Using adhesion to probe viscoelasticity of polymer film surfaces:: A quartz crystal microbalance study

We have used a quartz crystal microbalance to monitor the adhesion of small particles to the surface of polystyrene films. This technique is shown to provide a signature of viscoelastic relaxation processes at the polymer surface. For M-n values less than 10(4) and greater than 10(5), this signature occurs at a temperature similar to 12.5 K above the measured bulk glass transition temperature, T-g. For intermediate values of M-n however, the relaxation signature occurs at temperatures as low as 7 K above the bulk T-g. This observation suggests a slight decrease in the T-g value near the polymer surface. A model incorporating the effects of enrichment of polymer chain ends at the free surface was considered and found to provide a quantitative description of the data.