Confinement effects in polystyrene and poly(methylmethacrylate) films and nanocomposites are studied by fluorescence. The ability to employ an intensive measurable, the excited-state fluorescence lifetime, in defining the glass transition temperature, Tg, of polymers is demonstrated and compared to the use of an extensive measurable, fluorescence intensity. In addition, intrinsic fluorescence from the phenyl groups in polystyrene is used to determine the Tg-confinement effect in films as thin as ∼15 nm. The decrease in Tg with decreasing film thickness (below ∼60 nm) agrees well with results obtained by extrinsicpyrene fluorescence. Dye label fluorescence is used to quantify theenhancement in Tg observed with decreasing thickness (below ∼90 nm) in poly(methyl methacrylate) films; addition of 2-4 wt% dioctyl phthalate plasticizer reduces or eliminates the Tg-confinement effect in films down to 20 nm thickness. Intrinsic polystyrene fluorescence, which is sensitive to local conformation, is used to quantify the time scales (some tens of minutes) associated with stress relaxation in thin and ultrathinspin-coated films at Tg + 10 K. Finally, the shape of the fluorescence spectrum of pyrene doped at trace levels in polystyrene films and polystyrene-silica nanocomposites is used to determine effects of confinement on microenvironment polarity.
Bibliographical noteFunding Information:
We acknowledge the support of the NSF-MRSEC program (Grant DMR-0076097 and Grant DMR-0520513), Northwestern University, and a Henderson Dissertation-Year Fellowship (to C.J.E.). We also acknowledge the use of the profilometer in the shared used facilities of the Materials Research Center at Northwestern University.