Why many polymers are so fragile: A new perspective.

Area of Science:

• Polymer physics
• Materials science
• Physical chemistry

Background:

• Polymers exhibit higher fragility (steeper temperature dependence of structural relaxation time) than small molecules.
• The underlying mechanism for this high fragility in polymers is not well understood.

Purpose of the Study:

• To investigate the correlations between polymer properties and fragility.
• To understand the mechanism behind the unusually high fragility observed in polymers.

Main Methods:

• Analysis of correlations between various polymer properties and fragility.
• Study of model polymer polystyrene across different molecular weights (MWs).

Main Results:

• Correlations between properties and fragility hold for oligomers but fail for higher MW polymers.
• Viscosity fragility, determined by chain relaxation, follows normal correlations across all MWs.
• Segmental relaxation, not chain relaxation, exhibits unusually high fragility in polymers.

Conclusions:

• Polymer chain relaxation behaves similarly to small molecules, with normal fragility.
• Unusually high polymer fragility stems from segmental relaxation, potentially due to limited ergodicity caused by chain connectivity.
• This suggests a new perspective on polymer dynamics, highlighting the role of ergodicity time and length scale.