(391a) Defining the Structure, Rheology and Properties of Colloidal Rod Systems during Dynamic Phase Transitions

Rheological modifiers are added in formulations to tune rheology, enable function and drive phase changes. These complex functions require an in-depth understanding of the structure and properties, especially during material evolution throughout a phase transitions. We characterize two colloidal rod systems during phase transitions using multiple particle tracking microrheology (MPT). These systems include a colloid (monodisperse polyamide or polydisperse HCO), surfactant (linear alkylbenzene sulfonate, LAS) and non-absorbing polymer (polyethylene oxide, PEO), which drives gelation by depletion interactions. To determine the role of the starting material microstructure in gelation the ratio of the stabilizing surfactant, LAS, to colloid is varied and has two regimes: LAS:colloid=16 and LAS:colloid>16. We measure changes in material property evolution at different LAS:colloid ratios, which indicates that the mechanism of gelation differs in the two regimes. At LAS:colloid=16, the colloids are stable single colloids prior to gelation that form a sample-spanning network during gelation. At LAS:colloid>16, the colloidal rods start as bundles and then those bundles form gel networks. We analyze colloidal gelation using time-cure superposition to quantitatively determine the critical values at the phase transition. The critical relaxation exponent, n, depends on LAS:colloid. n is lower at LAS:colloid=16 than LAS:colloid>16 indicating that by changing the LAS:colloid ratio the system structure transitions from a tightly associated to a loosely associated network at the phase transition. We hypothesize this is due to the microstructure of the starting material, where the rods are single stable rods at LAS:colloid=16 and bundle at LAS:colloid>16, which is verified by zeta potential measurements. We compare the results of PA and HCO, and find that they are similar during sol-gel transitions regardless of the polydispersity of the colloidal rods. This indicates that polydispersity does not affect the mechanism, rheology and structure of the material during a phase transition. This study will inform future product design by providing formulation guidance to reach desired rheological properties while minimizing trial-and-error experiments.