Defining the structure and properties of colloidal rod systems during dynamic phase transitions

Rheological modifiers are added to formulations to tune rheology, enable function and drive phase changes requiring characterization of material structure and properties. We characterize the dynamic evolution of a colloidal rod used for rheological modification using multiple particle tracking microrheology (MPT). MPT measures the Brownian motion of embedded probes to extract rheological properties. This system consists of polyamide (PA), linear alkylbenzene sulfonate (LAS) and non-absorbing polyethylene oxide, which drives gelation by depletion interactions. We characterize the role of the starting microstructure on gelation using time-cure superposition to determine the critical relaxation exponent, n. n indicates the system structure transitions from a tightly to a loosely associated network at the phase transition depending on the PA:LAS. This is due to the initial PA stability, which is verified by zeta potential measurements. We compare the rheology of PA and hydrogenated castor oil, a polydisperse fiber, and find that polydispersity does not change the rheology and structure of the material during a phase transition. This study will inform future product design by providing guidance to reach desired rheological properties while minimizing trial-and-error experiments.