Understanding, predicting, and tuning the fragility of vitrimeric polymers

Abstract

Fragility is an empirical property that describes how abruptly a glassforming material solidifies upon supercooling. The degree of fragility carries important implications for the functionality and processability of a material, as well as for our fundamental understanding of the glass transition. However, the microstructural properties underlying fragility still remain poorly understood. Here, we explain the microstructure-fragility link in vitrimeric networks, a novel type of high-performance polymers with unique bond-swapping functionality and unusual glass-forming behavior. Our results are gained from coarse-grained computer simulations and first-principles Mode-Coupling Theory (MCT) of star-polymer vitrimers. We first demonstrate that the vitrimer fragility can be tuned over an unprecedentedly broad range, from fragile to strong and even superstrong behavior, by decreasing the bulk density. Remarkably, this entire phenomenology can be reproduced by microscopic MCT, thus challenging the conventional belief that existing first-principles theories cannot account for non-fragile behaviors. Our MCT analysis allows us to rationally identify the microstructural origin of the fragile-tosuperstrong crossover, which is rooted in the sensitivity of the static structure factor to temperature variations. On the molecular scale, this behavior stems from a change in dominant length scales, switching from repulsive excluded-volume interactions to intrachain attractions as the vitrimer density decreases. Finally, we develop a simplified schematic MCT model which corroborates our microscopically-founded conclusions and which unites our findings with earlier MCT studies. Our work sheds new light on the elusive structure-fragility link in glass-forming matter, and provides a firstprinciples-based platform for designing novel amorphous materials with an on-demand dynamic response.Keyword 1 | Keyword 2 | Keyword 3 | ... Significance StatementThe intricate interplay between the molecular structure of a material and its emergent macroscopic dynamics lies at the heart of modern materials science. This interplay, however, remains notoriously poorly understood for glass-forming systems. The recent advent of vitrimers-a promising new class of recyclable high-performance polymers with anomalous glassforming properties -revives the urgency to gain a full understanding of the elusive structure-dynamics link in glassy matter. We combine computer simulations and first-principles theory to rationally expose the structural origins of the complex glassy dynamics in vitrimers. Our findings provide cues for a better fundamental understanding of glass formation, and may stimulate the development of novel, sustainable amorphous materials with on-demand functionalities.

Glassy dynamics from generalized mode-coupling theory: existence and uniqueness of solutions for hierarchically coupled integro-differential equations

“…Our results are fundamental for present and future studies that investigate the GMCT solutions often numerically, when analytical results are not available, as a means to study e.g. fragility and tunability of liquid models in schematic GMCT [14,19,25]. Existence and uniqueness of solutions is then essential.…”

Integral equation theory of thermodynamics, pair structure, and growing static length scale in metastable hard sphere and Weeks-Chandler-Andersen fluids

“…Our new results are relevant to constructing more accurate theories of the equilibrium behavior of ultra-dense hard sphere fluids, density functional theories of crystallization [27] and interfaces [28], phase separation of hard sphere mixtures [29,30] and polymer-colloid mixtures [31,32], etc. But our prime motivation is its relevance as input to the dynamical theories mentioned above, which to date have most often used the PY closure [10,14,[33][34][35]. Extent microscopic theories of glassy dynamics typically require to high accuracy the Fourier space static structure factor, ( ), or the direct correlation function, ( ), including at very large wavevectors not easy to accurately determine with simulation.…”

Fragile Glasses Associated with a Dramatic Drop of Entropy under Supercooling

“…Glasses possessing the most dramatic slowdown are classified as fragile, whereas the opposite are referred to as strong. Several models of glasses have been able to reproduce a range of kinetic fragilities [12][13][14][15][16]. A closely related thermodynamic fragility [17] has also been defined and is based on how dramatically the entropy drops as the temperature decreases.…”