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Publicatie
Supramolecular Polymeric Solutions in Shear and Extensional Flows
Boek - Dissertatie
The behaviour of transient polymer networks in strong flows is poorly understood.
Those transient polymer networks are candidates for, e.g. self-healing and recyclable materials, easily tunable viscosity modifiers, and stimuli-responsive materials with applications in foodstuff, coatings, cost-efficient processes or biomedical areas.
To explore their potential and define strategies for designing novel materials amenable to contemporary needs, a fundamental understanding of their very complex and diverse multi-scale supramolecular structure and dynamics is needed.
The thesis aims to contribute to the understanding of metal-ligand bonded polymeric solutions with respect to non-equilibrium structures and strong flows. With this purpose in mind, the behaviour under large deformation was studied employing rheometry, such as orthogonal superposition rheology, capillary break-up, rheo-optics, and a toolbox of molecular characterisation techniques. The behaviour of telechelic polymers during capillary break-up was studied. To this aim, a framework towards more accurate capillary break-up measurements was developed. It was found that the metal-ligand bonds withstand the strong flow fields and do not break. Moreover, the average number of assemblies in solution was determined as a function of polymer concentration, metal cation nature, and metal cation concentration. The largest effective molecular weight is obtained at stoichiometric concentration, whereas a cation excess will decrease the average number of assemblies, depending on the binding constant. A rheological device capable of performing orthogonal superposition measurements was re-developed. The working principle was demonstrated by comparing linear and branched worm-like micelles. The response of supramolecular hydrogels to shear as well as the equilibration after cessation of flow was studied shining light onto the responsiveness of transient polymer networks. Non-monotonic stress relaxation after cessation of shear flow was revealed.
The increase in stress is associated with the redistribution of energy after the flow has stopped. When broken bonds are re-established after flow cessation, the released energy is partly used to locally increase the elastic energy by the formation of deformed domains. If shear has induced order such that these elastic domains are partly aligned, the re-establishing of bonds gives rise to an increase of the overall stress. Furthermore, a transition from predominantly intramolecular to predominantly intermolecular bonds under shear is indicated by an increase of the orthogonal storage modulus, failure of the Cox-Merz rule, and an increase in stress during uniaxial extensional flow. Whereas the increase in plateau modulus as well as the stress increase at intermediate Hencky strains could be linked to non-linear stretching of elastically active chains, the failure of the Cox-Merz rule points towards flow induced structure formation. At the same time, a shear induced reduction of the bond lifetime is observed, causing shear thinning, despite the enhanced structure. The findings will pave the way towards a clearer understanding of supramolecular structures, and the introduced techniques will open new avenues of inquiry to study complex materials.
The thesis aims to contribute to the understanding of metal-ligand bonded polymeric solutions with respect to non-equilibrium structures and strong flows. With this purpose in mind, the behaviour under large deformation was studied employing rheometry, such as orthogonal superposition rheology, capillary break-up, rheo-optics, and a toolbox of molecular characterisation techniques. The behaviour of telechelic polymers during capillary break-up was studied. To this aim, a framework towards more accurate capillary break-up measurements was developed. It was found that the metal-ligand bonds withstand the strong flow fields and do not break. Moreover, the average number of assemblies in solution was determined as a function of polymer concentration, metal cation nature, and metal cation concentration. The largest effective molecular weight is obtained at stoichiometric concentration, whereas a cation excess will decrease the average number of assemblies, depending on the binding constant. A rheological device capable of performing orthogonal superposition measurements was re-developed. The working principle was demonstrated by comparing linear and branched worm-like micelles. The response of supramolecular hydrogels to shear as well as the equilibration after cessation of flow was studied shining light onto the responsiveness of transient polymer networks. Non-monotonic stress relaxation after cessation of shear flow was revealed.
The increase in stress is associated with the redistribution of energy after the flow has stopped. When broken bonds are re-established after flow cessation, the released energy is partly used to locally increase the elastic energy by the formation of deformed domains. If shear has induced order such that these elastic domains are partly aligned, the re-establishing of bonds gives rise to an increase of the overall stress. Furthermore, a transition from predominantly intramolecular to predominantly intermolecular bonds under shear is indicated by an increase of the orthogonal storage modulus, failure of the Cox-Merz rule, and an increase in stress during uniaxial extensional flow. Whereas the increase in plateau modulus as well as the stress increase at intermediate Hencky strains could be linked to non-linear stretching of elastically active chains, the failure of the Cox-Merz rule points towards flow induced structure formation. At the same time, a shear induced reduction of the bond lifetime is observed, causing shear thinning, despite the enhanced structure. The findings will pave the way towards a clearer understanding of supramolecular structures, and the introduced techniques will open new avenues of inquiry to study complex materials.
Jaar van publicatie:2019
Toegankelijkheid:Open