Stéphanie Engelen

Aromatic esters are amongst the oldest known chemical motifs that allow for thermal (re)processing of thermosetting polymers. Moreover, phenyl esters are generally known as activated esters that do not require a catalyst to undergo acyl transfer reactions. Even though dynamic aromatic esters find applications in commercialized thermoset formulations, all‐aromatic esters have found limited use so far in the design of covalent adaptable networks (CAN) as a result of their high glass transition temperature (T_g) and specific curing process. Here, a strategy to include partly aromatic esters as dynamic cross‐links inside low T_g (−40 °C) thermosetting formulations, using aliphatic esters derived from para‐hydroxybenzoic acid, which serves as a highly activated phenol or as a reactive “phenylogous anhydride” is reported. A small molecule study shows that the activated phenyl ester bonds can readily exchange with free phenol moieties at 200 °C under catalyst‐free conditions, while the addition of a catalyst allows for a faster exchange. Robust and hydrophobic polymer networks are conveniently prepared via rapid thiol‐ene UV‐curing of unsaturated phenol esters. The obtained networks show high thermal stability (350 °C), fast processability, good water resistance, and low creep up to 120 °C, thus showing good promise as a platform for CAN.

Vinylogous urethane (VU_O) based polymer networks are widely used as catalyst‐free vitrimers that show rapid covalent bond exchange at elevated temperatures. In solution, vinylogous ureas (VU_N) undergo much faster bond exchange than VU_O and are highly dynamic at room temperature. However, this difference in reactivity is not observed in their respective dynamic polymer networks, as VU_O and VU_N vitrimers prepared herein with very similar macromolecular architectures show comparable stress relaxation and creep behavior. However, by using mixtures of VU_O and VU_N linkages within the same network, the dynamic reactions can be accelerated by an order of magnitude. The results can be rationalized by the effect of intermolecular hydrogen bonding, which is absent in VU_O vitrimers, but is very pronounced for vinylogous urea moieties. At low concentrations of VU_N, these hydrogen bonds act as catalysts for covalent bond exchange, while at high concentration, they provide a pervasive vinylogous urea ‐ urethane (VU) network of strong non‐covalent interactions, giving rise to phase separation and inhibiting polymer chain dynamics. This offers a straightforward design principle for dynamic polymer materials, showing at the same time the possible additive and synergistic effects of supramolecular and dynamic covalent polymer networks.

Vinylogous urethane (VU_O) based polymer networks are widely used as catalyst‐free vitrimers that show rapid covalent bond exchange at elevated temperatures. In solution, vinylogous ureas (VU_N) undergo much faster bond exchange than VU_O and are highly dynamic at room temperature. However, this difference in reactivity is not observed in their respective dynamic polymer networks, as VU_O and VU_N vitrimers prepared herein with very similar macromolecular architectures show comparable stress relaxation and creep behavior. However, by using mixtures of VU_O and VU_N linkages within the same network, the dynamic reactions can be accelerated by an order of magnitude. The results can be rationalized by the effect of intermolecular hydrogen bonding, which is absent in VU_O vitrimers, but is very pronounced for vinylogous urea moieties. At low concentrations of VU_N, these hydrogen bonds act as catalysts for covalent bond exchange, while at high concentration, they provide a pervasive vinylogous urea ‐ urethane (VU) network of strong non‐covalent interactions, giving rise to phase separation and inhibiting polymer chain dynamics. This offers a straightforward design principle for dynamic polymer materials, showing at the same time the possible additive and synergistic effects of supramolecular and dynamic covalent polymer networks.

Natural fiber‐reinforced composites are gaining increased interest for their significantly reduced carbon footprint compared to conventional glass or carbon fiber‐based counterparts. In this study, natural fibers are used in a resorcinol‐based epoxy resin that is thermally reshapable at higher temperatures (>180 °C) by using fast exchanging siloxane bonds, catalyzed by 1,5,7‐triazabicyclo[4.4.0]dec‐5‐ene. Stress relaxation times of only about 6 s at 220 °C can be reached. A resorcinol‐based epoxy compound is selected because it can be derived from cellulose, opening ways for more sustainable and reshapable composite materials. In a last step of the research, the low viscosity vitrimer formulation (<200 mPa s) is applied to make a flax fiber‐reinforced composite using an industrially relevant vacuum‐assisted resin infusion process. A section of this composite is successfully reshaped, which allows for envisioning a second life for natural fiber‐reinforced composites.

A vanillin derivative containing β-hydroxy-amine functionalities was introduced into vitrimer materials using vinylogous urethane chemistry. These new materials have a high bio-based content and show fast reprocessability.