Crystallization in vitrimers probed by time-domain NMR and complementary methods

Partial crystallization in vitrimers can improve their mechanical properties via suppressing creep. However, the interplay with the degree of crosslinking requires consideration [1-3]. Here, we discuss results of NMR studies on two different kinds of semicrystalline vitrimers [2,3], providing the absolute level of crystallinity and addressing the interplay of crystallization and the bond exchange reaction.

In one case, the polymorphism of "ethylene vitrimers" made from (α,ω)-diamino alkanes and boric acid [2] can be probed via changes in dipolar NMR line width. We confirm crystallinities beyond 90% and observe the crystallization kinetics as well as crystal-crystal transformations in real time [4]. A long-time increase of T_m and the heat of melting upon long-time annealing is found to occur at constant high crystallinity, suggesting that both are governed by the crystal perfection, as confirmed by NMR. In the other case of thiol-ene crosslinked vitrimers featuring thiol-thioester exchange reactions along the network chains [3], we elucidate the interference of the main-chain random isomeric structures resulting from the bond exchange with the crystallization.

[1] Y. Golitsyn et al., Polymer 165, 72 (2019)

[2] B: Soman, C. Evans, Soft Matter 17, 3569 (2022)

[3] A.S. Kuenstler, C.N. Bowman, ACS Macro Lett. 12, 133 (2023)

[4] K. Saalwächter et al., Front. Soft Matter 3, 1208777 (2023)