Recyclability and reprocessability of permanently cross-linked polymeric materials have received considerable scientific and technological attention in view of the environmental pollution and sustainable development. By introducing dynamic covalent bonds, vitrimers are emerging as a promising attempt to address this pressing challenge. However, there is still a lack of thermodynamic and kinetic understanding of the bond exchange reactions (BERs) of vitrimers at the molecular level. Herein, by employing coarse-grained molecular dynamics simulations, we successfully construct a model vitrimer system composed of a polymer network formed from linear chains, which can rearrange the network topology via BERs. In this study, we examine the effect of the bond swap energy barrier (ΔE sw ) on a variety of mechanical properties. We find that ΔE sw critically controls the dynamics of the linear chains and the reactive beads located on the linear chain. Our results indicate that the best mechanical performance characteristics are achieved at an intermediate value of ΔE sw . Meanwhile, stress relaxations are examined for different ΔE sw systems. By performing a triaxial deformation to induce the cavities, the vitrimer exhibits excellent self-healing capability by decreasing ΔE sw , as well as increasing the self-healing time and temperature. Lastly, extrusion of polymer vitrimer is simulated, and we find that the extrusion rate tends to increase linearly as ΔE sw decreases. In general, our results provide rational guidelines for designing high-performance vitrimers with good mechanical properties, excellent self-healing ability, and good reprocessability.
Attributed to its strain-induced crystallization (SIC), natural rubber (NR) exhibits more excellent mechanical properties compared to other elastomeric materials and has been attracting numerous scientific and technological attention. However, a systematical understanding of the structure–mechanics relation of NR is still lacking. Herein, for the first time, we employ molecular dynamics simulation to examine the effects of the key structural factors on the SIC and mechanical properties at the molecular level. We examine the effects of phospholipid and protein mass fraction (ω), the strength of hydrogen-bond interaction (εH), and the strength of non-hydrogen-bond interaction (εNH) on structural morphology, dynamic behavior, and mechanical properties. NR tends to form local clusters due to the hydrogen-bond interaction formed between phospholipids or proteins and chain ends, which is absent in the case of cis-1,4-polyisoprene (PIP). The polymer chain mobility of NR is retarded due to the formed clusters or even physical network at great εH and high ω. Interestingly, we find that the stress–strain behavior of NR is greatly manipulated by εH and ω, as evidenced by the increase of the chain orientation and the SIC, compared with the cases of PIP. This underlying mechanism results from the alignment of the molecular chains induced by the formed clusters along the deformed direction, and the clusters during the deformation become more stable, particularly at great εH. Lastly, we adopt a machine learning algorithm named extreme gradient boosting via data augmentation, finding that εH has the most significant influencing weight factor on the stress–strain behavior of NR. In general, this work demonstrates a detailed molecular-level structure–mechanics relation of NR and provides some rational guidelines for experimentally designing and synthesizing biomimetic NR.
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.
Understanding polymer–substrate interfacial dynamics at the molecular level is crucial for tailoring the properties of polymer ultrathin films (PUFs). Herein, through coarse-grained molecular dynamics simulation, the effect of length (N loop) and rigidity (K loop) of loop chains on the dynamics of linear chains is systematically explored, in which the loop chains are adsorbed on a solid substrate and the linear chains are covered on the loop chains. It is found that there is an optimal K loop, which strongly confines the motion of the linear chains. Meanwhile, compared to increasing the rigidity of the loop chains, increasing the length of the loop chains can more effectively confine the motion of the linear chains. More interestingly, we observe that the mismatch of the length (ΔN) and rigidity (ΔK) between the loop and linear chains leads to dynamic asymmetry (ΔD c). The relationship between the ΔN, ΔK, and ΔD c are found to follow the mathematical expression of ΔD c ∼ (ΔN)α(ΔK)β, in which the values of α and β are around 4.58 and 0.83, separately. Remarkably, using the Gaussian process regression model, we construct a master curve of diffusion coefficient on the segmental and chain length scales of the linear chains as a function of N loop and K loop, which is further validated by our simulated prediction. In general, this work provides a fundamental understanding of polymer interfacial dynamics at the molecular level, enlightening some rational principles for manipulating the physical properties of PUFs.
Densely grafted polymer chains onto spherical nanoparticles produce a diverse range of conformations. At high grafting densities, the corona region near the nanoparticle surface undergoes intense confinement due to a high concentration of chains in the concentrated polymer brush (CPB) region, which results in strong stretching for portions of the chains located within. In contrast, a semi-dilute polymer brush (SDPB) forms farther away from the core and offers reduced confinement for the polymer and more ideal conformations. However, conventional experimental methods are limited in their ability to provide detailed information on individual segments of grafted polymers in these regions; hence, molecular dynamics (MD) simulations are essential for gaining comprehensive insights into the behavior of the grafted chains. This study aims to explore the variations in polymer structure and dynamics that occur along the contour of the grafted chains as influenced by spatial confinement. We focus on the motions and relative positions of each bead along grafted polymers. Our results show that only the initial few grafted beads near the nanoparticle surface exhibit the strong stretching attributed segments in the CPB region of the brush. Increased grafting density or decreased chain flexibility leads to more stretched grafted chains and more aligned bond vectors. As a result, the relaxation dynamics of local regions of the polymer are also strongly influenced by these parameters. Although the grafted beads in the interior of the CPB region are highly sensitive to these parameters, those farther from the nanoparticle core experience significantly diminished effects. In comparison to the Daoud–Cotton (DC) model’s predictions of CPB size, beads near the nanoparticle surface show slower dynamic decay, especially in high grafting densities, aligning with the DC model’s estimates. Finally, we compare our simulations to previous works for additional insight into polymer-grafted nanoparticles.
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