Abstract
Many polymers exhibit much steeper temperature dependence of their structural relaxation time
(higher fragility) than liquids of small molecules, and the mechanism of this unusually high fragility
in polymers remains a puzzle. To reveal additional hints for understanding the underlying mechanism,
we analyzed correlation of many properties of polymers to their fragility on example of model polymer
polystyrene with various molecular weights (MWs).We demonstrate that these correlations work
for short chains (oligomers), but fail progressively with increase in MW. Our surprising discovery
is that the steepness of the temperature dependence (fragility) of the viscosity that is determined
by chain relaxation follows the correlations at all molecular weights. These results suggest that the
molecular level relaxation still follows the behavior usual for small molecules even in polymers,
and its fragility (chain fragility) falls in the range usual for molecular liquids. It is the segmental
relaxation that has this unusually high fragility. We speculate that many polymers cannot reach an
ergodic state on the time scale of segmental dynamics due to chain connectivity and rigidity. This
leads to sharper decrease in accessible configurational entropy upon cooling and results in steeper
temperature dependence of segmental relaxation. The proposed scenario provides a new important
insight into the specifics of polymer dynamics: the role of ergodicity time and length scale. At
the end, we suggest that a similar scenario can be applicable also to other molecular systems with
slow intra-molecular degrees of freedom and to chemically complex systems where the time scale
of chemical fluctuations can be longer than the time scale of structural relaxation.
