Justin Pye - Physical Aging in Ultrathin Polystyrene Films: Evidence of a Gradient in Dynamics at the Free Surface and Its Connection to the Glass Transition Temperature Reductions

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  Publication Details (including relevant citation   information):

  Pye, J. E.; Rohald, K. A.; Baker, E. A.; Roth, C. B.   Macromolecules. 2010, 43,   8296-8303.


  We have measured the temperature dependence of the physical aging   rate β of thick (2430 nm) and thin (29 nm) polystyrene (PS) films   supported on silicon using a new streamlined ellipsometry   procedure that we have recently developed. The physical aging   rates β(T) for the ∼30 nm thick films are found to be reduced at   all temperatures, which is not consistent with a simple shift in   β corresponding to the average reduced glass transition   temperature (Tg) of these films. Instead, the β(T) results   correspond well with there being a gradient in dynamics near the   free surface. Our β(T) results can be well fit by both a   two-layer model and a gradient model. The temperature-dependent   length scale (of order 10 nm) that characterizes the depth to   which the enhanced dynamics near the free surface propagate into   the film is similar to that found previously by Forrest and   Mattsson [Phys. Rev. E 2000, 61, R53−R56] for the molecular   weight (MW)-independent Tg reductions of low-MW free-standing PS   films, strongly suggesting that the same mechanism is responsible   for both effects. This length scale grows with decreasing   temperature, suggesting that the mechanism is cooperative in   nature.

  Address (URL): http://pubs.acs.org/doi/abs/10.1021/ma101412r