Model Polyisoprene Vitrimers

Abstract

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 ).

Segmental and Chain Dynamics of Polyisoprene-Based Model Vitrimers

“…Unfortunately, the present results cannot give us any indication of how this process takes place, other than the fact that it is much slower than the slower characteristic frequency identified by the dielectric relaxation results. The linear rheology experiments reported in the Supporting Information of ref show that at temperatures as high as 363 K, the onset of the terminal relaxation must occur at frequencies below mHz, which is also consistent with the present findings and supports the above framework.…”

“…Our results show no clear indication of this final relaxation process due to the overlapping contribution of conductivity to the measured signal. An estimate of the longest relaxation time (lowest relaxation frequency) in the PI6k-vit can be obtained from the viscosity (see Figure ) and rheology data reported in ref , which at T = 350 K would be η ≈ 4 × 10 10 Pa s, and G ≈ 0.5 MPa, giving rise to f slowest ≈ G /η ≈ 10 μHz. , This frequency is certainly in a range where the conductivity-related losses greatly dominate the dielectric relaxation of the PI-vitrimers (see Figure ). Therefore, the characteristic frequency of the detected slow dielectric relaxation component has to be related mainly with limited fluctuations of the PI chain-ends (see Scheme ) that would take place either within the clusters and/or because of motions/fluctuations of the whole clusters (see below).…”

“…Three model vitrimers based on well-defined polyisoprene chains (Scheme ) with three different molecular weights were investigated. The detailed synthesis and molecular characterization of precursors and final vitrimers are given elsewhere . We note that the samples were synthesized with the stoichiometry in mol corresponding to a small excess (about 5%) of primary amines, to facilitate bond exchange reactions.…”

“…For this purpose, the detailed study of model vitrimers is crucial to correctly understand the structure-dynamics-property relationships with the dynamic bond network. In this context, the synthesis and characterization of vitrimers based on polyisoprene (PI) has recently been presented . In these vitrimers, the aldehyde groups at the α, ω-ends of the PI chains react with the amino groups of the triamine cross-linkers to form imine bonds.…”

“…In this way, model vitrimers with a constant polymer chain length between the imine linkages were obtained. By using PI chains of different molecular weights, vitrimers with varying concentrations of triamine cross-linkers were obtained . The structure and vitrimeric transition of these materials were investigated in a recent work .…”