Influence of Molecular Architecture on the Thermoresponsive Behavior of Linear and Star Highly Fluorinated Homopolymers

Abstract

Macromolecules that display a lower critical solution temperature (LCST) in aqueous solution have in common several structural elements, namely, hydrophobic groups exposed to the aqueous environment and polar moieties that undergo strong hydrogen bonding with water. The temperature of the transition depends on the relative contributions of the hydrophobic and hydrophilic groups and, as explored in this work, the molecular weight and architecture of the polymer. Notably, the influence of the incorporation of fluorinated groups into the polymer structure has been scarcely reported. The partly fluorinated polymer prepared in this study, poly(N-(2-((2,2,2-trifluoroethyl)sulfinyl)ethyl)acrylamide) (PFSAM), is soluble in water at ambient temperature, but on modest heating, the solution displays a cloud point transition (T_CP). NMR and molecular dynamics (MD) studies reveal that the hydrophobic fluorinated methyl of the side chain is an important driver of the thermal transition, participating in dynamic F–F interactions both below and above T_CP. Passing above T_CP, the polymer side chain undergoes conformational changes; however, the dynamics of the CF₃ group are little affected. Analysis of the NMR line shape and MD simulations reveal heterogeneous chain collapse and levels of hydration along the polymer chain. Star polymers display lower transition temperatures and stars with short arms even lower, highlighting the importance of interarm interactions in thermoresponsive star polymers. This detailed spectroscopic and computational study provides an unparalleled level of information about the impact of the polymer chemical structure and chain topology on the thermoresponsive transition in polymers in solution.