Oligosiloxane-Based Epoxy Vitrimers: Adaptable Thermosetting Networks with Dual Dynamic Bonds

Abstract

Embedding dynamic covalent bonds into polymer compositions transforms static thermosets into active materials with the reprocessability of thermoplastics and the bulk properties of cross-linked networks. This class of next-generation materials, called covalent adaptable networks, shows significant promise in composites, soft optoelectronics, and robotics. Herein, we synthesized two oligosiloxane-based epoxy networks that provide fast dynamic bond exchange. Oligosiloxane diepoxides were cured with stoichiometric amounts of 1,2-phenylenediacetic acid to generate epoxy acid networks with two dynamic covalent bonding mechanisms. The resulting polymer networks provided access to fast stress-relaxation times (1−10 min) at temperatures of only 130 °C with excellent reprocessability.

Efficiency and Mechanism of Catalytic Siloxane Exchange in Vitrimer Polymers: Modeling and Density Functional Theory Investigations

“…Indeed, the ultimate energy values obtained from CCSD­(T) calculations stand at 18.56 kcal/mol in the gas phase and 17.12 kcal/mol in the pentanol solvent. Remarkably, these values match closely to the determined experimental values as well as the energy barrier (∼19 kcal/mol) observed in the earlier experimental investigations. , Prior studies have shown that TBD is an effective catalyst for many different types of transesterification processes, including vitrification, ring-opening polymerization, and depolymerization of aromatic esters. ,− It is also hypothesized that the TBD molecule’s bifunctionality affects the RC development . The nitrogen atom gives electron density as a nucleophile, while the hydrogen atom receives electron density as an electrophilic group.…”

“…In our previous work, we utilized a two-element Maxwell model to determine the activation energies for the fast and slow modes of 15.7 and 16.6 kcal/mol for Si2-PDAA and 21.9 and 22.9 kcal/mol for Si8-PDAA vitrimer networks . The results are consistent with previously demonstrated siloxane-containing vitrimer networks catalyzed by TBD. , While these values correspond to siloxane exchange and transesterification occurring within the material, determining which activation energy corresponds to which dynamic bond mechanism was not feasible.…”

“…This groundbreaking work demonstrates the synthesis of vitrimers commencing from a readily accessible amine hardener with siloxane molecules, specifically 1,3-bis­(3-aminopropyl)-1,1,3,3-tetramethyl disiloxane (BAS), in conjunction with bisphenol A diglycidyl ether (DGEBA) epoxy resin. In a more recent study, we reported the synthesis of two siloxane-containing vitrimer networks by combining oligosiloxanes with a dicarboxylic acid in the presence of the TBD catalyst, to create networks with siloxanes and esters, as depicted in Scheme . In this study, we calculated the dynamic bond activation energies using traditional stress-relaxation experimentations and the highest occupied molecular orbital–lowest unoccupied molecular orbital (HOMO–LUMO) energies of compounds mimicking network substructures …”

“…In a more recent study, we reported the synthesis of two siloxane-containing vitrimer networks by combining oligosiloxanes with a dicarboxylic acid in the presence of the TBD catalyst, to create networks with siloxanes and esters, as depicted in Scheme . In this study, we calculated the dynamic bond activation energies using traditional stress-relaxation experimentations and the highest occupied molecular orbital–lowest unoccupied molecular orbital (HOMO–LUMO) energies of compounds mimicking network substructures …”