Temperature-based fluorescence measurements of pyrene in block copolymer micelles: Probing micelle core glass transition breadths

Glass transition temperature (T _g) breadths are reported for polystyrene (PS) micelle cores in two series of micelle-forming block copolymers [PS-poly(ethylene oxide) and PS-poly(methyl methacrylate)] with an ionic liquid solvent (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide). An increased level of fluorescent molecules was induced within the cores upon rapid cooling followed by aging. Using fluorescence to monitor dye release with relaxation of this state upon heating, transition onset and end-point temperatures were defined. The system with the lowest PS-block molecular weight showed no evidence of a transition above 25 °C; however, in every other case, transitions were observed beginning at ∼40-45 °C and ending at ∼60-85 °C. These temperatures closely match PS-block T _g results measured by differential scanning calorimetry in semidilute solutions of the same materials, suggesting that the transition temperature range correlates strongly to the transition of the cores from fully glassy to fully rubbery. Differences in transition end-points were related to PS-block molecular weights and relative copolymer fractions of PS.