(326a) Rheological Properties of Phase Transitions in Polydisperse and Monodisperse Colloidal Rod Systems

Rheological modifiers are used to tune rheology or induce phase transitions of products with small amounts of material to reach desired rheological behavior. To efficiently design products, an understanding of the rheological properties and structure of the material is necessary. Colloidal rods are desirable as rheological modifiers due to their ability to change rheological properties with minimal additional material. This work characterizes the rheological properties and sol-gel transitions for two colloidal rod systems: monodisperse polyamide (PA) and polydisperse hydrogenated castor oil (HCO). These measurements characterize the structure and rheological properties of both systems during phase transitions to assess their effectiveness as rheological modifiers. Multiple particle tracking microrheology (MPT) is used to characterize gelation of HCO and PA, which is driven by depletion interactions. In MPT, fluorescent probe particles are embedded in the sample and their Brownian motion is measured and related to rheological properties. Our system consists of a colloid (HCO or PA), a surfactant (linear alkylbenzene sulfonate, LAS), and a non-absorbing polymer (polyethylene oxide, PEO) used to induce depletion interactions and gel the systems. PEO concentration is increased for both PA and HCO from 0.05 to 0.75 c/c^* ( c^* is critical overlap concentration) at different LAS : colloid. Using MPT, we measure a gradual decrease in the logarithmic slope of the mean-squared displacement (MSD) with increasing PEO concentration. This indicates that particle mobility is decreasing and the system structure is becoming more arrested with increasing depletion interactions. Measurements of probe particle diffusivity, indicate that the growing structure of the gel network depends on the LAS : colloid. To determine the gel point and critical values, time-cure superposition (TCS) is used to analyze MPT data. PA and HCO undergo sol-gel transitions with increasing depletion interactions at all concentrations, but the evolution of the rheology is dependent on LAS : colloid. The critical depletant concentration (c_c) is constant at all LAS : colloid with the exception of 0.2 wt% colloid at LAS : colloid=16. This indicates that the amount of PEO required to drive gelation is not significantly affected by LAS : colloid. The critical relaxation exponent, is a measure of the structure of the material at the phase transition and is dependent on LAS : colloid. is lower for LAS : colloid = 16 (n_PA = 0.34 ± 0.07 and n_HCO = 0.37 ± 0.14) than LAS : colloid > 16 (n_PA = 0.74 ± 0.13 and n_HCO = 0.83 ± 0.10). This indicates that at LAS : colloid = 16, the colloidal gels are tightly associated networks at the phase transition. At LAS : colloid > 16, the colloidal gels are loosely associated networks at the phase transition point. The difference in structure between LAS : colloid=16 and LAS : colloid > 16 is due to a difference in the zeta potential. This measures that there is a change in electrostatic forces induced by LAS stabilization of the colloidal rods. Our characterization determines that monodispersity and polydispersity does not affect rheological behavior between these two samples. Instead, LAS : colloid modifies the rheology and microstructure at the sol-gel transition for both of the systems by tuning the electrostatic forces between colloidal rods. This work will inform future product design to specify desired rheology and minimize trial-and-error experiments.