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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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Effect of coordination geometry on the structure and dynamics of mussel-inspired side-chain metallo-supramolecular polymer hydrogels Nature employs a wide library of transient bonds in hierarchical structures to function adaptively on multiple length and timescales. A prime example is the mussel’s byssus thread, which is composed of a histidine-rich soft core and a multi-phase catechol-rich hard shell. The formation of coordinative bonds with transition metal ions that are selectively absorbed from the seawater and establishment of elastically active and reversible physical bonds upon the exposure to basic pH values results in superior properties like strong adhesion to rocks, internal cohesion, and self-healing. Inspired by such designs chemists have developed bio-mimicking transient hydrogels. However, the replication of complex functions of biological systems requires a deep knowledge of the correlation between microscopic characteristics and macroscopic properties. To this end, we study the effect of pH-controlled coordination geometry on the structure and dynamics of mussel-inspired hydrogels. In Phase (I), using telechelic linear and star chains, we have established the conditions at which the coordination geometry can be controlled. In Phase (II), we would like to use this knowledge to study “the effect of coordination geometry on the structure and dynamics of mussel-inspired side-chain metallo-supramolecular polymer hydrogel”. Vinylic monomers grafted with the mussel-inspired ligands should be synthesized and employed in the controlled polymerization with a hydrophilic monomer. Hydrogels should be formed at various preparation conditions, and the rheological properties should be evaluated. Supervisor: Mostafa Ahmadi (ahmadi[a]uni-mainz.de)
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Microscopic evidences for the selective formation of heteroleptic complexes Heteroleptic complexes are widely employed in supramolecular complexes for obtaining unique catalysis, luminescence and structural properties or to induce nanoscopic rearrangements upon application of external stimuli that change the coordination-geometry preference. Despite this potential, they are rarely employed in the development of metallo-supramolecular polymer networks; this is unfortunate, as they would provide a tool to build highly homogeneous model-type networks that could form a basis for both a myriad of elementary investigations on transient networks and for their use in rational soft-functional materials design. In this regard, we have already demonstrated macroscopic evidences for the partial free formation of heteroleptic complexes between phenanthroline- and terpyridine-functionalized tetraPEG precursors, depending on the coordination geometry preference of the utilized metal ion. In this work, we would like to provide further “microscopic evidences for the selective formation of heteroleptic complexes”. To this end, we study the diffusion of fluorescence-dye labeled polymeric sticky tracers inside polymer networks that are formed by their complementary ligands. The project aims to synthesize the desired sticky tracers and measure their diffusion inside polymer hydrogels by the fluorescence recovery after photo-bleaching (FRAP) technique. 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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Adjustment of the osmotic pressure of polymeric hydrogels for utilizing in a forward osmosis process for water desalination Among various applications of hydrogels, their function as draw agents in forward osmosis processes for water desalination has attracted numerous interests. In this process, hydrogels are placed on one side of a semi-permeable membrane, where the other side of the membrane is in contact with a feed, saline solution. The diffusion of water through the semi-permeable membrane occurs only if the osmotic pressure of the hydrogels is higher than the osmotic pressure of the feed solution. In general, the osmotic pressure of hydrogels has enthalpic, entropic, and ionic contributions, whereby the latter is the topic of this research. This project, which can be a Bachelor thesis or a research module, targets at adjustment of the osmotic pressure of hydrogels by incorporating nanoparticles having high ionic group density at their surface. As a consequence of nano or sub-micron scale in diameter, in a fixed volume fraction, the provided charge density with these particles is significantly higher compared to sub-millimeter particles or charged comonomers. The project involves both preparative polymer-chemistry aspects, for example, preparation of nanocomposite hydrogels with different polymeric backbone and nano particles, and analytical polymer-physics work, for example, assessment of the gel-sample structure by FT-IR and gel mechanics by rheology. Supervisor: Amir Jangizehi (amir.jangizehi[a]uni-mainz.de)
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The increasing local temperature inside thermo-responsive hydrogels upon natural/artificial sunlight radiation by (nano) light-absorbers Hydrogels can act as draw agents in forward-osmosis water desalination to adsorbed water from a saline feed solution through a semi-permeable membrane. The adsorbed water, which is the product of this process, should be subsequently released from hydrogels by hydrogel shrinking. If the hydrogels are thermo-responsive in an LCST-type fashion the hydrogel shrinking is obtained by heating above this critical temperature. Regarding sustainability and CO2 footprint, however, the temperature increase should be obtained ideally by exposing the hydrogels to sunlight radiation. Considering the common low-heat transfer in polymeric hydrogels, achieving this target requires modification of hydrogel´s molecular structure, for example, by incorporation of (nano) light absorbents within hydrogels like carbon derivatives. In this research project, which can be a done as a Bachelor thesis or a research module, hydrogels with different types and amounts of (nano) light absorbers should be prepared. The dispersion of (nano) additives should be characterized by microscopy techniques like TEM. Moreover, the influence of (nano) absorbents on VPTT and the improvement of water releasing should be determined by light microscopy or DSC. Supervisor: Amir Jangizehi (amir.jangizehi[a]uni-mainz.de)
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Preparation of microporous/foam-like superabsorber hydrogels for utilizing as draw agent in forward osmosis water desalination Hydrogels as draw agents in the forward osmosis process for water desalination should possess some critical features. First, they should have a high capacity of water adsorption, ideally with fast kinetics. Second, the shrinking of the hydrogels, and with that, the release of adsorbed water, should be achieved by relatively low amount of energy consumption. Microporous / foam-like superabsorber hydrogels are one of the candidates that can potentially demonstrate these key features. Such hydrogels have high water adsorption capacity due to their hydrophilic, charged microstructure. In addition, the microporous structure commonly affects the kinetics of water adsorption by providing significantly extra water diffusion channels compared to non-porous hydrogels. In addition, soft, foam-like hydrogels are deformed under moderate pressure (as one example, consider the easy deformation of a sponge under hand-driven pressure). This research project, which can be a Bachelor thesis or a research module, targets the synthesis of such hydrogels via in-situ foaming through gelation of monomers like acrylic acid and acrylamide, during which CO2 should be formed. The gas formation can be done by mixing of acids (like acetic acid) and bases (like sodium bicarbonate). The softness of hydrogels can be adjusted by the density of porosity as well as by addition of other comonomers into molecular scaffold of the hydrogels. The porosity density should be analyzed with microscopy techniques like SEM. The mechanical properties should be analyzed with 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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