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Fragility is an empirical property that describes how abruptly aglass-forming material solidifies upon supercooling. The degree of fragilitycarries important implications for the functionality and processability of amaterial, as well as for our fundamental understanding of the glass transition.However, the microstructural properties underlying fragility still remainpoorly understood. Here, we explain the microstructure-fragility link invitrimeric networks, a novel type of high-performance polymers with uniquebond-swapping functionality and unusual glass-forming behavior. Our results aregained from coarse-grained computer simulations and first-principlesMode-Coupling Theory (MCT) of star-polymer vitrimers. We first demonstrate thatthe vitrimer fragility can be tuned over an unprecedentedly broad range, fromfragile to strong and even superstrong behavior, by decreasing the bulkdensity. Remarkably, this entire phenomenology can be reproduced by microscopicMCT, thus challenging the conventional belief that existing first-principlestheories cannot account for non-fragile behaviors. Our MCT analysis allows usto rationally identify the microstructural origin of the fragile-to-superstrongcrossover, which is rooted in the sensitivity of the static structure factor totemperature variations. On the molecular scale, this behavior stems from achange in dominant length scales, switching from repulsive excluded-volumeinteractions to intrachain attractions as the vitrimer density decreases.Finally, we develop a simplified schematic MCT model which corroborates ourmicroscopically-founded conclusions and which unites our findings with earlierMCT studies. Our work sheds new light on the elusive structure-fragility linkin glass-forming matter, and provides a first-principles-based platform fordesigning novel amorphous materials with an on-demand dynamic response.

Understanding, predicting, and tuning the fragility of vitrimeric polymers