Phase Behavior of Athermal Colloidal Mixtures of Chains and Monomers

Through extensive Monte Carlo simulations, we study the phase behavior of systems composed of freely jointed, hard-sphere polymers and monomers at different number fractions. This work is inspired by the fact that, despite their similarities in crystallization, the melting point of hard-sphere chains [1] is higher than that of their monomeric counterparts [2].

System configurations are generated, equilibrated, and successively analyzed through the Simu-D software [3]. The equilibration part is primarily based on chain-connectivity-altering [4] and identity-exchange Monte Carlo algorithms, especially designed for mixtures of chains and monomers of the same chemical constitution. The structural identification of the computer-generated system configurations is performed through the characteristic crystallographic element (CCE) norm descriptor [5].

We systematically study how both the packing density and the relative number fraction affect the ability of the systems to crystallize. We further identify the entropic origins of the phase transition and the difference in the local environment between spheres belonging to chains and individual ones. Depending on the simulation conditions, different morphologies are established ranging from predominantly amorphous packings to crystal morphologies of mixed hexagonal close-packed (HCP) and face-centered cubic (FCC) character. Extensions of the present work include the molecular simulation of athermal mixtures based on semi-flexible polymers.

[1] N. C. Karayiannis, K. Foteinopoulou, and Manuel Laso, Phys. Rev. Lett 103, 045703 (2009).

[2] B. J. Alder, and T. E. Wainwright, J. Chem. Phys. 27, 1208 (1957).

[3] M. Herranz et al., Int. J. Mol. Sci. 22, 12464 (2021).

[4] P. M. Ramos, N. C. Karayiannis, and M. Laso, J. Comput. Phys. 375, 918 (2018).

[5] P. M. Ramos et al., Crystals 10, 2073 (2020).