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Dual-Dynamic Physical Networks In the conventional polymer sciences, the mechanical properties of polymeric materials are engineered by manipulating different aspects of the chain architecture, such as the molar mass, branching, and chemical composition for thermoplastics, as well as the network microstructure in crosslinked gels. In contrast, in supramolecular polymers, the final properties are controlled through manipulation of non-covalent bonds, selected from a vast library with tunable association thermodynamics and kinetics. This approach is less demanding, since the bulk material can be tuned by varying small molecule components only, through the established molecule-to-material design concept. Recently, double network hydrogels have been developed based on the synergy of having two interpenetrating chemical networks, the first one highly crosslinked and the second one loosely crosslinked. At an optimal ratio of the two networks, the mechanical properties boost significantly, due to the ability for sacrificial breakage of the chemical bonds in the first network. This hierarchy of structure is a well-known trick in nature to achieve multi-facet function. A similar approach has been widely used in the design of supramolecular double-network hydrogels, where a brittle first chemical network is replaced by different types of reversible supramolecular assemblies. In this project, which can be a research module or a Bachelor thesis, we aim to mimic the structure of double-network hydrogels by hierarchical design of a dually crosslinked physical hydrogel. For this purpose, tetra-PEG building blocks with different molar masses will be functionalized by ligands with significantly different association tendencies. A network with diverse and tunable dynamics can be obtained by simultaneous introduction of metal ions with different complexation affinity. We will then to study the structure and dynamics of the resulting materials using static/dynamic light scattering and rheology. Supervisor: Mostafa Ahmadi (ahmadi[a]uni-mainz.de)
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Stimuli-Responsive Supramolecular Hydrogels Hydrogels are 3D network materials made of crosslinked hydrophilic polymer chains. Charged hydrogels have a high capacity for water uptake, up to 1000 times of their own weight; these materials are called superabsorbers and are commercially utilized in hygiene applications. Another class of hydrogels are supramolecular hydrogels in which hydrophilic polymer building blocks self-organize into a network by non-covalent bonds such as hydrogen bonding or ionic interactions. Due to the dynamic nature of these physical bonds, the sol–gel transition of supramolecular hydrogels can occur dependent on the environment such as temperature or pH. Therefore, different applications such as drug delivery, tissue engineering, and 3D printing can profit from them. This research project, which can be a Bachelor thesis or a research module, targets at combining the utility of both these types of hydrogels. Prototype samples will be fabricated by droplet-based microfluidics, and their water swelling capacity as well as their swelling/deswelling temperature range will be tuned by physical bonds and network architecture parameters. The project involves both preparative polymer-chemistry aspects, for example, preparation of hydrophilic polymer backbones with non-covalent cross-linking motifs as side groups, and analytical polymer-physics work, for example, assessment of the gel-sample mechanics by rheology. Supervisor: Amir Jangizehi (amir.jangizehi[a]uni-mainz.de)
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Microrheology based on Fluorescence Correlation Spectroscopy Passive microrheology is an excellent tool to probe the local mechanical properties of a polymer system on a micrometer length scale. The method uses micrometer-sized embedded tracers that locally deform the sample due to their Brownian motion. Thus, in contrast to classical macroscopic rheology, only small shear forces are applied, ensuring measurement in the linear viscoelastic regime. Additionally, the accessible frequencies range up to the MHz-regime. This microrheology technique can be performed using the method of fluorescence correlation spectroscopy (FCS) together with fluorescent-labelled tracer particles. The goal of this project is to demonstrate the validity of FCS as passive microrheology method in viscous, viscoelastic and elastic systems such as water-glycerol mixtures, polymer solutions and polymer gels. Supervisor: Nora Fribiczer (nofribic[a]uni-mainz.de)
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