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.

Orientational Mapping Augmented Sub-Wavelength Hyper-Spectral Imaging of Silk

“…A systematic study on orientational properties of the building blocks of the crystalline-amorphous hierarchial structure of single silk fibers which is essential to understanding the properties of silk, e.g., why a faster reeling makes stronger fibers 15 , 16 and how it is linked to fragility and relaxation in polymers 17 is highly required. Structure of single spider silk fibers was investigated by XRD including changes due to water uptake 18 , 19 .…”

Fragility of short‐chain poly(lactic acid)s derivatives by combining dielectric spectroscopy and fast scanning calorimetry

“…The dependence of the segmental dynamics on the chain length has been the focus of several studies. [20][21][22][23][24] A correlation between the fragility index and the chain length is frequently observed. At the opposite, some systems show no dependence of the fragility index on the molecular weight.…”

“…In consistence with previous studies, 5 the PDLLA exhibits a fragile behavior ( m = 155), while the oligomers exhibit lower m , which decreases from 99 (OLA2700) down to 68 (OLA1400) with the M w decrease. The dependence of the segmental dynamics on the chain length has been the focus of several studies 20–24 . A correlation between the fragility index and the chain length is frequently observed.…”

Universality of steady shear flow of Rouse melts

“…The CM (Ngai 1979, Ngai andWhite 1979) is an approach to relate the dynamical relationship between systems, and it was used to express correlations between different segmental chain dynamics processes with different temperature-dependent shift factors. Plazek and Ngai (1991) showed that the coupling parameter n α increases with the steepness index, S. The steepness index or fragility index m of a polymer is the characterisation of the temperature-dependent gradient of viscosity or structural relaxation time near the glass transition temperature T g and the degree to which the system is non-Arrhenius (Angell 1985;Dalle-Ferrier et al 2016). Santangelo and Roland (1998) demonstrated through a series of dynamic mechanical and calorimetric measurements of polystyrene that T g and the fragility of PS increase with increasing molecular weight.…”