title: Why many polymers are so fragile: A new perspective.
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.
authors: Laboratoire Léon Brillouin, UMR 12, CEA-CNRS, 91191 Saclay, France.
authors: C
authors: C
authors: Dalle-Ferrier
authors: Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA.
authors: A
authors: A
authors: Kisliuk
authors: Institute of Natural Sciences & Department of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai 200240, China.
authors: L
authors: L
authors: Hong
authors: IPCF del CNR, UOS di Messina, I-98158 Messina, Italy.
authors: G
authors: G
authors: Carini
authors: Dipartimento di Fisica e Scienze della Terra, Università di Messina, I-98166 Messina, Italy.
authors: G
authors: G
authors: Carini
authors: Dipartimento di Fisica e Scienze della Terra, Università di Messina, I-98166 Messina, Italy.
authors: G
authors: G
authors: D'Angelo
authors: Laboratoire Léon Brillouin, UMR 12, CEA-CNRS, 91191 Saclay, France.
authors: C
authors: C
authors: Alba-Simionesco
authors: Department of Chemistry and Joint Institute for Neutron Sciences, University of Tennessee, Knoxville, Tennessee 37996, USA.
authors: V N
authors: VN
authors: Novikov
authors: Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA.
authors: A P
authors: AP
authors: Sokolov