John H. Dillon Medal Talk: Probing Glass Physics Through Measurements of Polymer Dynamics in Thin films and in Strongly Confined Systems*

Extensive research in the past two decades has shown that the free surfaces of glasses have dramatically faster dynamics compared to the bulk dynamics with much lower activation energies for rearrangement. In ultrathin films of polymers and molecular glasses, the enhanced surface mobility results in large changes in the value of the glass transition temperature (T_g), enhanced overall dynamics, as well as other property changes such as elasticity, mechanical response, and aging rate.

In this presentation, I will discuss how as the film thickness is reduced below ~30 nm, the dynamics at the two interfaces strongly correlate such that the bulk-like dynamics disappear, and the free surface dynamics are influenced by the substrate dynamics and vice versa. In molecular glasses, supported on weakly interacting substrates these effects can cause a sharp transition from bulk dynamics to liquid-like behavior resulting in dewetting of 30nm films at temperatures well below T_g. These long-range dynamical correlations cannot be simply explained by changes in local interaction potentials. Furthermore, the thickness of this transition remains independent of chemical structure or substrate interactions and appears to be set by the bulk glass properties. As such, the strong perturbation at the free surface can be used as a probe of fundamental aspects of dynamics in bulk super-cooled liquids close or below their T_g.

In contrast, in highly confined systems such as polymers infiltrated into dense nanoparticle packings, while strong changes in T_g and viscosity are still observed, the geometric confinement effect is only significant when the pore sizes are a few nanometers. This is a much shorter length scale than what is observed in systems with free surfaces, suggesting that geometric confinement and free surfaces can have distinctly different effects on the dynamics.