Liquids near their glass transition are supercool-ed! Investigating the dynamic heterogeneities of supercooled glycerol using single molecule microscopy

Amorphous systems lacking long range order in the temperature range between the melting temperature and glass transition temperature (Tg) are known as supercooled liquids. Many different types of substances including metals, organic molecules, and polymers exhibit striking similarities in the supercooled regime such as nonexponential relaxation. It is assumed that this dynamic behavior is caused by spatial and temporal heterogeneities. Spatial heterogeneity refers to distinct regions in the supercooled liquid exhibiting exponential relaxations with time constants differing significantly from their neighboring regions, and temporal heterogeneity refers to alterations of dynamics of a spatial region or the whole system in time. To help distinguish between these types of dynamic heterogeneity, it becomes necessary to study such systems on the single molecule level. Using wide field single molecule fluorescence microscopy, we have studied glycerol embedded with perylene diimide probes near its Tg. Temperatures ranging from 198K (1.04Tg) –212K (1.12Tg) are investigated to observe the effects of probe size and the probe-host interactions. Rotational relaxation times of each probe are measured across the above temperature range. Surprisingly, the largest PDI probe rotates the fastest, and reports the greatest breadth of heterogeneity across all temperatures probed. Temporal heterogeneities in glycerol near Tg are also studied and analyzed using two complementary methods. In contrast to previous studies, exchange was observed on the shortest time scales probed. Taken together, our data indicates that exchange times found via these analyses are upper bounds on the time scale of dynamic exchange in the system.