Strain-Dependent Stress Relaxation and Mechanical Recovery in Multiphase Polyethylene Vitrimers

Polyethylene vitrimers combine reprocessability with dynamic covalent networks, but microphase separation can couple chemical relaxation with physical structural evolution. Here, polyethylene vitrimers containing rigid phthalic acid (PA) or flexible decanediol crosslinkers are compared. The PA system forms a more ordered phase-separated structure and exhibits a higher effective elastic crosslink density. Stress-relaxation data at 1, 5, and 10% strain were initially analyzed using an Arrhenius framework. Subsequent cyclic tests on PEV-PA at 180 °C gave modulus recoveries of 89.2 and 22.3% after 5 and 10% intermediate strains, respectively. The 10% history also markedly retarded subsequent relaxation at 1% strain. These results highlight persistent changes following the 10% history and support interpreting the fitted quantities as apparent macroscopic relaxation parameters. Loading history and mechanical recovery must therefore be considered when evaluating their chemical kinetic meaning.