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Title: Why many polymers are so fragile: A new perspective

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). Here, 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 also 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. Our proposed scenario providesmore » 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.« less
 [1] ;  [2] ;  [3] ;  [4] ;  [5] ;  [5] ;  [1] ;  [6] ;  [2] ;  [7]
  1. National Centre for Scientific Research-Mixed Organizations (CNRS-UMR), Paris (France)
  2. Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States). Chemical Sciences Division
  3. Shanghai Jiao Tong Univ. (China). Inst. of Natural Sciences and Dept. of Physics and Astronomy
  4. Inst. for the Chemical and Physical Processes (IPCF), Messina (Italy)
  5. Univ. of Messina (Italy). Dept. of Physics and Earth Science
  6. Univ. of Tennessee, Knoxville, TN (United States). Dept. of Chemistry and Joint Inst. for Neutron Sciences
  7. (United States). Dept. of Chemistry and Joint Inst. for Neutron Sciences
Publication Date:
Grant/Contract Number:
AC05-00OR22725; DMR-1408811
Accepted Manuscript
Journal Name:
Journal of Chemical Physics
Additional Journal Information:
Journal Volume: 145; Journal Issue: 15; Journal ID: ISSN 0021-9606
American Institute of Physics (AIP)
Research Org:
Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
Sponsoring Org:
USDOE Office of Science (SC); National Science Foundation (NSF)
Country of Publication:
United States
OSTI Identifier: