Title: Single Molecule Studies of Dynamic Heterogeneities in Supercooled Liquids
Description: We describe a set of single molecule fluorescence experiments that directly demonstrate the existence of spatial and temporal heterogeneity in two different small molecule glass former, glycerol and ortho-terphenyl (OTP) as well as the polymeric glass former polystyrene near their glass transition temperatures. The rotational dynamics of a set of perylene diimide probes are investigated in each small molecule glass former in a temperature range near their glass transition temperature. For all probes, the temperature dependence of their median rotational relaxation times (τc) reflect that of the structural relaxation of glycerol and OTP. The distribution of relaxation times for each probe at each temperature span around one decade and remain constant across all temperatures probed. In both glass formers, a trend as a function of probe rate of rotation occurs, where the fastest rotating probes exhibit the broadest τc distributions. Unexpectedly, a correlation between the rotational dynamics and the strength of the probe's intermolecular interactions with the host is seen.

In OTP, the fastest rotating probe is the smallest probe, with the lowest molecular weight, as expected. But in glycerol, the largest probe exhibits the fastest rotational dynamics. This counterintuitive result arises from the apparent inhibition of hydrogen bonding between the probe and host due to bulky non-polar groups sterically hindering the polar carbonyl groups on the probe. Analysis of dynamic exchange of probes on long time scales in glycerol (102 - 106 times the structural relaxation) does not reveal the presence of temporal heterogeneity on this time scale. Another technique employed to assess exchange on a shorter time scale reveals that ~30 % of molecules exhibit temporally heterogeneous behavior. Single molecule experiments on polystyrene (PS) near its glass transition temperature are also presented. Here, the rotational and translational dynamics of perlyene diimide probes in 100 nm PS films near its glass transition are studied. As in glycerol and OTP, average rotational relaxation times are found to mimic the temperature dependence of the host structural relaxation.

These studies, intended as control experiments for confined film SM studies, reveal spatial and temporal heterogeneity in PS dynamics. The measured distribution of rotational relaxation times spans 1.5 decades and remains constant across all temperature probed. These distributions fall between the expected distribution width for the purely spatially and temporally heterogeneous cases, suggesting the distributions are comprised of combination of spatial and temporal components. The median stretching exponent (β) from fitting SM trajectories results in β = 0.63 and a "quasi-ensemble" result of β = 0.58 found from combining SM linear dichroism autocorrelation functions. These represent the smallest stretching exponents reported for single molecule studies in supercooled liquids to date, indicating that the probe employed truly mirrors the dynamic heterogeneity of the host. The SM rotational relaxation rates are found to be correlated to their stretching exponents i.e. the lowest relaxation rates also have, on average, the lowest β values.

Additionally, small stretching exponents are correlated with long trajectories, suggesting that the rate of rotation together with the length of the trajectory dictate the degree of heterogeneity the probe is able to sample. Surprisingly, a mobile layer is observed in the films at temperatures near the glass transition. Translating molecules in this region are tracked and represent ~10% of the total molecules evaluated in this film. Molecules in the mobile region appear to be diffusing at rates that are magnitudes greater than the molecules rotating in the bulk region of the film.