Role of Dynamic\nCovalent Bond Density on the Structure\nand Properties of Vitrimers

Vitrimers, as a novel class of dynamic polymers, require\nprecise\nregulation of their dynamic and mechanical properties to achieve optimal,\ncomprehensive performance. This study utilizes a coarse-grained vitrimer\nelastomer model, composed of simple linear polymer chains by varying\ndynamic cross-link densities (ρ), to systematically investigate\nthe structural, dynamic, and mechanical properties of vitrimers. The\nresults demonstrate that the system number density decreases with\nincreasing ρ due to the repulsion between reactive sites. The\ncharacteristic glass transition temperature (<i>T</i><sub>g</sub>) increases linearly with cross-link densities such as <i>T</i><sub>g</sub> ∼ ρ, while the characteristic\ntopological transition temperature <i>T</i><sub>v</sub> exhibits\nnonmonotonic changes versus ρ. Interestingly, the bond exchange\nautocorrelation function indicates that the bond exchange rate reaches\nthe maximum at intermediate cross-link densities attributed to the\ncompetition between the number and the mobility of the reactive sites.\nMean square displacement analysis reveals that the mobility of beads\nand polymer chains decreased with ρ, and the logarithm of the\ndiffusion coefficients linearly decreases with ρ, ln <i>D</i> ∼ −ρ. The chain segment relaxation\ntimes with different cross-link densities can be described with an\nexponential equation τ<sub>α</sub> ∼ exp(ρ/<i>C</i>), while the whole chain relaxation times are more sensitive\nto the cross-link density. The linear viscoelasticity via equilibrium\nmolecular dynamics simulations indicate that higher cross-link densities\nlead to greater elasticity and higher energy dissipation capabilities,\nas indicated by the storage and loss moduli, and the derived viscosity\nversus the cross-link density for various energy barriers (Δ<i>E</i><sub>sw</sub>) exhibits the universal following relation\nln η<sup>0</sup> ∼ ρ. Uniaxial and triaxial tensile\ntests both show that high ρ and high Δ<i>E</i><sub>sw</sub> systems exhibit higher tensile strength at low strains\ndue to the tight and stable network structure, whereas at high strains,\nthe voids occur due to the stress concentration and the breakage of\nthe dynamic covalent bonds, which reduces the maximum tensile strength,\ntoughness, and elongation at break, as compared to the lower ρ\nand lower Δ<i>E</i><sub>sw</sub> systems. Therefore,\nthe optimal tailoring of the cross-link density and the exchange barrier\nshow the capability to achieve the most excellent comprehensive performance\nof the vitrimer. These findings provide crucial theoretical guidelines\nfor the design and optimization of high performance vitrimers.