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Understanding the deformational and failure behaviors of thermoplastic semicrystalline polymers is crucial due to the practical usages in various engineering applications. Taking isotactic polypropylene (iPP) as a semicrystalline polymer model system, atomistically informed coarse‐grained (CG) molecular dynamics (MD) simulations are employed to investigate the creep behavior of iPP. The simulations reveal that there exists a threshold stress of about 20.0 MPa, above which the maximum strain of iPP within the simulation time span increases dramatically. From the strain‐time analysis, it is observed that the iPP exhibits an initial elastic deformation stage and a subsequent plastic stage at lower stress levels, while a three‐stage creep behavior including a third fracture stage is observed at higher stress levels. Specifically, at lower stress levels, the bonded energy increases continuously as the chains stretch steadily, while the nonbonded energy shows an initial increase followed by a steady decrease due to the interchain sliding. At higher stress levels, both bonded and nonbonded energies change dramatically at the third stage, resulting from accelerated chain stretching, unfolding, sliding, and breaking. This study provides physical insight into the creep behavior of iPP at a fundamental molecular level and highlights the important role of microstructural evolution of chains in the deformation of semicrystalline polymer materials. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 1779–1791

With the incorporation of carbon nanotubes (CNTs), CNT/polypropylene (PP) nanocomposites are found to possess enhanced mechanical properties, but the reinforcing effect is reduced at large added CNT weight percentages due to CNT aggregation. Optimizing the properties of a nanocomposite requires a fundamental understanding of the effects of CNT dispersion on the nanocomposite. In this work, coarse-grained molecular models of CNT/PP nanocomposites are constructed, which consist of randomly dispersed or aggregated CNT bundles. Our simulation results reveal that with randomly dispersed CNT bundles, the nanocomposite shows properties that continuously improve with increasing CNT contents due to the effective CNT/PP interface and the reinforcing effect of CNTs. By comparison, the nanocomposite with aggregated CNT clusters exhibits a decline in yield strength at CNT contents over 3 wt%, which results from a reduced CNT load-carrying capacity due to the formation of structural voids in the interfacial region. This study achieves an in situ observation of the structural void evolution of loaded nanocomposites, provides valuable insights into the effects of CNT dispersion on the mechanics of CNT/PP nanocomposites, and paves the way for optimizing the design of nanocomposites with superior mechanical properties by designing the CNT dispersion in the structure.

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