Abstract
The glass transition is a long-standing unsolved problem in materials science. For polymers, our understanding of glass formation is particularly poor because of the added complexity of chain connectivity and flexibility; structural relaxation of polymers thus involves a complex interplay between intramolecular and intermolecular cooperativity. Here, we study how the glass-transition temperature varies with molecular weight for different polymer chemistries and chain flexibilities. We find that is controlled by the average mass (or volume) per conformational degree of freedom and that a “local” molecular relaxation (involving a few conformers) controls the larger-scale cooperative relaxation responsible for . We propose that dynamic facilitation where a local relaxation facilitates adjacent relaxations, leading to hierarchical dynamics, can explain our observations, including logarithmic dependences. Our study provides a new understanding of molecular relaxations and the glass transition in polymers, which paves the way for predictive design of polymers based on monomer-scale metrics.
- Received 22 December 2021
- Revised 7 March 2022
- Accepted 28 March 2022
DOI:https://doi.org/10.1103/PhysRevX.12.021047
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
Polymer glasses pervade our lives, from consumer products to medical implants. However, the molecular mechanisms behind glass formation are hotly debated, and the links between the glass-transition temperature and the chemical structure, or molecular weight, are poorly understood. We demonstrate that this temperature-weight dependence follows a near-universal master curve, controlled by a characteristic mass that encodes local chain flexibility and bulkiness. Chain motions on this local scale facilitate larger-scale rearrangements, which in turn determine the glass-transition temperature. Our work expands glass-transition theories to incorporate cooperative motions both within and in between molecules, thus widening opportunities for predictive polymer design.
We use experiments and computer simulations to show that the master curve for glass-transition temperature as a function of molecular weight is parametrized by the mass involved in the smallest possible chain rearrangement and has three characteristic regimes, two of which follow a logarithmic relation. We explain this as an interplay between inter- and intramolecular chain motions: Local chain motions, on the monomer scale, couple to larger-scale motions via chain relaxations that enable neighboring regions (along the molecule or in space) to move. This allows mobility to spread, resulting in structural relaxation, and thus setting the transition temperature.
This new framework could pave the way for more effective predictions of polymer properties based on monomer structure. An understanding of the hierarchical nature of glassy dynamics could transform applications such as electrolyte membranes in a battery or gas-separation membranes, where chain relaxations aid the transport of ions and gases.
