![]() ![]() As a result, the dominant failure mechanism of composite samples depends strongly on the length of the grafted chains, with disentanglement being the dominant mechanism for short chains, while bond breaking is the failure mode for chain lengths >10N e, where N e is the entanglement length. In the craze growth regime, the presence of “grafted chain” sections of ≈100 monomers alters the mechanical response of composite samples, giving rise to smaller extension ratios and higher drawing stresses than for the homopolymer matrix. Increasing the attraction between nanoparticle core and the grafted polymer inhibits more » void nucleation and leads to a higher yield stress. ![]() The yield behavior is found to be mostly controlled by the local nanoparticle-grafted polymer interfacial energy, with the grafted polymer-polymer matrix interfacial structure being of little to no relevance. We focus on the three key differences in the crazing behavior of a composite relative to the pure homopolymer matrix, namely, a lower yield stress, a smaller extension ratio, and a grafted chain length dependent failure stress. The crazing behavior of polymer nanocomposites formed by blending polymer grafted nanoparticles with an entangled polymer melt is studied by molecular dynamics simulations. Our results show that a few entanglements across the interface are sufficient to resist interfacial chain pullout and enhance the mechanical strength. Chain stiffness increases the density of entanglements, which increases the strength of the interface. When the strength of the interface saturates, the number of interfacial entanglements scales with the corresponding bulk entanglement density. They are less entangled and as a result they mechanically weaken the interface. At saturation, cut chains remain near the healing interface. However, the saturation strength of the damaged film is below the bulk strength for the polymer sample. For both healing and welding, the interfacial strength saturates as the bulk entanglement density is recovered across the interface. As a result, the interfacial strength of the healing film increases more slowly than for welding. Though interfacial entanglements increase more rapidly for the damaged films, a large fraction of these entanglements are near chain ends. We find that the diffusion across the interface is signifcantly faster in the damaged film compared to welding because of the presence of short chains. more » The mass uptake and formation of entanglements, as obtained from primitive path analysis, are extracted and correlated with the interfacial strength obtained from shear simulations. The recovery of the damaged film was followed as time elapsed and polymer molecules diffused across the interface. A polymer sample was cut into two separate films that were then held together in the melt state. These two processes differ from each other in their interfacial structure since damage leads to increased polydispersity and more short chains. Using molecular dynamics simulations we probe the healing of polymer films and compare the results with those obtained for thermal welding of homopolymer slabs. Self-healing of polymer films often takes place as the molecules diffuse across a damaged region, above their melting temperature. =, to cause energy dissipation through craze formation. Even small degrees of immisciblity reduce interfacial entanglements enough that failure occurs by chain pullout and G I << G b. Immiscibiltiy limits interdiffusion and more » thus suppresses entanglements at the interface. Before saturation, G I is proportional to the areal density of interfacial entanglements. As in previous studies of shear strength, saturation coincides with the recovery of the bulk entanglement density. As in experiment, G I increases as t 1/2 before saturating at the average bulk fracture energy G b. The interfacial fracture energy G I is calculated by coupling the simulation results to a continuum fracture mechanics model. The failure stress of the craze rises with welding time and the mode of craze breakdown changes from chain pullout to chain scission as the interface approaches bulk strength. Once chains have formed an average of about one entanglement across the interface, a stable craze is formed throughout the sample. At small t welded interfaces are not strong enough to support craze formation and fail at small strains through chain pullout at the interface. Bulk polymers fail through craze formation, followed by craze breakdown through chain scission. Changes in the tensile stress, mode of failure and interfacial fracture energy G I are correlated to changes in the interfacial entanglements as determined from Primitive Path Analysis. Large-scale molecular simulations are performed to investigate tensile failure of polymer interfaces as a function of welding time t. ![]()
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