(195h) Characterizing Rheological Properties for Both Polydisperse and Monodisperse Colloidal Rod Systems

Rheological modifiers are used to tune the rheology of products. To effectively use rheological modifiers, the material must be well understood in terms of their structure and rheological properties. Hydrogenated castor oil (HCO) is currently used as a rheological modifier in home care products, but these rods are polydisperse leading to complex heterogeneous structures. To determine the effect of polydispersity on rheological modification, this work characterizes HCO and polyamide (PA), a monodisperse colloidal rod, using multiple particle tracking microrheology (MPT). In MPT, fluorescent probe particles are embedded in the sample and their Brownian motion is measured. 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 drive gelation through depletion interactions. We measure the diffusivity and logarithmic slope of the mean-squared displacement (MSD) using MPT with increase depletion interaction to gel the systems. We find that at LAS : colloid > 16, the diffusivity of these systems rapidly decrease when the PEO concentration is increased from 0.3 to 0.35 c/c*, but the logarithmic slope of the MSD has a consistent decrease with PEO concentration. The diffusivity indicate that the phase transition happens in between 0.3 and 0.35 c/c*. Therefore, we analyze gelation using time-cure superposition to determine the critical phase transition point. It is determined that the critical depletant concentration is 0.34 ± 0.02 c/c* and 0.36 ± 0.04 c/c* for HCO and PA, respectively. This critical depletant concentration is not depend on LAS : colloid, which indicates that the amount of PEO that required to gel the systems is constant when varying the system component ratio . We also calculate the critical relaxation exponent for each system at different combinations of colloid and LAS concentration. The critical relaxation exponent indicates that gelation of both systems is dependent on LAS : colloid. The critical relaxation exponent is lower for LAS : colloid = 16 (tightly associated network) than LAS : colloid > 16 (loosely associated network). This study shows that the rheology and microstructure of monodisperse and polydisperse change during the sol-gel transition when change the surfactant to colloid ratio which provide guidance for the future product design.