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 ~23nm 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 gold spheres to partially embed themselves in the film. 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.
It is worth noting that the very small size of the gold nanospheres (~23nm) is an important consideration in this experiment as it gives the lengthscale on which chain mobility will be measured as well as a depth-scale in which this mobility exists. The glass transition temperature of the bulk film was measured by standard techniques and was found to be in agreement with that of bulk PS. If polymer mobility exists at the surface, surface tension will cause the deformation to relax, with the eventual restoration of the the flat surface.
The figure above (Fig. 2) shows the time evolution of the dimples when the system was held at 293K, well below the glass transition temperature (369K). The relaxation is clearly visible in this figure indicating enhanced surface mobility of the polymer chains, even at temperatures far below the glass transition. The relaxation was quantified by measuring the time evolution of the holes' depths, which was repeated for various temperatures below the glass transition. Fitting the data to a simple exponential model, the relaxation timescale and thus the characteristic timescale of polymer surface motion could be compared with the bulk at various temperatures. The results are striking: Near the glass transition temperature, the surface relaxation time is comparable to the bulk, however, while the bulk mobility sharply drops off to zero (relaxation time goes to infinity) in the bulk, the surface relaxation time is found to increase only by two orders of magnitude over 80 K below the transition temperature.