Vitrimers: permanent organic networks with glass-like fluidity

Abstract

Most covalent adaptable networks give highly interesting properties for material processing such as reshaping, recycling and repairing. Classical thermally reversible chemical cross-links allow for a heat-triggered switch between materials that behave as insoluble cured resins, and liquid thermoplastic materials, through a fully reversible sol-gel transition. In 2011, a new class of materials, coined vitrimers, was introduced, which extended the realm of adaptable organic polymer networks. Such materials have the remarkable property that they can be thermally processed in a liquid state without losing network integrity. This feature renders the materials processable like vitreous glass, not requiring precise temperature control. In this mini-review, an overview of the state-of-the-art in the quickly emerging field of vitrimer materials is presented. With a main focus on the chemical origins of their unique thermal behavior, the existing chemical systems and their properties will be discussed. Furthermore, future prospects and challenges in this important research field are highlighted.

Fig. 1. CANs are divided in two groups: (a) dissociative, and (b) associative, based on the exchange reactions that proceed respectively with or without a temporary net loss of cross-link density. Vitrimers belong to the group of associative CANs.

Fig. 2. Angell fragility plot, showing the viscosity as a function of the inverse temperature, scaled with T _g (or T _v for vitrimers when T _v > T _g). Thermoplastics such as polystyrene (PS) are characterized with a narrow glass transition temperature and thus a very fast decrease in viscosity near T _g. In contrast, vitrimers (epoxy-anhydride based, epoxy-acid based and vinylogous urethanes) show an Arrhenius-like dependence of the viscosity, which results in a gradual viscosity decrease similar to vitreous silica.

Fig. 3. Representation of the viscoelastic behavior of vitrimers with (a) a glass transition, T _g, lower than the topology freezing transition temperature, T _v. Upon heating, the vitrimer evolves from a glassy solid (T < T _g) to an elastomer (T _g < T < T _v) to a viscoelastic liquid (T > T _v) that follows the Arrhenius law. (b) A hypothetical T _v is situated well below T _g. Upon heating, the vitrimer evolves from a glassy solid to a viscoelastic liquid with a viscosity that is first controlled through diffusion (WLF) and then by the exchange kinetics (Arrhenius).

Fig. 4. (a) The reaction between epoxides and carboxylic acids results in a repeating unit, containing both an ester and a hydroxyl function. (b) The catalytic transesterification reaction that can occur at elevated temperatures.

Fig. 5. (a) Synthesis of vinylogous urethane; this reaction can even be performed in water. (b) Exchange of vinylogous urethanes and amines occurs at temperatures above 100 °C without catalyst via a Michael addition.

Fig. 6. Polymerisation of α-azide-ω-alkyne monomers with bisfunctional alkylating agents results in polymer networks via the formation of 1,2,3-triazoles followed by the cross-linking process through alkylation to 1,2,3-triazoliums. The resulting networks can rearrange their topology through exchange between 1,2,3-triazoles and 1,2,3-triazoliums.

Fig. 7. Siloxane silanol exchange reaction through addition and elimination of a silanolate anion.

Fig. 8. Olefin metathesis exchange reaction, (a) insertion of the Grubbs' second generation catalyst into an alkyl chain (b) exchange of two alkene bonds (c) left: structure of 2^nd generation Grubbs' catalyst; right: the associative exchange mechanism of a metathesis reaction.

Fig. 9. Aromatic disulfide metathesis proceeds at room temperature without any catalyst. The quadruple H-bonds of the urea groups prevent the network to flow at low temperatures.

Fig. 10. Reversible processes associated with imine chemistry. (a) Equilibrium of imine formation, via intermediate hemi-aminal formation; (b) transamination between imines and amines via intermediate aminal formation; (c) imine metathesis.

Wim Denissen

Johan M. Winne

Filip E. Du Prez