Entropy-driven crystallization of flexible chains of hard spheres under confinement

Through extensive Monte Carlo simulations, we study the heterogeneous crystallization of linear, fully flexible chains of tangent hard spheres under confinement in one or three dimensions. Confinement is realized through the presence of flat, impenetrable, and parallel walls. The phase behavior is studied by employing a wide range of systems with varied average chain lengths and packing densities. For that, crystal nucleation and growth are analyzed through the Characteristic Crystallographic Element (norm) descriptor. The phenomenon is naturally split into two distinct contributions: surface crystallization on the walls, and bulk crystallization far from them. Depending on the simulation conditions, different morphologies are established in the bulk zone ranging from random hexagonal close-packed layers with a unique stacking direction, perpendicular to the walls (confinement in one dimension) to predominantly hexagonal close-packed (confinement in all dimensions) crystals. In parallel, surface crystals show perfection with a predominantly triangular character. Finally, we analyze the entropic origins of the phase transition and identify the similarities and differences with respect to the bulk, homogeneous crystallization.