Surface dynamics of a thin polystyrene film probed by low-energy muons

Abstract

Low-energy muons have been used to probe the local motion in a thin-film sample of polystyrene as a function of depth below the free surface. The muon spin relaxation in zero magnetic field is dominated by a muoniated radical state which is formed on the polystyrene ring. The muon spin-relaxation rate $\ensuremath{\lambda}$ is found to be highly sensitive to the dynamical state of the polymer, and $\ensuremath{\lambda}$ provides a local probe of changes in the dynamical correlation time around the glass transition. When high-energy muons are used to study the bulk properties, the glass-transition temperature ${T}_{g}$ is revealed as a change in the temperature dependence of $\ensuremath{\lambda}$. Reducing the muon implantation depth, while holding the sample a few degrees below the bulk ${T}_{g}$, leads to a drop in $\ensuremath{\lambda}$, signifying a reduction in the local ${T}_{g}$ on approaching the surface. The width of the dynamical surface layer is found to be enhanced over previous estimates made at much lower temperatures, providing experimental evidence for critical divergence of the layer width on approaching the bulk ${T}_{g}$.