Fragile Glass Formation and Non-Arrhenius Upturns in Ethylene Vitrimers Revealed by Dielectric Spectroscopy

Abstract

Vitrimers, dynamic networks with bonds that exchange without breaking, are an emerging class of reprocessable and recyclable polymers. The dynamics in such materials are complex and span from a single bond exchange or alpha relaxation event up to bulk flow. Most prior work has focused on investigations of stress relaxation times or creep experiments, but little has been pursued to investigate more local dynamics over a wide range of temperatures. A series of precise ethylene vitrimers are synthesized with four to seven carbons between dynamic bonds, and broadband dielectric spectroscopy is used to probe the segmental dynamics. Three distinct modes are identified in the dielectric spectra�an alpha process, beta process, and a normal mode assigned to strand motion in the network between dynamic bonds. The last mode corresponds within error to the rheological crossover time, indicating that this process is responsible for bulk flow at higher temperatures. At lower temperatures, approaching the glass transition causes a positive deviation of the crossover time from Arrhenius behavior in the networks at roughly the same distance above T g . Finally, we analyze our networks in the context of a previously developed theory for bond dissociation in associating polymers and find evidence that the non-Arrhenius behavior reflects strong decoupling of the bond exchange barrier crossing event with the segmental or alpha relaxation. This implies the bond exchange event that conserves dynamic cross-link density experiences a local frictional resistance due to the surrounding polymer matrix that is smaller and much less temperature dependent than the primary structural relaxation process, and to a larger degree than observed in most associating copolymer melts where physical bond breaking is a dissociative process.

Time–Temperature Superposition of Polybutadiene Vitrimers

“…For measurements collected over wide time or frequency ranges, however, the short-and long-time dynamics do not superpose into a single master curve. 39,51,53,54 As evidenced by the different shift factors for SAOS and creep experiments in this study, the thermorheological behavior of PB vitrimers agrees with these trends.…”

“…Another transient network theory is the model of Ghosh and Schweizer (GS), in which local frictional resistance of the backbone segments attenuates the overall temperature dependence of the network relaxation. 54,80 Combining the RBLM and GS models, we develop alternative forms of the cross-link mobility and long-time dynamics activation energy (σ GS and E a GS )…”

Polymeric Dynamic Crosslinker for Upcycling of Fragile Low-Molecular-Weight Polypropylene

“…Specifically, vitrimers are an emerging class of crosslinked materials with unique reprocessability, enabled by the associative dynamic bond-exchange mechanism, which can retain the same crosslinking density at elevated reprocessing temperatures. Over the past few years, a number of vitrimer chemistries have been demonstrated with polyolefins, such as transesterification, , disulfide crosslinking, , imine exchange, , thio-anhydride, silyl ether exchange, , and dioxaborolane metathesis; − most rely on reactive extrusion to simultaneously process and functionalize plastics. A recent review article by Ahmadi et al, systematically discussed recent progress in polyolefin-based vitrimers …”

Time-temperature superposition of polybutadiene vitrimers

“…Another transient network theory is the model of Ghosh and Schweizer (GS), in which local frictional resistance of the backbone segments attenuates the overall temperature dependence of the network relaxation. 54,79 Combining the RBLM and GS models, we can develop alternative forms of the cross-link mobility and long time dynamics activation energy (𝜎 𝐺𝑆 and 𝐸 𝑎 𝐺𝑆 )…”

“…41,42,43,44,45,46,47,48,49,50 The temperature dependence over a wide range cannot be described by a single activation energy. 48,51,52,53,54 Responding to these observations, experimental, computational, and theoretical efforts over the past five years have focused on illuminating the relationship between vitrimer viscoelasticity and temperature. Generally, the relaxation rate expresses two different temperature regimes, where the estimated activation energy of the low temperature regime is less than that of the high temperature regime.…”