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This study examines the origin of the widely different length scales,
h
t
—nanometers to micrometers—that have been observed for the propagation of the near-surface enhanced mobility in glassy polymers. Mechanical relaxations of polystyrene films with thicknesses,
h
, from 5 nm to 186 μm have been studied. For
h
< ~1 μm, the films relaxed faster than the bulk and the relaxation time decreased with decreasing
h
below ~100 nm, consistent with the enhanced dynamics originating from a near-surface nanolayer. For
h
> ~1 μm, a bulk-like relaxation mode emerged, while the fast mode changed to one that extended over ~1 μm from the free surface. These findings evidence that the mobile surface region is inhomogeneous, comprising a nanoscale outer layer and a slower microscale sublayer that relax by different mechanisms. Consequently, measurements probing the enhanced mobility of different mechanisms may find vastly different
h
t
’s as shown by the literature.
Surface glass-transition temperature (T
g
surf) and
transition
width (W) within 1 nm of the surface were measured
by monitoring a qualitative change in the contact angle or density
of end-groups (by time-of-flight secondary ion mass spectrometry)
with temperature. Polystyrene (PS) films with various thicknesses
(h) and molecular weights were studied. For unannealed
PS supported on oxide-coated silicon or poly(dimethyl siloxane) with h > ∼60 nm, T
g
surf approached a plateau value
of ∼25 K below the bulk T
g; below
60 nm, T
g
surf decreased with decreasing h. Separately, W exhibited a stepwise increase when h was decreased below the radius of gyration of the polymer.
Upon thermal preannealing or deposition on a PS brush or adsorbed
layer, the films ceased to exhibit T
g
surf reductions
or stepwise change in W. We discuss how an out-of-equilibrium
density profile with a deficit near the substrate and de Gennes’
sliding mode may explain these observations.
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