Fatigue Behavior of Polymer Nanocomposites under Low-Strain\nCyclic Loading: Insights from Molecular Dynamics Simulation

Understanding\nthe structural evolution and bond-breaking behavior\nunder cyclic loading is crucial for designing polymer nanocomposites\n(PNCs) with superior fatigue resistance. Coarse-grained models of\nPNCs filled with spherical carbon black nanoparticles (NPs) at varying\nfilling fractions of φ were successfully constructed using molecular\ndynamics simulations. Structural and dynamic simulation results reveal\nthat higher φ leads to increased aggregation of NPs and markedly\nrestricts the relaxation behavior of the polymer matrix. Subsequently,\nfatigue testing of PNCs was conducted under low-strain cyclic tensile\ndeformation, and the real-time bond-breaking behavior was tracked.\nThe decay behavior of the bond number autocorrelation function was\nfound to be accurately described by the KWW equation, enabling precise\ndetermination of the characteristic lifetime τ<sub><i>f</i></sub>. With increasing φ, the dominant factor influencing\nbond-breaking behavior gradually shifts from the polymer network,\nincluding entanglements and cross-linking networks, to the filler\nnetwork. This suggests the presence of a critical filling fraction\nφ<sub><i>c</i></sub> where τ<sub><i>f</i></sub> is maximized. For low-strain failure mechanisms, temperature\nfield observations at varying cycles reveal that localized temperature\nrise emerges as the predominant factor. Furthermore, the mobility\nof both polymers and NPs increases with cycles. Specifically, the\ndiffusion coefficient of polymer monomers shows a clear power-law\nrelationship with the bond-breaking rate, characterized by D∼fbroken1.5. Finally, the stiffness of polymer chains\nsignificantly influences the fatigue behavior, evidenced by an initial\nincrease followed by a decrease in the τ<sub><i>f</i></sub> with increasing bending energy <i>k</i>. This behavior\nis attributed to the competitive relationship between high entanglement\ndensity at low <i>k</i> and enhanced preorientation at high <i>k</i>. In summary, this study provides a general paradigm for\ndescribing failure behavior under cyclic deformation and offers insights\ninto fatigue mechanisms at the molecular level, thereby guiding the\ndevelopment of improved fatigue-resistant PNCs.