Investigating Aqueous PEO-PPO-PEO Triblock Copolymer Dispersion Dynamics with Colloidal Sphere Thermal Motion

Abstract

The objective of the thesis is to investigate the structural and dynamical behavior of aqueous poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock surfactants, commercially known as PluronicsTM, by studying the Brownian motion of embedded tracer probe particles. Recent advances in the understanding of diffusive light transport in highly scattering media have seeded a novel dynamic light scattering technique in the multiple scattering limit known as diffusing wave spectroscopy (DWS). This technique allows for the determination of probe motion at frequencies greater than 1 MHz and at a spatial resolution of several angstroms. Thus, DWS provides unique access to early thermal relaxation modes in Pluronic samples, which is the proposed origin of viscoelastic behavior observed on a macroscopic level.

In general, Pluronic-type macromolecular surfactants associate into spherical micelles, leading to complex structures and rich dynamic behavior when dispersed in aqueous solution. In the first part of the thesis, we demonstrate the utility of DWS microrheology to studying the short-time dynamics of aqueous Pluronic L64 [(EO)13(PO)30(EO)13] solutions, revealing a temperature independent high frequency viscosity above the micellization temperature. The dynamics at high temperatures are dominated by an apparent attractive inter-micellar potential, consistent with our inverse osmotic pressure measurements. We confirm the presence of a short-lived elastic gel at high temperatures, which we attribute to a spanning cluster indicating the crossing of a dynamic percolation threshold.

In the second part of this thesis, we employ a high-pressure scattering cell to examine the phase space of aqueous Pluronic P85 [(EO)25(PO)40(EO)25] solutions. We demonstrate that subtle changes in water by densification via increased hydrostatic pressure, by increasing thermal energy, or even by isotopic substitution, lead to discernable large-scale effects in aqueous P85 Pluronic samples. More generally, we show the utility of DWS tracer studies to explore complex fluids in high pressure and temperature environments, allowing for the construction of phase diagrams based on dynamical pathways.