45952-G6
Do Probe Molecules Alter Host Dynamics in Glassy Systems?
The research undertaken during the granting period began to clarify the subtleties of extracting dynamics of small molecule glass formers just above the glass transition temperature (Tg) through the use of single molecule (SM) experiments. In this temperature regime, the very interesting but poorly understood slow dynamics are believed to be related to spatiotemporal heterogeneities. SM experiments hold the potential to directly assess the size and time scales associated with the persistence of these heterogeneities. This distinguishes them from other experiments currently being performed on these systems.
We proposed to examine dynamics in glass formers just above Tg by following the rotation of fluorescent probe molecules embedded in these systems. These experiments minimize ensemble averaging over spatial heterogeneity and thus can provide information on the distribution of timescales of molecular orientational relaxation in glassy matrices at an unprecedented level of detail. However, these experiments also engender new questions about whether the probe samples and reports on all domains within the heterogeneous system in an unbiased manner. Thus, we proposed to perform a set of SM experiments and complementary molecular dynamics (MD) simulations. The MD simulations performed embedded spherical probes of varying size within a Kob-Anderson system of spheres at volume fractions in which the probe-less system is well known to exhibit phenomenology associated with molecular supercooled liquids. Our simulations showed that large probes can affect the dynamics of the surrounding particles, and not always in an intuitive manner. For example, we found that embedding a smooth probe of large diameter (greater than or equal to three times the diameter of the rest of the particles in the system) caused the surrounding particles to speed up, whereas these particles slowed down in the presence of identically sized rough probes. Moreover, the probes themselves exhibited dynamics with different temperature dependences as a function of probe size: large, smooth probes made their surroundings less glassy and this was evident not only in the motion of the surrounding particles but also in the translational dynamics of the probes themselves.
On the experimental side, we proposed SM experiments using a variety of probes including Rhodamine6G, terrylene, fluorescein, and pyrene embedded in the glass former o-terphenyl (OTP). While we had anticipated only moderate signal to noise in the wide field epi-fluorescence measurements we proposed, we did not anticipate that the signal to noise ratio would be as poor as it was. In part this high level of noise came from impurities in the OTP itself that we could not resolve with standard purification techniques. This problem simultaneously led us to devise a new way of pre-bleaching impurities in samples and also to move our studies to glycerol, another well studied glass former. Similarly, we found that the probes we first suggested were not ideal and for SM experiments, and we moved to three other probes: rubrene, Nile Red, and perylene diimide. To date, most of the SM experiments we have performed have been on rubrene in glycerol. We find that for this system the temperature dependence of the rotational constant of rubrene embedded in glycerol is consistent with that measured in both probe-bearing and probe-free ensemble experiments. We also find significant spatial heterogeneity in evidence, with single rubrene molecules exhibiting rotational dynamics that differ by up to three orders of magnitude at the same time in the same sample. This strongly suggests that rubrene, though it is quite large compared to a single glycerol molecule, feels at least a subset of heterogeneous dynamics displayed by the surrounding glycerol. Moreover, we find the breadth of heterogeneities increases with decreasing temperature, which is not consistent the principle of time-temperature superposition that some theories suggest exist in supercooled liquids. We also find that rubrene in glycerol exhibits dynamic heterogeneities. This is in contrast to the only other published study at the single molecule level of glycerol near Tg. We believe this discrepancy is due to different temperature histories of the glycerol. Moreover, we believe that the dynamic exchanges we measure are indeed due to sporadic cooperative rearrangements of small sets of molecules in supercooled glycerol. We have not yet determined the timescale of the dynamic exchanges relative to the average rotational correlation time of the glycerol molecules, but data currently being collected will reveal the magnitude and temperature dependence of this quantity. Because this quantity has been reported to be anywhere from ~1 and strongly temperature dependent to 106 and temperature independent, single molecule measurements with a variety of probes are expected to be crucial in resolving this discrepancy.
