Insights into Uniaxial Tension and Relaxation of Nanorod-Filled Polymer Vitrimer Nanocomposites: A Molecular Dynamics Simulation

Abstract

The mechanical properties of polymer nanocomposites (PNCs) depend sensitively on the structure (e.g., orientation, dispersion, and so on) of the incorporated nanofillers. Many studies have shown that the alignment of anisotropic nanofillers can improve the mechanical properties of PNCs. However, achieving this alignment typically requires complex preparation processes. To address this challenge, researchers have introduced dynamic covalent bonds to form reversible cross-linked polymer systems, which would lead to unique properties, such as self-healing, recyclability, and reprocessibility. In addition, inspired by the above ideas, we introduce nanorods as fillers into a linear vitrimer system to form nanorod vitrimer composites (NVCs). In NVCs, we can easily manipulate the alignment of the nanorods due to bond exchange reactions (BERs) in the vitrimer matrices. By using coarse-grained molecular dynamics (CGMD) simulations, we systematically investigate the factors affecting the nanorod orientation in NVCs. We find that the main factor affecting the nanorod orientation is the network rearrangement caused by BERs. Specifically, the BER potential barrier (ΔE sw) directly determines the probability of BERs’ occurrence. At the same time, the increase of the interfacial interaction between polymer chains and nanorods (εnp) confines the motion of the active beads, which slows down the rate of BERs. Additionally, the impact of temperature (T) and aspect ratio (ld) on the orientation of nanorods during uniaxial stretching or stress relaxation are also discussed. Finally, by combining uniaxial stretching and stress relaxation processes, we elucidate the orientation retention mechanism of NVCs and demonstrate the mechanical property enhancement phenomenon of the pre-oriented NVC systems. This work provides a simple strategy for manipulating the nanorod alignment in vitrimer matrices and uncovers guidelines for designing new functional polymer vitrimer nanocomposites at the molecular level.

Unveiling the Multiscale Dynamics of Polymer Vitrimers Via Molecular Dynamics Simulations

“…However, computer simulations provide an effective means to accomplish this task. In this study, we utilized the bond number autocorrelation function C sw ( t ), which has been previously used in our work, , to systematically study the kinetics of bond swap behavior C normals normalw ( t ) = ⟨ N b ( t 0 + t ) · N b ( t 0 ) ̅ ⟩ where N b ( t 0 + t ) is the number of dynamic bonds which were initially present at moment t 0 and still exist after time t . The bar and brackets denote averaging over time and the ensemble, respectively.…”

“…However, computer simulations provide an effective means to accomplish this task. In this study, we utilized the bond number autocorrelation function C sw (t), which has been previously used in our work, 35,36 to systematically study the kinetics of bond swap behavior…”

“…This approach has been validated as an efficient method for simulating a wide range of problems in polymer physics. , The harmonic potential is defined as U bond = K ( r – r 0 ) 2 , where r 0 is the equilibrium bond distance and K is the bond interaction strength. We chose K = 200.0ε/σ 2 , as previously employed in our works, , to guarantee a certain stiffness of the bonds while avoiding high-frequency modes and chain crossing. Every polymer chain consists of 25 monomers, with each linear chain including 7 reactive beads, as shown in Figure .…”

“…Motivated by the above issues, in this work, we have developed a coarse-grained model of a vitrimer system composed of a polymer network formed from linear chains, which can rearrange the network topology via the BERs algorithm. The BERs algorithm has been verified to be effective in probing the relaxation, mechanics, and fragility of the vitrimer system. ,,, Subsequently, we systematically investigated the dynamical behavior of vitrimer systems at different time and length scales, such as bond exchange, polymer bead mobility, chain segmental relaxation, dynamical heterogeneity, whole chain relaxation, and viscoelastic properties, and established a general correlation between different dynamical behaviors. To gain further insights, we assessed the applicability of the TTSP and time-energy barrier superposition principles (TESP) to vitrimer materials using the corresponding characteristic relaxation time to normalize each dynamical behavior.…”

Modeling Exchange Reactions in Covalent Adaptable Networks with Machine Learning Force Fields

“…As a result, conventional force fields do not enable bond formation and bond breaking, and equilibrium bond lengths are preserved. To address this problem, we manually altered the system topology relying on a cutoff distance criterion in our previous work. , Recently, Sciortino and co-workers proposed a coarse-grained three-body potential that allows for bond swaps using standard MD driven by force and energy. − However, these methods do not consider the chemical details of the bond rearrangement in CANs.…”