Molecular Dynamics Simulation of the Fracture in Polymer-Exfoliated Layered Silicate Nanocomposites

Revealing the toughening mechanism of graphene-polymer nanocomposite through molecular dynamics simulation

By employing united atom molecular dynamics simulation, we have investigated the effects of polymer-graphene interaction ε(np) volume fraction of grapheme φ thermodynamics of polymer matrix (rubbery versus glassy), interfacial interaction in the case of the same dispersion state, shape of nanoparticles (NPs) such as C60 CNT and graphene at the same loading on the toughening efficiency of polymer nanocomposites. By beginning with the pure polymer, we observe that a plateau stress occurs at long chain length because entangled polymer chains in fibrils cannot become broken. We find that the work needed to dissipate during the failure increases with the increase of ε(np) and φ and the yield point in the stress-strain behavior occurs at a smaller strain for an attractive NPs filled system compared to the pure case, attributed to the more mechanically heterogeneous environment. The thermodynamics of the polymer matrix (below and above Tg) seems to have a significant effect on the toughening efficiency of graphene sheets. In the case of the same dispersion state, stronger interfacial interaction always induces long and highly orientated polymer fibrils along the deformation direction, with graphene sheets being encapsulated in these fiber-like bundles. By characterizing the interaction energy between polymer-polymer and polymer-graphene as a function of the strain, we find that the separation of polymer chains from the graphene sheets cease immediately after the yield point, followed by the continuous propagation of the cavities by excluding surrounded polymer chains and graphene sheets together. We also find that at the same attractive interfacial interaction and same loading, the toughening efficiency exhibits the following order: graphene > CNT > C60 Generally, the toughening mechanism of graphene sheets results from the formation of long and highly orientated polymer fibrils to prevent the occurrence of the rupture, which can be greatly improved by the strong interfacial interaction and the large surface area compared to CNT and C60 This also indicates that polymer matrices with high flexibility and mobility of polymer chains tend to be better toughened. It is hoped that this simulation work will provide rational guidance for fabricating high performance of polymer nanocomposites with excellent toughness.

Molecular dynamics simulation of the rupture mechanism in nanorod filled polymer nanocomposites

Through coarse-grained molecular dynamics simulation, we aim to uncover the rupture mechanism of polymer-nanorod nanocomposites by characterizing the structural and dynamic changes during the tension process. We find that the strain at failure is corresponding to the coalescence of a single void into larger voids, namely the change of the free volume. And the minimum of the Van der Walls (VDWL) energy reflects the maximum mobility of polymer chains and the largest number of voids of polymer nanocomposites. After the failure, the stress gradually decreases with the strain, accompanied by the contract of the highly orientated polymer bundles. In particular, we observe that the nucleation of voids prefers to occur from where the ends of polymer chains are located. We systematically study the effects of the interfacial interaction, temperature, the length and volume fraction of nanorods, chain length, bulk cross-linking density and interfacial chemical bonds on the rupture behavior, such as the stress at failure, the tensile modulus and the rupture energy. The rupture resistance ability increases with the increase of the interfacial interaction, rod length, and bulk cross-linking density. With an increase in the interfacial interaction, it induces the rupture transition from mode A (no bundles) to B (bundles). The transition point of the stress at failure as a function of the temperature roughly corresponds to the glass transition temperature. At longer chain length, a non-zero stress plateau occurs. And excessive chemical bonds between polymers and nanorods are harmful to the rupture property. We find that an optimal volume fraction of nanorods exists for the stress-strain behavior, which can be rationalized by the formation of the strongest polymer-nanorod network, leading to the slowest mobility of nanorods.

COMPUTATIONAL SIMULATION OF ELASTOMER NANOCOMPOSITES: CURRENT PROGRESS AND FUTURE CHALLENGES

In the field of elastomer nanocomposites (ENCs), computational simulation technique is becoming more and more essential, as a result of its ability to provide important and clear information at the molecular level, which is always difficult to obtain or not accessible through experimental investigations. We focus on summarizing the progress achieved in the simulation research of three critical topics of ENCs, namely, (i) the dispersion mechanism (particularly polymer-mediated interparticle interaction, the “many-body” effect at high filler loading), (ii) the characterization of the nanoscale/microscale structure and dynamics [the modified chain configuration in the presence of nanoparticles (NPs), the interfacial binding strength determining the efficiency of the stress transfer, the possibly altered interfacial chain structure, interfacial segmental dynamics leading to the shift of the glass transition temperature Tg, the formation of the filler network and its structure, the chemical cross-linking process], and (iii) the macroscopic viscoelasticity (the Payne effect), mechanical reinforcement, and physical property (thermal conductivity). Since recently only limited simulation work has been carried out pertaining to ENCs, we discuss these three topics in light of the simulation and theoretical achievements of polymer nanocomposites (mainly polymer melts filled with NPs). Meanwhile, some relevant experimental studies are also included for better illustration. Furthermore, for each topic, three typically different reinforcing fillers, such as three-dimensional spherical, two-dimensional sheet, and one-dimensional rod NPs, separately corresponding to carbon black or silica, clay sheets, and carbon nanotubes intensively used in the practical applications of ENCs, are illustrated in order. In order to realize a comprehensive understanding of the structure–property relation and in the meantime to provide more practical guidelines for the engineering applications of ENCs, we investigate future simulation opportunities and difficulties.