Dynamic Heterogeneity in Plasticized Polystyrene Thin Films

Polymer dynamics near the glass transition temperature (T_g) is complex, and a complete molecular scale theory for the glass transition is lacking. Relaxation timescales that characterize polymer dynamics near T_g display highly non-exponential behavior and span several orders of magnitude on approaching T_g. Rotational dynamics of individual fluorescent probes provide insight into host nanoscale environments, thereby reporting local dynamics typically obscured in ensemble measurements. Modulation of a polymer matrix network’s rigidity by swelling with good solvents is an understudied route to accessing dynamics in the glassy regime, and such experiments can shed light on plasticization processes. We perform controlled swelling of polystyrene with toluene, monitored via a quartz crystal microbalance (QCM), to target effective temperatures near T_g as monitored by the average rotational correlation time of perylene diimide probe molecules. Characterizing the distribution of rotational correlation time and degree of non-exponentiality of individual probe molecule relaxation reveals clear differences in the degree and temporal persistence of heterogeneity in solvent-swollen compared to temperature-controlled polystyrene in the rubbery regime.