Linear Viscoelasticity and Flow of Self-Assembled Vitrimers: The Case of a Polyethylene/dioxaborolane System

Abstract

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 adhe...

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