Reprocessing of Covalent Adaptable Polyamide Networks through Internal Catalysis and Ring-Size Effects

Abstract

Here, we report the introduction of internally catalyzed amide bonds to obtain covalent adaptable polyamide networks that rely on the dissociation equilibrium between dicarboxamides and imides. While amide bonds are usually considered to be robust and thermally stable, the present study shows that their dynamic character can be activated by a smart choice of available building blocks without the addition of any external catalyst or other additives. Hence, a range of polyamide-based dynamic networks with variable mechanical and viscoelastic properties have been obtained in a systematic study, using a straightforward curing process of dibasic ester and amine compounds. Since the dissociation process involves a cyclic imide formation, the correlation between ring size and the thermomechanical viscosity profile was studied for five- to seven-membered ring intermediates, depending on the chosen dibasic ester monomer. This resulted in a marked temperature response with activation energies in the range of 116–197 kJ mol–1, yielding a sharp transition between elastic and viscous behavior. Moreover, the ease and versatility of this chemistry platform were demonstrated by selecting a variety of amines, resulting in densely cross-linked dynamic networks with T g values ranging from −20 to 110 °C. With this approach, it is possible to design amorphous polyamide networks with an acute temperature response, allowing for good reprocessability and, simultaneously, high resistance to irreversible deformation at elevated temperatures.

Suppressing Creep and Promoting Fast Reprocessing of Vitrimers with Reversibly Trapped Amines

“…More specifically, in this temperature range the small amount of released amines do not exchange fast enough within a reasonable time frame, yielding an apparent more elastic response to deformation. The strong reduction in viscous behaviour was ascribed to the relatively thermally demanding dissociation of a glutaramide‐type structure to a six‐membered glutarimide and amine [39] . Note that this apparent liquid to solid behaviour is similar to that of hybrid networks containing both dynamic and non‐dynamic covalent bonds, [44, 45] although residual stresses can still relax for the modified VU system by simply heating to higher temperatures.…”

“…In this work, we report our highly encouraging findings in this area of arresting low temperature deformation while keeping reprocessability, by adopting a very recently developed reversible covalent amide linkage within VU vitrimers (Scheme 1). [39] By adding a small amount (0.2 to 4 wt%) of a dibasic ester co‐monomer, amines can be fully reacted and trapped into the polymer matrix, resulting in creep‐resistant VU vitrimers (Scheme 1a). In essence, by adding a second covalent dynamic exchange, a dissociative connection is incorporated in associative VU networks (Scheme 1b).…”

Synthesis and characterization of new bio-based poly(acylhydrazone) vanillin vitrimers

“…37,38 Nevertheless, the fact that T v s are lower than T g s does not affect the applicability as thermosets at room temperature due to the high values of the T g obtained for these materials, in contrast to what happens in other vitrimers where the creep resistance behavior could be compromised. [39][40][41]…”

Suppressing Creep and Promoting Fast Reprocessing of Vitrimers with Reversibly Trapped Amines

“…In a second step, some modified VU materials were designed whereby (protonated) chain‐end amines could be released by the previously established dissociation equilibrium between dicarboxamides and imide‐amines [39] . This was achieved by adding dimethyl glutarate (DBE‐5, 4 ) to an excess of amine 2 and further curing with acetoacetate 1 (Figure 1b).…”