Vitrimers -a class of polymer networks which are covalently crosslinked and insoluble like thermosets, but flow when heated like thermoplastics -contain dynamic links and/or crosslinks that undergo an associative exchange reaction. These dynamic crosslinks enable vitrimers to have interesting mechanical/rheological behavior, self-healing, adhesive, and shape memory properties.We demonstrate that vitrimers can self-assemble into complex meso-and nanostructures when crosslinks and backbone monomers strongly interact. Vitrimers featuring polyethylene (PE) as the backbone and dioxaborolane maleimide as the crosslinkable moiety were studied in both the molten and semi-crystalline states. We observed that PE vitrimers macroscopically phase separated into dioxaborolane maleimide rich and poor regions, and characterized the extent of phase separation by optical transmission measurements. This phase separation can explain the relatively low insoluble fractions and overall crystallinities of PE vitrimers. Using synchrotronsourced small-angle X-ray scattering (SAXS), we discovered that PE vitrimers and their linear precursors micro-phase separated into hierarchical nanostructures. Fitting of the SAXS patterns to a scattering model strongly suggests that the nanostructures -which persist in both the melt and amorphous fraction of the semi-crystalline state -may be described as dioxaborolane maleimide rich aggregates packed in a mass fractal arrangement. These findings of hierarchical meso-and nanostructures point out that incompatibility effects between network components and resulting self-assembly must be considered for understanding behavior and the rational design of vitrimer materials.Thermoplastics and thermoplastic elastomers are soluble in good solvents and flow when heated above the glass or melting temperature. Thermosets and rubbers do not flow and are not soluble.Vitrimers, a class of polymers introduced by Leibler and collaborators in 2011, flow when heated, but remain insoluble. 1 Vitrimers are made of polymer networks which contain covalent crosslinks that undergo dynamic associative exchange reactions. The covalent crosslinks in a vitrimer maintain network connectivity at all times and temperatures. Unlike materials employing dissociative crosslinking mechanisms, 2 vitrimers cannot be completely dissolved -even in good solvents. 1,3 Associative exchange reactions permit the network topology to fluctuate and the system to flow when stress is applied, and exchange reaction kinetics control the vitrimer relaxation dynamics and viscosity. 2,4,5,6 The initial reports of vitrimer systems focused on epoxy networks that reorganized via metal-catalyzed transesterification. 1,4,6 Today, the library of dynamic exchange reactions has expanded to include chemistries that are catalytically-controlled (olefin metathesis and transcarbonation) 7,8 or catalyst-free (transamination, 9,10,11 trans-N-alkylation, 12,13 reversible addition of thiols, 14 imine exchange, 15,16 addition-fragmentation chain transfer, 17,18 boronic est...

For vitrimer systems obtained by dynamic cross-linking of polymer chains, incompatibility effects between the cross-links and polymer backbone can lead to microphase separation, resulting in a network made of cross-linked aggregates. Additionally, when there is a wide distribution of the number of cross-links per chain, macrophase separation can occur, mostly due to entropic effects.Here, we investigate the linear viscoelasticity and flow of a polyethylene (PE) vitrimer that has cross-linkable dioxaborolane maleimide grafts, which self-assemble into a hierarchical nanostructure. To elucidate the role of self-assembly, we first studied dioxaborolane graft functionalized PE that was non-cross-linked. It had a terminal relaxation time that was orders of magnitude larger than both neat PE and partially peroxide cross-linked PE. When dioxaborolane cross-linker was added to form the vitrimer, the resulting material could not achieve terminal relaxation within 8 h. The soluble and insoluble portions of the PE vitrimer were then isolated and characterized. The soluble portion, which was graft-poor, expressed similar flow behavior as neat PE, while the insoluble portion -which was a graft-rich network of cross-linked aggregatesrelaxed very little over 8 h. When the insoluble and soluble portions were blended, the rheological behavior of the original vitrimer was basically recovered, showing that the soluble portion acts as a lubricant. When the insoluble portion was blended with neat PE, the material relaxed much more stress, but still did not reach steady-state flow within 8 h. When high stresses were applied, however, PE vitrimer flowed. Nonlinear rheology experiments revealed melt fracture at high strains and suggested that flow is enabled by rapid healing, which follows fracture events. The presence of macroscopic phase separation facilitated flow. IntroductionVitrimers are covalent networks that engage in thermoactivated associative exchange reactions. 1,2,3 Under cooling, they undergo a reversible topology freezing transition, analogous to the glass transition of amorphous silica. In contrast to dissociative cross-links, the associative cross-links of vitrimers preserve network connectivity at all times and temperatures below degradation conditions, yet still enable the network topology to fluctuate. 1,2,3 The consequences of the topology freezing transition on materials properties were investigated for epoxy/acid systems that underwent catalyst-driven transesterification. 1 Since then, the literature has focused on altering the exchange reaction (either catalytically controlled or catalyst-free) and adapting chemistries to different polymer backbones. Other studies explored the properties of vitrimer composites and the addition of nondynamic cross-links to the network. 14,28,29,30,31,32,33,34,35 These research efforts established that modifying vitrimer chemistry provides a pathway for tuning mechanical properties, 36,37,38 stress relaxation, 39,40,41,42 shape memory, 43,44,45 and the ability to self-heal and adher...