Entropy-Driven Heterogeneous Crystallization of Hard-Sphere Chains under Unidimensional Confinement.

Pablo Miguel Ramos,Manuel Laso,Nikos Ch Karayiannis,Miguel Herranz,Katerina Foteinopoulou

doi:10.3390/polym13091352

Pablo Miguel Ramos, Manuel Laso + Show 3 more

Open Access

https://doi.org/10.3390/polym13091352

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Journal: Polymers	Publication Date: Apr 21, 2021
Citations: 10	License type: CC BY 4.0

Affiliation: Universidad Politécnica de Madrid

Abstract

We investigate, through Monte Carlo simulations, the heterogeneous crystallization of linear chains of tangent hard spheres under confinement in one dimension. Confinement is realized through flat, impenetrable, and parallel walls. A wide range of systems is studied with respect to their average chain lengths (N = 12 to 100) and packing densities (φ = 0.50 to 0.61). The local structure is quantified through the Characteristic Crystallographic Element (CCE) norm descriptor. Here, we split the phenomenon into the bulk crystallization, far from the walls, and the projected surface crystallization in layers adjacent to the confining surfaces. Once a critical volume fraction is met, the chains show a phase transition, starting from regions near the hard walls. The established crystal morphologies consist of alternating hexagonal close-packed or face-centered cubic layers with a stacking direction perpendicular to the confining walls. Crystal layer perfection is observed with an increasing concentration. As in the case of the unconstrained phase transition of athermal polymers at high densities, crystal nucleation and growth compete with the formation of sites of a fivefold local symmetry. While surface crystallites show perfection with a predominantly triangular character, the morphologies of square crystals or of a mixed type are also formed. The simulation results show that the rate of perfection of the surface crystallization is not significantly faster than that of the bulk crystallization.

Highlights

The phase and morphology of a system dictate its macroscopic properties
We present a description of the mechanism of the crystal nucleation and growth of polymers of tangent hard spheres under unidimensional confinement, as observed in extensive Monte Carlo (MC) simulations
We analyze crystallization through two different mechanisms: surface and bulk crystallization, each quantified through the application of the Crystallographic Element (CCE)-norm descriptor

Summary

Introduction

The phase and morphology of a system dictate its macroscopic properties. understanding crystal nucleation and growth as a function of processing history and conditions could lead to the design of novel materials with improved properties. And pioneering molecular dynamics simulations have unmistakably shown that hard sphere systems crystallize [12]. For such phase transition to take place, the two critical conditions are: (i) a critical volume fraction is reached; and (ii) the evolution of the system is tracked for a sufficiently long time [13]. Crystal morphologies of athermal polymers adopt random hexagonal close-packed layers of hexagonal closepacked and face-centered cubic characters. These alternating layers possess a unique stacking direction and are free of twin defects, as the latter are incompatible with the entropic barriers imposed by chain connectivity [15]. The critical phase transition of hard-sphere chains is heavily affected by factors, such as bond gaps [16,17] and/or chain stiffness [18]

Results

Discussion

Conclusion