How do Interfacial Potential and Surface Tension Tune the Solubility of Thermoresponsive Polymers

In this project, Dr. Paul S. Cremer of The Pennsylvania State University will explore the liquid-liquid phase separation of polymer molecules in aqueous solutions in the presence of Hofmeister salts and osmolytes. The central question to be addressed is how ion-specific properties are reflected in solubility changes of dissolved polymers as the salt or small molecule concentration is increased. This study will provide foundational insights into a wide range of critical problems, including protein folding, materials assembly, and the formation of membraneless organelles inside cells. Beyond its scientific implications, this project will train students in an interdisciplinary field involving polymer chemistry, physical chemistry, biomaterials chemistry, and advanced spectroscopy. Furthermore, the research team will partner with high school chemistry teachers to develop laboratory modules that could be performed in their own classrooms using simple, inexpensive chemicals and readily available equipment. This should help significantly broaden the types of experiments on noncovalent chemistry that are available at the high school level. From a technical perspective, the project will test the hypothesis that ion-specific changes are primarily governed by the direct interactions of ions with polymer molecules rather than by changes to bulk water structure. The research will aim to elucidate the distinctions between water-mediated and ion-polymer phase separation mechanisms while identifying key differences in behavior between polymers that possess an upper critical solution temperature (UCST) and those that have a lower critical solution temperature (LCST). Cloud point measurements will be performed on a variety of polymers, including resilin-like polypeptides that contain an eight-residue polypeptide repeat. These phase transition data will be correlated with thermodynamic data from electrolyte solutions to resolve the mechanisms of phase separation. Studies will be conducted as a function of cation and anion identity, and additional measurements will be made with various osmolytes and polymer chain chemistries. Next, multicurve resolution Raman and vibrational sum frequency generation spectroscopy will be performed to explore how cloud point measurements correlate with water structure around specific polymer moieties. Finally, all-atom molecular dynamic simulations will be performed to explicitly understand how noncovalent forces, such as hydrogen bonding, charge pairing, dispersion, and cation-π interactions, regulate polymer solubility in aqueous solutions. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.