University of Connecticut

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PhD Dissertation Defense

Monday, June 13, 2022
12:30pm – 2:30pm

Storrs Campus

Graduate Student Rasika Dahanayake, Department of Physics, University of Connecticut

Temperature and Solvent induced coil-globule transition of Polypropylene Oxide in solutions and its behavior within self-assembled nanostructures

Bio-compatible amphiphilic polymers are actively studied due to their many biomedical and industrial applications. Polypropylene oxide (PPO) is a polymer that exhibits an interesting thermal responsiveness. The mechanism of temperature induced coil-globule transition hard to assess with traditional methods, therefore molecular dynamics (MD) simulations can provide new insights. Here using all-atom MD simulations, we investigate the coil-globule transition and demonstrate that it is the temperature-induced increase in the sequence length of monomers that are not hydrogen bonded to water that drives the transition. Longer chains can create longer sequences, which serve as nucleation sites for hydrophobic cluster formation causing the transition to occur at lower temperatures.

Coil-globule transition of PPO can also be induced by addition of isobutyric acid (IBA) to aqueous solution. Adding a very small amount of IBA caused PPO to lose hydrogen bonds with water and create more stable hydrogen bonds with IBA which act as a nucleation site for the polymer collapse. With further addition of IBA, PPO expands and resides at the IBA/water interface to retain some fraction of hydrogen bonds with water along with IBA-water hydrogen bonds. In contrast to polyethylene oxide (PEO), where a significant fraction of water is doubly-bonded to polymer, PPO is mostly hydrated via singly bonded water molecules and loses hydration upon a temperature increase. These results illustrate that hydrogen bonding and a polymer’s capability to maintain its hydration shell are the key factors in polymer responsiveness to external triggers.

This polymer responsiveness leads to temperature sensitive micelles used in biomedical applications. The structure and properties of cholesterol-polyethylene oxide-cholesterol micelles were studied using atomistic and coarse-grained MARTINI MD simulations. We also investigated using all-atom MD simulations the structure of di-block PPO-PEO and tri-block Pluronic micelles in aqueous solutions and analyzed micelle interactions with co-solvents such as alcohols and IBA. We found that di-block copolymers were able pack much tighter into spherically symmetric micelles compared to triblock copolymers. Depending on their hydrophobicity, co-solvents can localize at the interface or penetrate within the PPO core of micelles that can affect micelle properties and stability.

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Prof. E. Dormidontova

Physics Department (primary), College of Liberal Arts and Sciences, UConn Master Calendar

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