Which probes can report intrinsic dynamic heterogeneity of a glass forming liquid?

Abstract

Using extrinsic probes to study a host system relies on the probes' ability to accurately report the host properties under study. Probes have long been used to characterize dynamic heterogeneity, the phenomenon in which a liquid near its glass transition exhibits distinct dynamics as a function of time and position, with molecules within nanometers of each other exhibiting dynamics that may vary by orders of magnitude. The spatial and temporal characteristics of dynamic heterogeneity demand the selection of probes using stringent criteria on their size and dynamics. In this report, we study the dynamic heterogeneity of the prototypical molecular glass former o-terphenyl by investigating single molecule rotation of two perylene dicarboximide probe molecules that differ in size and comparing this to results obtained previously with the probe BODIPY268. It is found that a probe's ability to accurately report dynamic heterogeneity in o-terphenyl depends on whether the reported distribution of dynamics overlaps with the intrinsic dynamics of the host, which is naturally related to the width of the intrinsic dynamics and the magnitude of dynamical shift in probe dynamics relative to the host. We show that a probe that rotates ≈15 times more slowly than the intrinsic dynamics of the host o-terphenyl senses the slowest ≈5% of the full dynamic heterogeneity whereas one that rotates ≈65 times more slowly than the host fails to report dynamic heterogeneity of the host. Published by AIP Publishing. https://doi.

How Tethered Probes Report the Dynamics of a Polymer near the Glass Transition

“…However, unlike PMMA-6-PDI-6-PMMA where the β value at T g and its temperature dependence are comparable to the free probe of 5-PDI-5, only dβ/dT of tbPDI was comparable. The β value at T g reported by tbPDI was significantly increased from ≈0.35 to 0.58 which indicates the inherent time distribution became narrower since the β value is directly related to the width the distribution by inverse Laplace transform where the distribution become narrower as the β value approaches the unity. , These results are similar to the same free probes in different matrices. , In polystyrene, tbPDI rotates ≈1 decade slower than the 5-PDI-5 with the same temperature dependence of dynamics, whereas the β value changes from ≈0.50 to ≈0.85 …”

“…51,54 These results are similar to the same free probes in different matrices. 34,55 In polystyrene, tbPDI rotates ≈1 decade slower than the 5-PDI-5 with the same temperature dependence of dynamics, whereas the β value changes from ≈0.50 to ≈0.85. 34 The mechanisms of how probes report the dynamics of host polymer are depicted in Figure 4.…”

Glass Dynamics Deep in the Energy Landscape

“…We then go on to describe how the RFOT theory illuminates these experiments and suggests new possibilities for laboratory investigation. While there are other promising new experiments 14 and a range of theoretical approaches for describing glasses, for this Perspective we have restricted our attention, attempting to provide a short and coherent overview.…”

Environmentally Sensitive Luminescence Reveals Spatial Confinement, Dynamics, and Their Molecular Weight Dependence in a Polymer Glass

“…Historically, one of the first practices in the field employed probes for fluorescence spectroscopy correlation studies. By using these fluorophores, it was possible to track molecular mobility and dynamic heterogeneity in polymer glasses − and characterize chain dynamics and local viscosity . These probes largely acted as stable light sources allowing to track their transport, diffusion, and emission anisotropy.…”