Entropy-controlled cross-linking in linker-mediated vitrimers

Abstract

Recently developed linker-mediated vitrimers based on metathesis of dioxaborolanes with various commercially available polymers have shown both good processability and outstanding performance, such as mechanical, thermal, and chemical resistance, suggesting new ways of processing cross-linked polymers in industry, of which the design principle remains unknown [M. Röttger et al., Science 356, 62–65 (2017)]. Here we formulate a theoretical framework to elucidate the phase behavior of the linker-mediated vitrimers, in which entropy plays a governing role. We find that, with increasing the linker concentration, vitrimers undergo a reentrant gel–sol transition, which explains a recent experiment [S. Wu, H. Yang, S. Huang, Q. Chen, Macromolecules 53, 1180–1190 (2020)]. More intriguingly, at the low temperature limit, the linker concentration still determines the cross-linking degree of the vitrimers, which originates from the competition between the conformational entropy of polymers and the translational entropy of linkers. Our theoretical predictions agree quantitatively with computer simulations, and offer guidelines in understanding and controlling the properties of this newly developed vitrimer system.

Effect of bond exchange rate on dynamics and mechanics of vitrimers

“…50 Lei et al, proposed a new simulation framework for predicting phase behavior of vitrimers created by the metathesis reaction of dioxaborolanes where they found that the mechanical properties of the vitrimer in the low-temperature regime are controlled by the concentration of the free linker. 51 Molecular dynamics simulations in conjunction with mode-coupling theory (MCT) have also been used to understand and predict the glass behavior of vitrimeric polymers. 52 More recently, we proposed a simulation methodology that utilizes coarse-grained MD in conjunction with a Monte Carlo (MC) method to capture the associative bond exchange in vitrimers.…”

“…Later on, with the aid of a coarse‐grained model, Ciarella et al, demonstrated that the existence of defects in the model vitrimer facilitates the bond exchange and eventually accelerates stress relaxation in the network 50 . Lei et al, proposed a new simulation framework for predicting phase behavior of vitrimers created by the metathesis reaction of dioxaborolanes where they found that the mechanical properties of the vitrimer in the low‐temperature regime are controlled by the concentration of the free linker 51 . Molecular dynamics simulations in conjunction with mode‐coupling theory (MCT) have also been used to understand and predict the glass behavior of vitrimeric polymers 52 .…”

Entropy driven thermo-gelling vitrimer

“…S1c). This non-monotonic dependence originates from an entropic effect recently found in linker-mediated vitrimers [34,37,38]. Moreover, the position and height of f P2C can be also tuned by changing the concentration of B and D molecules, i.e., βµ B and βµ D (Fig.…”

“…For the second reaction in Fig. 1a, we double the forward reaction flux to equalize the flux between the two states before and after the reaction [34]. Furthermore, for mimicking the actual topology and reaction dynamics of vitrimers, i.e., the covalent bondexchange dynamics in Fig.…”

“…The rate of these reactions is governed by temperature, and some of them, e.g., silyl ether metathesis [28], thio-disulfide exchange reaction [29,30], dynamic Schiff base [31,32], are biocompatible and applicable in vivo. In this work, based on a recently developed linker-mediated vitrimer using metathesis reactions [33,34], we propose a new reversible thermo-gelling polymer. The polymer remains as an un-crosslinked liquid at low temperature and solidifies to a covalently crosslinked elastomer with increasing temperature.…”