface modification, additional chemical strategies for achieving mild polymer functionalisation were assessed too. PMMA activation was accordingly achieved through peroxidative copper catalysis, followed by sodium borohydride reduction to result in surface hydroxylation. As was described in Chapter 5, this offered a platform for the robust growth of SiHCl3-based silicone nanofilaments, while maintaining polymer transparency. Due to their intricate nanostructure, these silicone nanofilaments granted superhydrophobicity (SWCA > 150°, sliding angles < 1°) to the material. The presence of Si-H moieties on the surface allowed for further functionalisation through hydrosilylation. As a proof of principle, we employed platinum-catalysed hydrosilylation to decorate the surface with extensively fluorinated alkenes and alkanes. This fluorinated exterior provided the material with protection towards hydrolytic degradation. We have thereby developed the first intrinsically superhydrophobic reactive silicone nanofilament-coated transparent polymer surface.

Finally, Chapter 6 summarises the highlights of previous chapters, while offering an in-depth discussion on possible improvements and future work.