Dynamics of Vitrimers: Defects as a Highway to Stress Relaxation

Ciarella, Simone; Sciortino, Francesco; Ellenbroek, Wouter G.

doi:10.1103/physrevlett.121.058003

Cited by 83 publications

(141 citation statements)

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“…2(e). This effect is not surprising, as enhanced vitrimeric functionality is known to lead to more efficient relaxation and more liquid-like behavior D R A F T (37,51). Interestingly, we find that the fragility index m along the glass transition line is virtually unaffected by our choice of ∆Eswap ( Fig.…”

Section: Resultsmentioning

confidence: 53%

“…Compared to Ref. (37), the model was optimized in order to avoid its rigidification at the topological transition Tv (3) by changing the mixture ratios to reduce the number of intra-star bonds. In this condition, the mechanism driving the dynamical arrest is the standard glass transition, so that the system can relax and reach equilibrium even below Tv.…”

Section: Methodsmentioning

confidence: 99%

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Understanding, predicting, and tuning the fragility of vitrimeric polymers

Ciarella

Biezemans

Janssen

2019

Proc. Natl. Acad. Sci. U.S.A.

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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.

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Section: Resultsmentioning

confidence: 53%

Section: Methodsmentioning

confidence: 99%

Understanding, predicting, and tuning the fragility of vitrimeric polymers

Ciarella

Biezemans

Janssen

2019

Proc. Natl. Acad. Sci. U.S.A.

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“…Analogous studies aimed at understanding the effect of crosslink kinetics on viscoelastic material responses have been performed for a number of synthetic polymeric materials. Besides investigating the effect of the crosslink properties themselves (Yount et al, 2005;Shen et al, 2007;Appel et al, 2014;Rossow et al, 2014;Grindy et al, 2015;Tunn et al, 2018), these studies have focused on the contributions of network defects such as dangling ends and loops (Annable et al, 1993;Rossow et al, 2014;Ciarella et al, 2018), crosslink functionality (Li et al, 2016;Gu et al, 2018;Tunn et al, 2019) and polymer length (Annable et al, 1993;Tan et al, 2017). Even though a direct comparison is difficult due to the different polymers used, it can generally be concluded that the number of elastically active chains and their ability to relax after crosslink dissociation are key parameters that determine the macroscopic relaxation time of a material.…”

Section: Introductionmentioning

confidence: 99%

Influence of Network Topology on the Viscoelastic Properties of Dynamically Crosslinked Hydrogels

Grad¹,

Tunn²,

Voerman³

et al. 2020

Preprint

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Biological materials combine stress relaxation and self-healing with non-linear stress-strain responses. These characteristic features are a direct result of hierarchical self-assembly, which often results in fiber-like architectures. Even though structural knowledge is rapidly increasing, it has remained a challenge to establish relationships between microscopic and macroscopic structure and function. Here, we focus on understanding how network topology determines the viscoelastic properties, i.e. stress relaxation, of biomimetic hydrogels. We have dynamically crosslinked two different synthetic polymers with one and the same crosslink. The first polymer, a polyisocyanopeptide (PIC), self-assembles into semi-flexible, fiber-like bundles and thus displays stress-stiffening, similar to many biopolymer networks. The second polymer, 4-arm poly(ethylene glycol) (starPEG), serves as a reference network with well-characterized structural and viscoelastic properties. Using one and the same coiled coil crosslink allows us to decouple the effects of crosslink kinetics and network topology on the stress relaxation behavior of the resulting hydrogel networks. We show that the fiber-containing PIC network displays a relaxation time approximately two orders of magnitude slower than the starPEG network. This reveals that crosslink kinetics is not the only determinant for stress relaxation. Instead, we propose that the different network topologies determine the ability of elastically active network chains to relax stress. In the starPEG network, each elastically active chain contains exactly one crosslink. In the absence of entanglements, crosslink dissociation thus relaxes the entire chain. In contrast, each polymer is crosslinked to the fiber bundle in multiple positions in the PIC hydrogel. The dissociation of a single crosslink is thus not sufficient for chain relaxation. This suggests that tuning the number of crosslinks per elastically active chain in combination with crosslink kinetics is a powerful design principle for tuning stress relaxation in polymeric materials. The presence of a higher number of crosslinks per elastically active chain thus yields materials with a slow macroscopic relaxation time but fast dynamics at the microscopic level. Using this principle for the design of synthetic cell culture matrices will yield materials with excellent long-term stability combined with the ability to locally reorganize, thus facilitating cell motility, spreading and growth.

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“…5,6,23,24,25,26 Theoretical models have been developed to clarify the relationship between the exchange reaction kinetics and resulting vitrimer material characteristics. 27,28,29,30,31,32 While much of the vitrimer literature focuses on optimizing exchange reactions, diversifying polymer backbones, and applications, 33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49 less attention has been directed towards exploring incompatibility effects. This question should be particularly important in vitrimer systems, for example, in which the crosslink functional groups and network strands have very different polarizabilities.…”

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confidence: 99%

Phase Separation and Self-Assembly in Vitrimers: Hierarchical Morphology of Molten and Semicrystalline Polyethylene/Dioxaborolane Maleimide Systems

2018

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Vitrimers -a class of polymer networks which are covalently crosslinked and insoluble like thermosets, but flow when heated like thermoplastics -contain dynamic links and/or crosslinks that undergo an associative exchange reaction. These dynamic crosslinks enable vitrimers to have interesting mechanical/rheological behavior, self-healing, adhesive, and shape memory properties.We demonstrate that vitrimers can self-assemble into complex meso-and nanostructures when crosslinks and backbone monomers strongly interact. Vitrimers featuring polyethylene (PE) as the backbone and dioxaborolane maleimide as the crosslinkable moiety were studied in both the molten and semi-crystalline states. We observed that PE vitrimers macroscopically phase separated into dioxaborolane maleimide rich and poor regions, and characterized the extent of phase separation by optical transmission measurements. This phase separation can explain the relatively low insoluble fractions and overall crystallinities of PE vitrimers. Using synchrotronsourced small-angle X-ray scattering (SAXS), we discovered that PE vitrimers and their linear precursors micro-phase separated into hierarchical nanostructures. Fitting of the SAXS patterns to a scattering model strongly suggests that the nanostructures -which persist in both the melt and amorphous fraction of the semi-crystalline state -may be described as dioxaborolane maleimide rich aggregates packed in a mass fractal arrangement. These findings of hierarchical meso-and nanostructures point out that incompatibility effects between network components and resulting self-assembly must be considered for understanding behavior and the rational design of vitrimer materials.Thermoplastics and thermoplastic elastomers are soluble in good solvents and flow when heated above the glass or melting temperature. Thermosets and rubbers do not flow and are not soluble.Vitrimers, a class of polymers introduced by Leibler and collaborators in 2011, flow when heated, but remain insoluble. 1 Vitrimers are made of polymer networks which contain covalent crosslinks that undergo dynamic associative exchange reactions. The covalent crosslinks in a vitrimer maintain network connectivity at all times and temperatures. Unlike materials employing dissociative crosslinking mechanisms, 2 vitrimers cannot be completely dissolved -even in good solvents. 1,3 Associative exchange reactions permit the network topology to fluctuate and the system to flow when stress is applied, and exchange reaction kinetics control the vitrimer relaxation dynamics and viscosity. 2,4,5,6 The initial reports of vitrimer systems focused on epoxy networks that reorganized via metal-catalyzed transesterification. 1,4,6 Today, the library of dynamic exchange reactions has expanded to include chemistries that are catalytically-controlled (olefin metathesis and transcarbonation) 7,8 or catalyst-free (transamination, 9,10,11 trans-N-alkylation, 12,13 reversible addition of thiols, 14 imine exchange, 15,16 addition-fragmentation chain transfer, 17,18 boronic est...

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Dynamics of Vitrimers: Defects as a Highway to Stress Relaxation

Cited by 83 publications

References 39 publications

Understanding, predicting, and tuning the fragility of vitrimeric polymers

Understanding, predicting, and tuning the fragility of vitrimeric polymers

Influence of Network Topology on the Viscoelastic Properties of Dynamically Crosslinked Hydrogels

Phase Separation and Self-Assembly in Vitrimers: Hierarchical Morphology of Molten and Semicrystalline Polyethylene/Dioxaborolane Maleimide Systems

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