Measuring the Surface Dynamics of Glassy Polymers
Original entry: Ian Burgess, Fall 2009
Z. Fakhraai and J. A. Forrest, "Measuring the Surface Dynamics of Glassy Polymers" Science 319, 600 (2008).
Polymer chain segments in bulk polystyrene (PS) cooled below the glass transition temperature experience little to no mobility, however this is not necessarily true at the polymer surface. The authors in this study measure the mobility of polymer chains near the surface of a film below the glass transition temperature. By partially embedding gold nanospheres at the surface and then removing them, leaving behind dimple-like impressions in the film, they observe the motion of polymer chains near the surface via the relaxation of the surface deformation.
A fundamental challenge of this type of experiment is that the surface deformation must be produced without inducing any stress in the film, as stress would also induce such a relaxation without requiring any fluid-like mobility in the chains. This is accomplished in their experiment by scattering the gold spheres across the top of a pre-spin-cast and annealed (PS) film and then heating the film to 378K, above the glass transition temperature, to allow the higher-density gold spheres to partially embed themselves in the film under the force of gravity. The authors point out that this temperature must be sufficiently high that the relaxation time of the polymer is shorter than the timescale associated with the falling gold particles. The particles were then removed without inducing stress by dissolving them in a mercury droplet placed on the surface which then is allowed to slide off by tilting the film. The surface relief patterns were then monitored by atomic force microscopy.
If polymer mobility exists at the surface, surface tension forces