Reprocessable Cross-Linked Polymer Networks: Are Associative Exchange Mechanisms Desirable?

Abstract

Covalent adaptable networks (CANs) are covalently cross-linked polymers that may be reshaped via cross-linking and/or strand exchange at elevated temperatures. They represent an exciting and rapidly developing frontier in polymer science for their potential as stimuli-responsive materials and to make traditionally nonrecyclable thermosets more sustainable. CANs whose cross-links undergo exchange via associative intermediates rather than dissociating to separate reactive groups are termed vitrimers. Vitrimers were postulated to be an attractive subset of CANs, because associative cross-link exchange mechanisms maintain the original cross-link density of the network throughout the exchange process. As a result, associative CANs demonstrate a gradual, Arrhenius-like reduction in viscosity at elevated temperatures while maintaining mechanical integrity. In contrast, CANs reprocessed by dissociation and reformation of cross-links have been postulated to exhibit a more rapid decrease in viscosity with increasing temperature. Here, we survey the stress relaxation behavior of all dissociative CANs for which variable temperature stress relaxation or viscosity data are reported to date. All exhibit an Arrhenius relationship between temperature and viscosity, as only a small percentage of the cross-links are broken instantaneously under typical reprocessing conditions. As such, dissociative and associative CANs show nearly identical reprocessing behavior over broad temperature ranges typically used for reprocessing. Given that the term vitrimer was coined to highlight an Arrhenius relationship between viscosity and temperature, in analogy to vitreous glasses, we discourage its continued use to describe associative CANs. The realization that the cross-link exchange mechanism does not greatly influence the practical reprocessing behavior of most CANs suggests that exchange chemistries can be considered with fewer constraints, focusing instead on their activation parameters, synthetic convenience, and application-specific considerations.

Molecular control over vitrimer-like mechanics – tuneable dynamic motifs based on the Hammett equation in polyimine materials

“…This observation is suggestive of an associative exchange mechanism, as a dissociative mechanism would typically result in a decrease in the crosslinking density, 85 although some dissociative CANs have been shown to display very similar vitrimer-like behaviour as common associative CANs, [86][87][88] which has raised the question whether the term vitrimer should be used selectively for associative CANs. 89 Full conversion of the polymer network was further concluded from FT-IR spectroscopy (Fig. S8 †), as disappearance of the aldehyde signal at 1700 cm À1 and concomitant appearance of the imine signal at 1640 cm À1 could be observed.…”

Recyclable Organocatalyzed Poly(Thiourethane) Covalent Adaptable Networks

“…Coefficients of variations are less than 7% for stress-and-strain results and less than 5% for the tensile moduli. 4 Temperature of the maximum of the tan δ peak. 5 Full width at half maximum of the tan δ peak.…”

“…In view of the increasing amount of plastic generated, the recycling of this type of polymers is extremely important to prevent them from ending up in landfills and contaminating the environment. The development of covalent adaptable networks (CANs), covalently crosslinked polymers with the ability to be reshaped, to flow and to self-repair, represents a promising approach to improve the lifetime and recyclability of the thermosetting polymers [1][2][3][4].…”

Dynamic Semi IPNs with Duple Dynamic Linkers: Self-Healing, Reprocessing, Welding, and Shape Memory Behaviors

“…Besides, according to the DMTA plots in Figure 7 c, the 6%CPSFTPU 1 sample shows a continually decreased storage modulus, as the temperature increased from 25 to 200 °C due to the presence of dissociated Diels−Alder adducts. Whereas, DMTA plots also displayed a rubbery plateau at 90~170 °C before network collapse, which is similar with associative vitrimers [ 55 ]. Consequently, semi IPNs should involve the reversible mechanism both associative and dissociated.…”