Structure-property relationships in transient network hydrogels [electronic resource]
Keywords: Chemical Engineering.

Dynamic covalent and non-covalent cross-link junctions provide a viable means of incorporating self-healing properties into gel networks. In this work two dramatically different gel networks are characterized based on the self-healing behavior of cross-link junctions present in both networks. The microstructure of self-assembled poly(MTC-OBn)-PEG-poly(MTC-OBn) gels is probed using small-angle X-ray scattering (SAXS), while the mechanical properties of these shear thinning gels are characterized using oscillatory shear rheology. The temperature-dependent dynamic behavior of PEG-hemiaminal dynamic covalent networks (HDCN) is explored using dynamic oscillatory shear rheology and compressive mechanical testing. The focus of this work is translating polymer molecular properties into meaningful applications of gel networks. A novel class of supramolecular hydrogels was produced by synthesizing a triblock poly(carbonate) material using organocatalytic ring-opening polymerization (ROP) of a poly(ethylene glycol) PEG macroinitiator with a cyclic monomer (MTC-OBn), then subsequently physically cross-linking triblock copolymers by hydrophobic collapse to form hydrogels in water. Controlling the molecular properties of triblock copolymer materials, primarily polycarbonate and poly(ethylene glycol) segment lengths, resulted in a viable platform for probing mesoscale material properties using small angle X-ray scattering (SAXS). The macroscale molecular properties of these supramolecular networks were characterized using oscillatory shear rheology. SAXS spectra show a strong scattering peak, indicating a molecular domain size of 20 - 25 nm. SAXS and rheological characterization indicates that the molecular weight of the PEG and polycarbonate segments affects the gel structure on both the mesoscale and macroscale. Additionally, we have developed a covalently cross-linked PEG-hemiaminal dynamic covalent network (HDCN) organogel that exhibits temperature-dependent dynamic covalent behavior. The reaction conditions for the synthesis of HDCN organogels were varied in order to produce materials with distinct mechanical properties. We propose that a temperature-dependent equilibrium between NMP-soluble formaldehyde and hemiaminal covalent cross-link junctions creates a unique dynamic system that can be trapped under the right conditions. Gels that form in this kinetically trapped state do not exhibit self-healing behavior, while gels that have a higher concentration of paraformaldehyde exist in a state of dynamic equilibrium, evidenced by the dynamic mechanical properties of these organogel networks. Model compound NMR studies were conducted to identify the chemical species that contribute to the unique mechanical properties of HDCNS. The mechanical properties of self-healing HDCNs and elastomeric HDCNs were characterized by compressive mechanical testing and oscillatory shear rheology coupled with temperature control. An elegant self-healing, dynamic system is presented in this work that demonstrates the capabilities of HDCNs as elastomeric organogel materials.