Crystallization of Flexible Chains of Tangent Hard Spheres under Full Confinement

We present results from extensive Monte Carlo simulations
on the
crystallization of athermal polymers under full confinement. Polymers
are represented as freely jointed chains of tangent hard spheres of
uniform size. Confinement is applied through the presence of flat,
parallel, and impenetrable walls in all dimensions. We analyze crystallization
as the summation of two contributions: one that occurs in the bulk
volume of the system (bulk crystallization), and one on the wall surfaces
(surface crystallization). Depending on volume fraction initially
amorphous (disordered) hard-sphere chain packings transit to the stable
crystal phase. The established ordered morphologies consist primarily
of hexagonal close-packed (HCP) crystals in the bulk volume and of
triangular
(TRI) crystals on the surface. As in the case of athermal packings
in the bulk (without confinement), a structural competition is observed
between the 5-fold local symmetry and the formation of close-packed
crystallites. Effectively, the full confinement inside a cube favors
the growth of the HCP crystal, as the FCC one is quite incompatible
with the imposed spatial constraints. Consequently, we observe the
formation of noncompact ordered motifs which grow from the surface
to the inner volume of the simulation cell. We further compare the
2D and 3D crystals formed by monomeric hard spheres under the same
simulation conditions. Significant differences are observed at low
densities that tend to diminish as concentration increases.