Semiflexible oligomers crystallize via a cooperative phase transition

Abstract

Semicrystalline polymers are ubiquitous, yet despite their fundamental and industrial importance, the theory of homogeneous nucleation from a melt remains a subject of debate. A key component of the controversy is that polymer crystallization is a non-equilibrium process, making it difficult to distinguish between effects that are purely kinetic and those that arise from the underlying thermodynamics. Due to computational cost constraints, simulations of polymer crystallization typically employ non-equilibrium molecular dynamics techniques with large degrees of undercooling that further exacerbate the coupling between thermodynamics and kinetics. In a departure from this approach, in this study, we isolate the near-equilibrium nucleation behavior of a simple model of a melt of short, semiflexible oligomers. We employ several Monte Carlo methods and compute a phase diagram in the temperature–density plane along with two-dimensional free energy landscapes (FELs) that characterize the nucleation behavior. The phase diagram shows the existence of ordered nematic and crystalline phases in addition to the disordered melt phase. The minimum free energy path in the FEL for the melt–crystal transition shows a cooperative transition, where nematic order and monomer positional order move in tandem as the system crystallizes. This near-equilibrium phase transition mechanism broadly agrees with recent evidence that polymer stiffness plays an important role in crystallization but differs in the specifics of the mechanism from several recent theories. We conclude that the computation of multidimensional FELs for models that are larger and more fine-grained will be important for evaluating and refining theories of homogeneous nucleation for polymer crystallization.

Molecular Dynamics Simulations of the Effects of Entanglement on Polymer Crystal Nucleation

“…Together with advanced sampling methods, MD simulations can also reveal the complex free energy landscapes for the phase transition. By equilibrium and nonequilibrium MD simulations, previous authors demonstrated the important roles of nematic order and precursors in crystal nucleation and suggested plausible multistage nucleation pathways. − Indeed, previous simulation studies greatly enhanced our understanding of the thermodynamics of polymer crystallization. However, the role of polymer dynamics in crystallization kinetics remains largely unexplored.…”

“…Indeed, crystalline order prefers to grow in the less entangled and partially ordered regions. Still, because local orientational order can promote crystal nucleation, ,− the effect of entanglement on polymer nucleation is still not clear.…”

Crystallization of Flexible Chains of Tangent Hard Spheres under Full Confinement

“…Key factors that dictate phase behavior of athermal polymers include the presence of gaps in bond lengths, , volume fraction, , and chain stiffness, − while chain length has a surprisingly minimal role. Simulations have clearly demonstrated that at the same volume fraction oligomers (of average length N = 12) or polymers deep in the polymeric regime ( N = 1000) crystallize with very similar mechanisms. , …”

Interfacial Oriented Precursor to Secondary Nucleation of Alkane Oligomer Crystals Revealed by Molecular Dynamic Simulations

“…In primary nucleation, the thermodynamic analysis by Kawak et al found a tandem growth of the global crystalline and nematic order near equilibrium, suggesting no equilibrium intermediate phase, while Zhang and Larson demonstrated a metastable nematic precursor to crystallization below an isotropic–nematic transition temperature predicted by theory. MD simulations of secondary nucleation by Bourque et al revealed a surface nucleation mechanism, wherein each new crystalline layer nucleates and spreads over the layer below it.…”

“…Milner found from a self-consistent field theory that nucleation from an intermediate nematic rotator phase is favored throughout the temperature range where crystal nucleation is observed. Kawak et al found from multiple Monte Carlo simulations that under near-equilibrium conditions the melt–crystal transition is smooth and cooperative, with nematic alignment and positional ordering occurring simultaneously. The phase transition behavior was speculated to differ under deeper quenches, based on their observation that the temperature-dependent polymer stiffness plays an important role in the cooperativity between nematic and crystalline order.…”