Research & Faculty

Default Header Image

Enhanced high-frequency molecular dynamics in the near-surface region of polystyrene thin films observed with [small beta]-NMR

TitleEnhanced high-frequency molecular dynamics in the near-surface region of polystyrene thin films observed with [small beta]-NMR
Publication TypeJournal Article
Year of Publication2015
AuthorsMcKenzie, I, Daley, CR, Kiefl, RF, Levy, CDPhilip, W. MacFarlane, A, Morris, GD, Pearson, MR, Wang, D, Forrest, JA
JournalSoft Matter
Volume11
Pagination1755-1761
Abstract

[small beta]-detected nuclear spin relaxation of 8Li+ has been used to probe the depth dependence of molecular dynamics in high- and low-molecular-weight deuterated polystyrene. The average nuclear spin-lattice relaxation rate{,} 1/Tavg1{,} is a measure of the spectral density of the polymer motion at the Larmor frequency (41 MHz at 6.55 T). In both samples{,} 1/Tavg1 is depth independent below [similar]200 K but above this temperature it decreases approximately exponentially with distance from the free surface{,} returning to bulk behavior for depths greater than [similar]10 nm. This is direct evidence for a region near the free surface with enhanced molecular dynamics compared with the bulk. The effective thickness of the surface region increases with increasing temperature and is finite even above the glass transition. These results present challenges for the current understanding of dynamics near the surface of polymer glasses.

URLhttp://dx.doi.org/10.1039/C4SM02245A
DOI10.1039/C4SM02245A