We use all the established methods of polymerization, however, controlled radical polymerization as ATRP and RAFT, often in combination with click reactions are the most frequently used techniques.
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The physicochemical characteristics of many polymers are strongly affected by external stimuli. For example, the solubility of polymers may depend strongly on the temperature, and not just in the way we know it for small molecules. Some polymers exhibit a so-called lower critical solution temperature (LCST), losing their solubility upon heating. Alternatively, some polymers exhibit the opposite type of behavior, an upper critical solution temperature (UCST), where the polymer dissolves upon heating and even combinations of both behavior can be realized.
These thermoresponsive properties are influenced by several factors, such as polymer concentration, molar mass and architecture and the presence of salts. Besides thermoresponsiveness, numerous polymers exist which respond to other external stimuli such as light, pH, redox and so on. The development of modern polymerization, modification and functionalization methods has enabled also the preparation of systems, which respond to multiple changes in the environment. The applications of responsive polymers include drug-delivery, smart coatings, hydrogels, sensors, bioconjugates and degradable materials.
A large proportion of drugs are poorly water-soluble, which is a major issue in pharmaceutical technology and contributes to expensive failures in drug development and to the high development costs of novel medicines. In addition, therapeutic peptides and proteins also often suffer from suboptimal dispersibility and stability issues, which makes the translation from research to actual medicines challenging and expensive. The issue of delivery of genetic material has come into the foreground with the development of mRNA based vaccines to fight the COVID-19 pandemic.
All these topics are addressed in the field of research called drug delivery. We develop for example drug carriers based on graft copolymers with polysaccharide backbones or micelle forming amphiphilic block copolymers. The polymer syntheses allow designing the architecture of the polymers in such a way that they form either spherical micelles, worm-like self-assemblies or larger vesicles called polymersomes. All these structures are highly interesting from drug delivery.
However, polymer micelles and other drug delivery systems have showed one prominent limitations over the last decades. They drug loading capacity is typically quite limited, meaning a lot a material is needed to delivery comparably little amount of drug. We are developing ultra-high drug loaded delivery systems, which are capable to carry and solubilize unparalleled amounts of drugs. This can drastically improve therapeutic efficacy, but further studies are needed to understand their structure-property relationships and how these affect the biological response to these systems.
The most often utilized method to prepare polymer colloids is emulsion polymerization where a monomer is polymerized via a free radical polymerization process in an aqueous medium in the presence of a stabilizer system. Other two variations are the microemulsion and miniemulsion polymerization processes which are used to prepare submicron-size polymer colloids. These techniques are utilized in creating various nano- and microsized particles with potential applications in catalysis and as carriers for active substances. Recently we have started to study also the polymerization induced self-assembling (PISA) processes. In this process, the polymerization conditions are chosen such that the self-assembly occurs and is controlled during the polymerization. This can help to drastically simplify the process and reduce use of possibly toxic and environmentally problematic solvents.
Modern synthetic methods have made it relatively easy to construct nanoscale hybrid materials, which combine inorganic and organic substances or synthetic and biological matter. Examples vary from inorganic entities as gold, silver, copper, silica nanoparticles grafted with synthetic polymers to polymeric bioconjugates. Also for 3D printing, organic-inorganic hybrid hydrogels are of great interest.
The development of human culture has been closely connected with the utilization of (natural) polymeric materials. The use of polymers extracted from non-food biomass as an alternative to synthetic polymers is one important way of utilizing biomass and reducing oil-based materials and their environmental impact. However, many advanced applications are difficult to realize using biopolymers in their native forms, but the properties of biopolymers can be altered by modification (chemical, physical, enzymatic) of the polymeric chains. Our group focuses on the macromolecular characterization of chemically modified hemicelluloses and chemically modified cellulose.
The group has excellent control of the most important structure and property characterization methods of polymers. Besides the thorough chemical analysis of the prepared products, studies on stimulus-responsive self-assembly, supramolecular gelation, nanoparticle formation, morphological features, thermal stability and mechanical properties are conducted using spectroscopic, scattering, and rheological methods. Techniques we routinely utilize are liquid and solid state NMR, fluorescence, light scattering, size exclusion chromatography, asymmetric flow field flow fractionation (AF4), MALDI mass spectroscopy, rheology and dynamic-mechanical analyses, conventional and microDSC. In addition, we routinely also access large international research infrastructure such as neutron research reactors as the FRM II in Garching, Germany.
Additive manufacturing has seen tremendous advances in recent years with new technologies and new materials developed at a rapid pace. Apart from some important applications in metal 3D printing, the majority of additive manufacturing involves polymers and colloids. An intimate knowledge of polymer synthesis and polymer properties are crucial for the control of the processes and the resulting materials properties. In addition, stimulus responsive polymers give access to 4D printing, in so far that a 3D printed constructs can change it´s shape or mechanical properties over time or on stimulation.
For this purpose we create new polymers and novel hybrid materials. Our focus is to enable new materials properties, improve printability, shape fidelity and resolution by smart polymer design and a deep understanding of the physico-chemical and rheological properties. In particular we are focusing on the utilizing of two different printing techniques: extrusion based printing of hydrogels and melt electrowriting.