Role of Dynamic Covalent Bond Density on the Structure and Properties of Vitrimers

Abstract

Vitrimers, as a novel class of dynamic polymers, require precise regulation of their dynamic and mechanical properties to achieve optimal, comprehensive performance. This study utilizes a coarse-grained vitrimer elastomer model, composed of simple linear polymer chains by varying dynamic cross-link densities (ρ), to systematically investigate the structural, dynamic, and mechanical properties of vitrimers. The results demonstrate that the system number density decreases with increasing ρ due to the repulsion between reactive sites. The characteristic glass transition temperature (Tg) increases linearly with cross-link densities such as Tg ∼ ρ, while the characteristic topological transition temperature Tv exhibits nonmonotonic changes versus ρ. Interestingly, the bond exchange autocorrelation function indicates that the bond exchange rate reaches the maximum at intermediate cross-link densities attributed to the competition between the number and the mobility of the reactive sites. Mean square displacement analysis reveals that the mobility of beads and polymer chains decreased with ρ, and the logarithm of the diffusion coefficients linearly decreases with ρ, ln D ∼ −ρ. The chain segment relaxation times with different cross-link densities can be described with an exponential equation τα ∼ exp(ρ/C), while the whole chain relaxation times are more sensitive to the cross-link density. The linear viscoelasticity via equilibrium molecular dynamics simulations indicate that higher cross-link densities lead to greater elasticity and higher energy dissipation capabilities, as indicated by the storage and loss moduli, and the derived viscosity versus the cross-link density for various energy barriers (ΔEsw) exhibits the universal following relation ln η0 ∼ ρ. Uniaxial and triaxial tensile tests both show that high ρ and high ΔEsw systems exhibit higher tensile strength at low strains due to the tight and stable network structure, whereas at high strains, the voids occur due to the stress concentration and the breakage of the dynamic covalent bonds, which reduces the maximum tensile strength, toughness, and elongation at break, as compared to the lower ρ and lower ΔEsw systems. Therefore, the optimal tailoring of the cross-link density and the exchange barrier show the capability to achieve the most excellent comprehensive performance of the vitrimer. These findings provide crucial theoretical guidelines for the design and optimization of high performance vitrimers.