(20c) Still Playing with Gels and Watching Paint Dry: Rheology of Associative Semicrystalline Polymers and Stratification in Colloidal Films

In this talk, I will briefly describe two current projects in the Bhatia group which are the most direct descendants of my time in the Russel lab. In the first half of the talk, I will discuss rheology and structural studies of associative micellar gels of poly(lactide)-poly(ethylene oxide)-poly(lactide) (PLA-PEO-PLA) triblock copolymers in water, with various ratios of L-lactide and D-lactide in the PLA blocks, leading to varying degrees of crystallinity in the PLA domains. The impact of stereochemistry on the rheology is very striking; with a maximum in the storage modulus for intermediate L/D ratios. We attribute the rheology to a competition between an increase in the time for PLA endblocks to pull out of micelles as the L/D ratio is increased and PLLA crystallization occurs, and a decrease in the number of bridging chains for micelles with crystalline PLA domains, as formation of bridges may be hindered by crowded crystalline PLA domains. In the second half of this talk, I will present recent results on a project in collaboration with and inspired by another former member of the Russel lab, Dr. Alex Routh: stratification in films prepared from binary colloidal dispersions containing large and small particles of varying size and initial volume fraction, and in polymer-nanoparticle films prepared by evaporative drying. Our results show evidence of different types of stratification behavior, including large-on-top (e.g., large particles migrating to the top surface of the film), small-on-top, and “sandwich”-like layering. I discuss these results in terms of recent theories for stratification during evaporative drying and present some initial studies showing that coatings with nanoscale roughness resist bacterial adhesion.