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.

Topological copolymers with a comblike structure have been found to have the ability to recover without the need for extraneous material components and the use of external stimuli; however, a systematic and fundamental understanding at the molecular level is still lacking. In this work, coarse-grained molecular dynamics (CGMD) simulation and noncovalent interaction (NCI) analysis were employed to investigate a model system of selfhealing comblike copolymers consisting of methyl methacrylate (MMA) and nbutyl acrylate (n-BA). Specifically, the different interactions of neighboring macromolecules were analyzed by NCI. Then, the influence of the following three parameters on the mechanical properties and interpenetration of side chains was probed: (1) a spacer between side chains l g , (2) average molecular weight per backbone bond m b , and (3) the flexibility of side chains k s . Based on the optimum of the three parameters, self-healing simulations were performed to examine the mechanical behavior of the system at different healing temperatures and healing times. The NCI results reveal that the sectional backbone chains are forced to stretch due to the steric repulsion and the side chains of neighboring molecules are interpenetrated due to the attraction of van der Waals force. Favorable steric repulsion and a spacer between side chains allow for interpenetration of side chains of neighboring macromolecules, leading to optimal mechanical properties with the graft polymer composition φ = l g /(l g + R sc ) ≅ 3/7, where R sc and l g are the length of side chains and the spacer separating two consecutive side chains along the polymer backbone, respectively. Interpenetration of side chains is negatively correlated with the average molecular weight per backbone bond (m b ) for graft chains. Comblike copolymers with stiff side chains tend to form lock-and-key structures, which are conducive to closer interpenetration and better mechanical properties.

No abstract

Exploring novel healing mechanisms is a constant impetus for the development of self-healing materials. Herein, we find that the side-chain interlocking of bottlebrush polymers can form a dynamic network and...