Revisiting Glassy Behavior in Polystyrene with Single Molecule Approaches

Polystyrene in the rubbery regime displays phenomenology also seen in small molecule supercooled liquids, and dynamic heterogeneity in this system has been well studied. We have revisited several aspects of glassy behavior and dynamic heterogeneity in polystyrene using single molecule approaches. In particular, we have characterized exchange time (τ_ex, the timescale a particular dynamical environment maintains a given dynamics) at a number of temperatures down to and including the glass transition temperature. These studies reveal that the ratio of τ_ex to the alpha relaxation time (τ_ex/τ_α) is independent of temperature in the range probed. More recently, we have characterized dynamic heterogeneity in polystyrene as a function of fragility as controlled by molecular weight. Over a broad range of molecular weights, we find a similar degree of dynamic heterogeneity and ratio of τ_ex to the alpha relaxation time in polystyrene. Finally, aiming to resolve long-standing questions regarding the origins of rotational-translational decoupling, we have combined rotational and translational measurements of single molecule probes in polystyrene. Initial results suggest slowly rotating molecules are typically also slowly translating molecules but continue to suggest overall translational diffusion is faster than would be expected given system viscosity.