Direct Measurement of Molecular Motion in Freestanding Polystyrene Thin Films

TitleDirect Measurement of Molecular Motion in Freestanding Polystyrene Thin Films
Publication TypeJournal Article
Year of Publication2011
AuthorsPaeng, K., S. F. Swallen, and M. D. Ediger
JournalJournal of the American Chemical Society
Volume133
Pagination8444-8447
ISBN Number0002-7863
Accession NumberWOS:000291414400015
KeywordsCONFINEMENT, DIFFUSION, DYNAMICS, GLASS-TRANSITION TEMPERATURE, HOLE GROWTH, LAYERS, MOBILITY, POLYMER-FILMS, PROBE, RELAXATION
Abstract

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.

DOI10.1021/ja2022834
Alternate JournalJ. Am. Chem. Soc.