title: Enhanced high-frequency molecular dynamics in the near-surface region of polystyrene thin films observed with β-NMR.
abstract: β-detected nuclear spin relaxation of (8)Li(+) 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/T(avg)(1), 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/T(avg)(1) is depth independent below ∼200 K but above this temperature it decreases approximately exponentially with distance from the free surface, returning to bulk behavior for depths greater than ∼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.
authors.affiliation: TRIUMF, Vancouver, V6T 2A3, B.C., Canada. iain.mckenzie@triumf.ca.
authors.forename: Iain
authors.initials: I
authors.lastname: McKenzie
authors.forename: Chad R
authors.initials: CR
authors.lastname: Daley
authors.forename: Robert F
authors.initials: RF
authors.lastname: Kiefl
authors.forename: C D Philip
authors.initials: CD
authors.lastname: Levy
authors.forename: W Andrew
authors.initials: WA
authors.lastname: MacFarlane
authors.forename: Gerald D
authors.initials: GD
authors.lastname: Morris
authors.forename: Matthew R
authors.initials: MR
authors.lastname: Pearson
authors.forename: Dong
authors.initials: D
authors.lastname: Wang
authors.forename: James A
authors.initials: JA
authors.lastname: Forrest