Model Polyisoprene Vitrimers

Vitrimers are an important class of cross-linked polymers that behave as conventional thermosets at low temperatures and exhibit viscoelastic liquid behavior at high temperatures. The currently available vitrimers are far to be well-defined. Therefore, there is a need for well-defined vitrimers (known molecular weight between cross-links and low dispersity of the chains) to establish accurate structure-property relationships. This work presents the successful synthesis of model polyisoprene (PI) vitrimers based on imine dynamic covalent bonds (DCB) via condensation reaction of α,ω-aldehyde-functionalized PI (OHC-PI-CHO) and tris(2-aminoethyl)amine. Four model PI vitrimers (PI_2K-Vit, PI_4K-Vit, PI_6K-Vit, and PI_11K-Vit) were synthesized with different molecular weights between the cross-links (M_c). Fourier transform-infrared (FT-IR) and solid-state proton NMR spectroscopies were used to demonstrate the formation of imine DCB in vitrimers. Preliminary mechanical and rheological studies of the vitrimers with thermally activated imine bonds were measured via dynamic mechanical analysis (DMA) and rheology. The highest activation energy found for PI_11K-Vit (88 kJ mol^-1) is attributed to the exchange reaction of the imine DCB and the topological constraints. The vitrimers having M_c lower than the entanglement molecular weight of PI (PI_6K-Vit) show a low activation energy (62 kJ mol^-1).