|Title||Direct Measurement of Molecular Motion in Freestanding Polystyrene Thin Films|
|Publication Type||Journal Article|
|Year of Publication||2011|
|Authors||Paeng, K., S. F. Swallen, and M. D. Ediger|
|Journal||Journal of the American Chemical Society|
|Keywords||CONFINEMENT, DIFFUSION, DYNAMICS, GLASS-TRANSITION TEMPERATURE, HOLE GROWTH, LAYERS, MOBILITY, POLYMER-FILMS, PROBE, RELAXATION|
An optical photobleaching technique has been used to measure the reorientation of dilute probes in freestanding polystyrene films as thin as 14 rim. Temperature-ramping and isothermal anisotropy measurements reveal the existence of two subsets of probe molecules with different dynamics. While the slow subset shows bulk-like dynamics, the more mobile subset reorients within a few hundred seconds even at T-g,T-DSC - 25 K (T-g,T-DSC is the glass transition temperature of bulk polystyrene). At T-g,T-DSC - 5 K, the mobility of these two subsets differs by 4 orders of magnitude. These data are interpreted as indicating the presence of a high-mobility layer at the film surface whose thickness is independent of polymer molecular weight and total film thickness. The thickness of the mobile surface layer increases with temperature and equals 7 nm at T-g,T-DSC.
|Alternate Journal||J. Am. Chem. Soc.|