Micrometer Mobile Surface Layer in a Glassy Polymer

It is well established that the near-surface dynamics of glassy polymers are enhanced relative to the bulk. However, the distance propagated by these dynamics, h_t, is still unsettled. In this study, we measured the mechanical relaxations of free-standing polystyrene films with thickneeses, h, from 5 nm to 186 μm. 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 bulklike relaxation mode emerged, while the fast mode changed to one that stretched over ~1 μm from the free surface. These findings evidence that the mobile surface region is inhomogeneous, comprising a nanoscale outerlayer and a slower microscale sublayer that relax by different mechanisms. Consequently, measurements probing the enhanced dynamics of different mechanisms may find vastley different values of h_t, as shown by the literature.