Linear Viscoelasticity and FLOW of Self-Assembled Vitrimers: The Case of a Polyethylene/Dioxaborolane System

We investigated the linear viscoelasticity of a polyethylene (PE) vitrimer that has cross-linkable dioxaborolane maleimide grafts. Strong interactions between the PE backbone and grafts cause the molten vitrimer to macro- and microphase separate into hierarchical structures. Small-amplitude oscillatory shear, stress relaxation, and creep measurements were used for characterization. Graft functionalized PE (which does not contain cross-linker, but still self-assembles) had a terminal relaxation time that was two orders of magnitude larger than neat PE. When cross-linker was added to form the vitrimer, the material exhibited a higher melt strength but did not achieve steady-state flow within 8 hr. The soluble graft-poor portion of PE vitrimer had similar flow behavior as neat PE, but its flow activation energy was twice as large. Conversely, the insoluble graft-rich fraction behaved as a viscoelastic solid that relaxed very little over 8 hr. Blend experiments suggested the interface between the graft-rich and graft-poor phases of PE vitrimer also influenced relaxation. These findings indicate that self-assembly/associative cross-linking interplay greatly impacts the rheology and, consequently, processability of vitrimers.