Chad Snyder - Nanoscale thermal–mechanical probe determination of ‘softening transitions’ in thin polymer films

Document created by Chad Snyder on Aug 28, 2014
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  Publication Details (including relevant citation   information):

  J. Zhou, B. Berry, J.F. Douglas, A. Karim, C.R. Snyder and C.   Soles, Nanotechnology 19, 495703   (2008).


  We report a quantitative study of the softening behavior of   glassy polystyrene (PS) films at length scales on the order of   100 nm using nano-thermomechanometry (nano-TM), an emerging   scanning probe technique in which a highly doped silicon atomic   force microscopy (AFM) tip is resistively heated on the surface   of a polymer film. The apparent ‘softening temperature’   Ts of the film is found to depend on the   logarithm of the square root of the thermal ramping rate   R. This relation allows us to estimate a   quasi-equilibrium (or zero rate) softening transition temperature   Ts0 by extrapolation. We observe marked   shifts of Ts0 with decreasing film thickness, but the   nature of these shifts, and even their sign, depend strongly on   both the thermal and mechanical properties of the supporting   substrate. Finite element simulations suggest that thin PS films   on rigid substrates with large thermal conductivities lead to   increasing Ts0 with decreasing film   thickness, whereas softer, less thermally conductive substrates   promote
  reductions in Ts0. Experimental observations   on a range of substrates confirm this behavior and indicate a   complicated interplay between the thermal and mechanical   properties of the thin PS film and the substrate. This study   directly points to relevant factors for quantitative measurements   of thermophysical properties of materials at the nanoscale using   this nano-TM based method.

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