Doctor of Philosophy
Schultz, Kelly M.
Characterizing the rheological properties of polymeric and colloidal gels during dynamic phase transitions is critical in developing targeted products for industrial and home care use. These materials require specific rheology that allow them to respond to external stimuli. The goal of this work is to characterize the rheological properties of polymeric and fibrous colloidal gels throughout gelation, degradation, and at equilibrium to infer their microstructure. An emphasis is made on hydrogenated castor oil (HCO), a fibrous colloid that undergoes heterogenous phase transitions and is subject to changes in microstructure when sheared during sample preparation. We use multiple particle tracking (MPT) microrheology to measure the rheological properties of the scaffolds. MPT is a passive microrheological technique that relies on the thermal motion of probe particles to measure material properties and is ideal for measuring heterogeneous phase transitions.We begin by characterizing the gelation of poly(ethylene glycol) (PEG) hydrogels at varying polymeric interactions and gelation mechanisms. The gels consist of a 20,000 g mol-1 4 arm star PEG backbone end-functionalized with either acrylate or maleimide for chain- and step-growth polymerization reactions, respectively. Both gels are cross-linked with 1,500 g mol-1 linear PEG-dithiol. The overlap concentration of the PEG backbone is determined using both bulk rheology and microrheology, and gels are formed below, at, and above the overlap concentration. PEG-maleimide gelation occurs spontaneously and is measured with MPT through time. PEG-acrylate gels require the formation of a free radical by exposure to ultraviolet light. The critical relaxation exponent, n, is calculated using time-cure superposition, the superposition of viscoelastic functions at different extents of reaction. The critical relaxation exponent decreases as the concentration and polymeric interactions increase to the overlap concentration, then remains constant above the overlap concentration. The value of n indicates that below the overlap concentration gelation occurs through a percolation mechanism, and forms a more tightly associated network as concentration increases.Microrheology is then used to measure the degradation and gelation of a fibrous colloidal gel, hydrogenated castor oil. HCO undergoes a heterogeneous phase transition based on an osmotic pressure gradient, making MPT an ideal measurement tool. The critical relaxation exponents, ndeg and ngel, are calculated for the degradation of a 4 wt% HCO gel and gelation of a 0.125 wt% HCO sol, respectively. The calculated values of ndeg and ngel are different from each other, indicating a change in microstructure potentially caused by colloidal rearrangement during phase transition or shear imparted on the sample during preparation. An analysis of the van Hove correlation functions determines that MPT is viable despite heterogeneities. Further analysis of the heterogeneous structure quantifies homogenous material microenvironments at the equilibrium phases and heterogeneous microenvironments during the critical transition. The non-Gaussian parameter, a measure of rheological heterogeneity, indicates the greatest heterogeneity occurs when the material is in the viscoelastic solid phase during the critical transition.To increase the amount of information gained from MPT experiments, we develop a technique to simultaneously track a bi-disperse population of probe particles. 0.5 and 2.0 ?m particles are tracked separately using a brightness based radius of gyration, Rg2, and ensemble-averaged mean-squared displacement curves are calculated from the separated populations. The viscosity of glycerol solutions at a range of concentrations verifies the technique for Newtonian fluids. The technique is then used to measure an 18wt% PEG-acrylate gelation, and the critical relaxation exponent is calculated. Both probe particle sizes determine the same value for n. Finally, the degradation of HCO gel is re-examined using bi-disperse probe sizes. 0.5 ?m probes measure the same heterogeneous phase transition previously measured, while 2.0 ?m probes exhibit no heterogeneity and do not measure a sol-gel transition.To determine if colloidal rearrangement during phase transition is effecting the scaffold structure of HCO, a ?2rheology, microfluidics with microrheology, is developed. A two-layer microfluidic device is designed to exchange fluid surrounding a sample, enabling phase transitions, while minimizing the shear imparted on the sample. A key component of the device is six symmetrically placed channels connecting a sample chamber and solvent basin, located directly above the sample chamber. During fluid exchange, the sample is locked in place due to the equal pressure surrounding it. An estimate of the amount of shear imparted on the sample is calculated to be below the yield stress of the sample. A total of nine phase transitions are measured on a single sample, and there is no change in the rheological properties when the material is at equilibrium.The effect of shear imparted on the sample during preparation is determined using a combination of microrheology, ?2rheology, and bulk rheology. Critical relaxation exponents for pre-sheared HCO gelation and non-sheared HCO degradation, ngel and ndeg, respectively, are related to a normalized elastic modulus of a polyamide fiber system. Accessible phases of HCO during gelation and degradation are related to the phases measured in the polyamide system. ?2rheology determines that the neither pre-sheared nor non-sheared HCO gels change rheological properties at equilibrium, however the shear does change the accessible phases of each system. Pre-sheared HCO makes a weaker network in the gel phase, but form a solution of bundled fibers. Non-sheared HCO forms a network of entangled fibers that can undergo a phase change but the osmotic pressure gradient cannot completely dissociate the fiber entanglements. Bulk rheology confirms these findings by showing that the non-sheared HCO degradation does not have a crossover point between storage and loss moduli, while the gelation of the pre-sheared sample has a crossover at the point predicted by the normalized modulus. Finally, a normalization of bulk and microrheology data show that the two methods are measuring the phase transitions at the same point.In all, this work presents characterization techniques for the dynamic phase changes of polymeric and colloidal gels. The information gained through this work enables engineering of materials through the addition of rheological modifiers to fit specific needs of home care and industrial products.
Wehrman, Matthew David, "Rheological properties of colloidal and polymeric scaffolds measured using multiple particle tracking microrheology" (2018). Theses and Dissertations. 4260.