(321b) Characterization of Glass Transition Temperatures In Block Copolymer/Ionic Liquid Micelle Cores

Achieving equilibrium structures in block copolymer micelles is significantly more difficult than in small-molecule surfactant micelles, due to the much higher thermodynamic barriers and slower kinetics associated with the long-chain structures of their unimers. Polymer chains can also become trapped or frozen within micelles in cases when the core-forming block has a glass transition temperature (Tg) at the temperature ranges of interest. Measurements of micelle core glass transitions are challenging, given the nanoscale volumes and small volume fractions being probed. Such measurements would not only provide insight into block copolymer micellar equilibration, but also have the potential to yield contributions to the field of polymer systems confined at the nanoscale. The physical properties of such systems can be significantly different from those of bulk systems; this has been demonstrated in a number of cases in terms of changes to the Tg of polymer thin films, but few studies have been carried out in other geometries. Frozen micelles present an important opportunity for expanding the range of studies in low-dimensional structures

We discuss the development of a fluorescence-based technique for the investigation of Tgs in polystyrene (PS) micelles cores in ionic liquid solvents. Specifically, we demonstrate the ability to detect the transition from their fully glassy state to their fully rubbery state in micelles of poly(styrene-b-ethylene oxide) and poly(styrene-b-methylmethacrylate). The use of non-volatile ionic liquids as the solvent allowed for studies over extended temperature ranges while remaining in a stable, liquid state. The systems are explored as a function of copolymer block molecular weight and composition and micelle preparation protocol. We find that the PS core Tgs are shifted 20–45 ºC below reported bulk PS homopolymer Tgs of equivalent molecular weight, and that they can also be significantly broader (up to 45 ºC in breadth).