  We focus on polymer-grafted nanoparticles (PGNP). A PGNP is composed of two
different layers: the hard core of a nanoparticle and the soft corona of
grafted polymers on the surface. It is predicted that PGNPs with these two
distinct layers will have similar behaviors as star polymers and hard spheres.
The interaction between PGNPs strongly depend upon their grafting density and
the length of the grafted polymer chains, N. Thus, PGNP may exhibit
polymorphism. Moreover, it is expected that crystals made from PGNPs will be
structurally tough due to the entanglement of grafted polymers. The crystal
polymorph of PGNP is explored using molecular dynamics simulations. We
succeeded in finding FCC/HCP and BCC crystals depending on the length of the
grafted polymer chain. When N is small, PGNPs behave like hard spheres. The
crystals formed are arranged in FCC/HCP structure, much like the phase
transition observed in an Alder transition. When N is large enough, the
increase in the free energy of grafted polymers can no longer be neglected.
Thus, the crystals formed in these systems are arranged in BCC structure, which
has a lower density than FCC/HCP. When N is not too small or large, FCC/HCP
structures are observed when the concentration of PGNPs is low, but a phase
transition occurs when the concentration of PGNPs becomes higher with the
compression of the system. Again, the increase in free energy of grafted
polymers can no longer be neglected and the crystals arrange themselves in a
BCC structure. Also, it can be revealed that the lattice spacing of PGNP
crystals can be controlled easily and widely by the chain length. These results
should play an important role in many future simulations and experimental
studies of PGNP crystals.
