Linking the kinetics of formation and growth to the mechanical properties of calcium carbonate composites

calcium carbonate;amorphous calcium carbonate;biomineralization;crystal growth, ,

Lopez Berganza, Josue Alejandro

关键词: calcium carbonate;    amorphous calcium carbonate;    biomineralization;    crystal growth, ,;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/105658/LOPEZBERGANZA-DISSERTATION-2019.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Calcium carbonate, the most common among other biogenic minerals, is widely used in skeletal components in marine organisms. These organisms exploit varied physical and chemical interactions between calcium carbonate and organic additives to direct the precipitation process through thermodynamic and kinetic pathways that yield mineral-organic complexes that are not only visually stunning but also highly functional. By carefully tuning the growth environment within their bodies, calcium carbonate can precipitate as one of its crystalline polymorphs, exhibiting high strength or as its amorphous state, which may contribute to an enhanced toughness in the material. The numerous calcium carbonate precipitation pathways in the presence of synthetic organic additives have garnered interest due to their potential to shed light into the mechanisms that drive biomineral formation and inspired the development of highly tunable composite materials. Despite recent leaps in the understanding of these systems, a clear connection linking the kinetics of calcium carbonate formation, microstructure and enhanced mechanical properties is still lacking. Inspired by the sophisticated mineral-organic structures with superior mechanical properties found in nature, this work aims to answer the following questions: 1) How do selected organic additives and biomimetic organic matrices exert control over the precipitation of calcium carbonate through molecular mechanisms? 2) How does the mineralization pathway dictate the microstructure and mechanical response of calcium carbonate-organic composites?To achieve this, the research presented in this thesis explored diverse interactions between calcium carbonate and organic matter at multiple length scales.At the atomic scale and based on first principles, the hydration shell of calcium carbonate was characterized, yielding insights into the structure and energetics of the water molecules that regulate the precipitation of calcium carbonate at the earliest stages of formation. The combined role of polyelectrolyte coated substrates and soluble organic matter on the kinetics of calcium carbonate nucleation was examined in experiments. The polyelectrolyte chemistry either promoted the formation of crystalline nuclei or the stabilization of amorphous calcium carbonate (ACC). On the other hand, soluble organic matter dictated the supersaturation through the binding of ions in solution as well as affecting the nucleation of crystals on the substrate by adsorbing on the polyelectrolyte films, and thereby, changing the effective surface energy. The knowledge gained in this study was leveraged to exploit ACC in a polyelectrolyte matrix; here nanoparticles with highly tunable nanogranular structures were synthesized. The concentration of polyelectrolyte in the particles was observed to toughen the particles by activating various energy dissipation mechanisms under stress and enhancing the cohesion between the nanogranules. These particles enabled us to fundamentally investigate ACC stabilization and toughening mechanisms, but they are not suitable to large-scale composites. For this reason, hydrogels were targeted as matrices for mineralization. A protocol to mineralize the hydrogels through the formation of ACC was explored. This strategy creates well-controlled conditions within the hydrogel and over a large range of ion concentrations. The nucleation and growth of calcite and the incorporation of agarose into the crystals were found to be tightly controlled by ACC. The mechanical response of these composites showed little to no improvement due to the sparse nucleation of calcite and the gradients in crystal distribution within the hydrogel. To arrest size and nucleation density gradients, magnesium was introduced to stabilize ACC, allowing additional time for ion diffusion to happen. This strategy permits the percolation of the amorphous phase through the network allowing for a homogeneous microstructure over the entire thickness of the hydrogel. Eliminating the presence of gradients through the use of magnesium also had an effect on the mechanical response of the hydrogels. Most notably, the hydrogels became more resilient as larger deformations were needed in order to disrupt the network, while their behavior tended towards more elastic and less (viscous) dissipative. While this improvement in microstructure control improves the resilience, it does not significantly strengthen the network. The future outlook presents a strategy to achieve a simultaneous strengthening of the hydrogel as well as an improvement of the hydrogel’s resilience through the control exerted by ACC precipitation and its solid-state transformation into calcium phosphate polymorphs within the hydrogels.