(240i) Criteria Governing Rod Formation and Growth in Polymer Micelles

Wormlike micelles (WLMs) are elongated, flexible self-assembled structures commonly used in drag reduction and cosmetic applications. While most commonly studied in amphiphilic small molecule surfactants, WLMs can also form in amphiphilic block polymer solutions, where self-assembly can be tuned via properties including molecular weight and block composition. Previous work on polymer WLMs examines limited block polymer compositions and mostly concerns the sphere-to-rod transition instead of WLM growth. Additionally, solution conditions such as temperature or additive concentration vary widely across these studies; thus, a holistic picture of the role of amphiphile parameters and solution conditions on polymer WLM formation and growth is lacking. Here, a series triblock poloxamers with polyethylene oxide (PEO) end blocks and polypropylene oxide (PPO) midblocks are examined comprehensively to determine the role of amphiphile characteristics (molecular weight, block molecular weight, PPO fraction) and solution conditions (temperature, inorganic salt concentration) on rod formation and WLM growth. In these systems, micelles undergo a sphere-to-rod transition and growth into WLMs with increasing temperatures and salt concentrations, eventually phase separating at the cloud point temperature. These morphological changes are examined and compared across a range of parameters, where local micelle structure is investigated using small-angle neutron scattering (SANS) and is connected with bulk properties via shear rheology. Interestingly, amphiphile characteristics weakly impact the local structure, yet bulk properties such as viscosity and relaxation time increase by orders of magnitude with increasing PPO and decreasing PEO block size. Similar to prior studies suggesting that temperature and salt are interchangeable for inducing transitions like micellization or the cloud point, we observe that the sphere-to-rod transition temperature decreases with increasing salt content while the local rod structure remains virtually unchanged. However, WLMs formed in the presence of salt exhibit a more dramatic viscosity increase due to increased WLM growth at lower temperatures vs. their counterparts at higher temperatures. Additionally, while sodium fluoride is a more effective salt for lowering the sphere-to-rod and cloud point transition temperatures, sodium chloride is more effective at inducing micelle growth and elongation. Thus while amphiphile characteristics and salt concentration only weakly impact the rod cross-sectional dimensions, these factors impart large changes in WLM growth and the bulk rheological response of poloxamer WLMs. These studies reveal fundamental insight on the interrelationship between parameters dictating rod formation and growth in poloxamer micelles, providing rational design guidelines for micellar fluids with desired properties and a comprehensive dataset for developing or refining computational models.