TY - THES

T1 - Nanocomposite Hydrogels - Fracture Toughness and Energy Dissipation Mechanisms

AU - Klein, Andrea

N1 - embargoed until null

PY - 2013

Y1 - 2013

N2 - Traditional hydrogels consist of organically cross-linked polymer networks containing a high fraction of water. Due to extraordinary characteristics including outstanding transparency as well as superior swelling and de-swelling properties they are attractive for diverse biomedical applications such as soft contact lenses. However, implementation in more sophisticated purposes such as artificial muscles is limited by poor mechanical properties, in particular low fracture toughness. Substitution of organic cross-linkers by clay, a multifunctional, inorganic cross-linker results in nanocomposite hydrogels. These materials exhibit high tensile strength combined with high elongation-at-break as well as high compliance while retaining the remaining unique characteristics of hydrogels. Still, tearing toughness of nanocomposite hydrogels has not yet been studied systematically. Hence, the overall objective of this study was to quantify the toughness and identify active mechanisms for dissipation of energy of nanocomposite hydrogels. Nanocomposite hydrogels were prepared by employing synthetic clay of type hectorite and monomer N,N-dimethylacrylamide by allowing in-situ free radical polymerization. Sample variables were clay content and sample thickness. For comparison a traditional hydrogel was prepared employing N,N’-methylenebis(acrylamide) as organic cross-linker. Sample preparation was validated by different morphological characterization techniques. The exfoliation of clay into disk-shaped nanoparticles and homogeneous distribution was confirmed by X-ray diffractometry and transmission electron microscopy. Thermogravimetric analysis verified the high water content of the prepared gels. Infrared spectroscopy confirmed similar polymerization processes for all hydrogels except the nanocomposite hydrogel with the smallest thickness. This is probably due to an incomplete polymerization process for this sample. Tensile testing proved extraordinarily high values for strain-at-break for nanocomposite hydrogels beyond the crosshead travel’s limit (strain-at-break greater than 1250 %) while traditional hydrogels exhibited brittle behaviour. Storage Modulus, Shear Modulus and the number of network chains between cross-links per unit volume were derived. All these parameters increased with rising clay content. In comparison to traditional hydrogels nanocomposite hydrogels exhibited a less stiff behaviour. Fracture toughness was determined by employing the pure shear test approach. Organically cross-linked hydrogels could not be tested due to their fragility. In contrast, nanocomposite hydrogels could quickly dissipate large amounts of energy. Extraordinarily high fracture toughness values were determined for nanocomposite hydrogels. One dissipation mechanism that was observed was blunting of the crack tip. So as to detect further dissipation mechanisms viscoelastic properties were characterized. Step cycle testing revealed that for low to moderate true strains the ratio between elastic and plastic strain remained constant. However, at a certain strain plastic strain almost com¬pletely dominated the deformation behaviour. Thus plastic deformation is presumably large in the whole sample and also close to adjacencies of a crack tip if a crack is present. Stress relaxation experiments revealed a stress relaxation time (the time at which maximal viscoelastic dissipation occurs) similar to the time necessary to rupture samples in fracture tough¬ness tests. Thus, viscoelastic dissipation accounts for high fracture energies. Viscoelastic dissipation was additionally confirmed by dynamic mechanical analysis and rheometry. Hence, investigations revealed different mechanisms contributing to the high fracture toughness of nanocomposite hydrogels. Firstly, blunting prevents expansion of existing cracks. Secondly, plastic deformation provides absorption of energy. Thirdly, viscoelastic

AB - Traditional hydrogels consist of organically cross-linked polymer networks containing a high fraction of water. Due to extraordinary characteristics including outstanding transparency as well as superior swelling and de-swelling properties they are attractive for diverse biomedical applications such as soft contact lenses. However, implementation in more sophisticated purposes such as artificial muscles is limited by poor mechanical properties, in particular low fracture toughness. Substitution of organic cross-linkers by clay, a multifunctional, inorganic cross-linker results in nanocomposite hydrogels. These materials exhibit high tensile strength combined with high elongation-at-break as well as high compliance while retaining the remaining unique characteristics of hydrogels. Still, tearing toughness of nanocomposite hydrogels has not yet been studied systematically. Hence, the overall objective of this study was to quantify the toughness and identify active mechanisms for dissipation of energy of nanocomposite hydrogels. Nanocomposite hydrogels were prepared by employing synthetic clay of type hectorite and monomer N,N-dimethylacrylamide by allowing in-situ free radical polymerization. Sample variables were clay content and sample thickness. For comparison a traditional hydrogel was prepared employing N,N’-methylenebis(acrylamide) as organic cross-linker. Sample preparation was validated by different morphological characterization techniques. The exfoliation of clay into disk-shaped nanoparticles and homogeneous distribution was confirmed by X-ray diffractometry and transmission electron microscopy. Thermogravimetric analysis verified the high water content of the prepared gels. Infrared spectroscopy confirmed similar polymerization processes for all hydrogels except the nanocomposite hydrogel with the smallest thickness. This is probably due to an incomplete polymerization process for this sample. Tensile testing proved extraordinarily high values for strain-at-break for nanocomposite hydrogels beyond the crosshead travel’s limit (strain-at-break greater than 1250 %) while traditional hydrogels exhibited brittle behaviour. Storage Modulus, Shear Modulus and the number of network chains between cross-links per unit volume were derived. All these parameters increased with rising clay content. In comparison to traditional hydrogels nanocomposite hydrogels exhibited a less stiff behaviour. Fracture toughness was determined by employing the pure shear test approach. Organically cross-linked hydrogels could not be tested due to their fragility. In contrast, nanocomposite hydrogels could quickly dissipate large amounts of energy. Extraordinarily high fracture toughness values were determined for nanocomposite hydrogels. One dissipation mechanism that was observed was blunting of the crack tip. So as to detect further dissipation mechanisms viscoelastic properties were characterized. Step cycle testing revealed that for low to moderate true strains the ratio between elastic and plastic strain remained constant. However, at a certain strain plastic strain almost com¬pletely dominated the deformation behaviour. Thus plastic deformation is presumably large in the whole sample and also close to adjacencies of a crack tip if a crack is present. Stress relaxation experiments revealed a stress relaxation time (the time at which maximal viscoelastic dissipation occurs) similar to the time necessary to rupture samples in fracture tough¬ness tests. Thus, viscoelastic dissipation accounts for high fracture energies. Viscoelastic dissipation was additionally confirmed by dynamic mechanical analysis and rheometry. Hence, investigations revealed different mechanisms contributing to the high fracture toughness of nanocomposite hydrogels. Firstly, blunting prevents expansion of existing cracks. Secondly, plastic deformation provides absorption of energy. Thirdly, viscoelastic

KW - nanocomposite

KW - hydrogels

KW - energy dissipation

KW - dissipation mechanisms

KW - clay

KW - fracture toughness

KW - pure shear test

KW - nanocomposite Hydrogele

KW - Bruchzähigkeit

KW - bruchmechanische Schubversuche

KW - Bruchdehnung

KW - Clay

M3 - Master's Thesis

ER -