Entropy-driven crystallization in dense systems of athermal chain molecules

Area of Science:

• Materials Science
• Computational Chemistry
• Statistical Mechanics

Background:

• Understanding crystallization in complex molecular systems is crucial for materials design.
• Dense packings of polymers and similar chain-like molecules present unique ordering challenges.
• Entropy's role in phase transitions is a fundamental concept in thermodynamics.

Purpose of the Study:

• To directly observe and characterize entropy-driven crystallization in simulations.
• To investigate the influence of chain length on crystal nucleation and morphology.
• To elucidate the entropic mechanisms governing the ordering transition in hard-sphere chains.

Main Methods:

• Molecular dynamics simulations of dense packings of linear hard-sphere chains.
• Analysis of crystal nucleation events and their spontaneous formation.
• Characterization of incipient nuclei morphology, including stacking and layering.
• Investigation of particle mobility and local free volume changes during ordering.

Main Results:

• Spontaneous formation of crystal nuclei observed in the phase coexistence region.
• Nucleation occurred independently of chain length for the simulated hard-sphere chains.
• Incipient nuclei consistently displayed well-defined, stack-faulted layered morphologies.
• Observed morphologies significantly differed from those of monomeric analogs.
• The ordering transition was confirmed to be driven by increased translational entropy.

Conclusions:

• Entropy is a primary driver for crystallization in dense packings of linear hard-sphere chains.
• The unique layered morphologies observed are a distinct feature of chain crystallization.
• Enhanced particle mobility and more symmetric free volume contribute to the entropic driving force for ordering.