  The crazing behavior of polymer nanocomposites formed by blending polymer
grafted nanoparticles with an entangled polymer melt is studied by molecular
dynamics simulations. 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 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.
Increasing the attraction between nanoparticle core and the grafted polymer
inhibits void nucleation and leads to a higher yield stress. 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. 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 greater
than 10Ne, where Ne is the entanglement length.
