“Plunger” Method for Simulating Crystal–Melt Interfacial Free Energies

Semiflexible oligomers crystallize via a cooperative phase transition

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.

A metastable nematic precursor accelerates polyethylene oligomer crystallization as determined by atomistic simulations and self-consistent field theory

Using PYS, TraPPE, OPLS-L, and Flexible-Williams (FW) force field models, atomistic simulations at temperatures ranging from 450 K to 600 K are performed to predict the melt density ρ, the persistence length N_p, the nematic coupling constant α, and crystallization dynamics for pentacontane (C50). The coupling constant α arises from packing entropy of rodlike Kuhn segments and increases with increasing ρ and N_p. Together with a self-consistent field theory, N_p and α are then used to predict the isotropic-to-nematic (IN) transition temperature for polyethylene (PE) oligomers as a function of chain length. The nematic phase is found to be metastable since the IN transition temperature lies below the crystal melting temperatures for C50 in simulations using different force fields. Finally, isothermal simulations of crystallization for PE C50 oligomers and C1000 polymers show that crystal nucleation may be much accelerated by quenching below the IN transition temperature, where chains in the isotropic state first rapidly form nematic ordered domains, within which crystalline order then grows. We also find that the PYS, TraPPE, and FW models overpredict the melting temperature for C50 by around 50 K, while the most flexible OPLS-L model gives a melting temperature within around 10 K of the experimental value. Although giving a more accurate melting temperature, the slow crystallization kinetics of the OPLS-L model may limit its application in direct simulations of PE crystallization.

Calculations of free energy of surface interactions in crystalline polyethylene

The surface free energy of the crystalline polyethylene (PE) is an important property related with wettability, adhesion, and crystal growth. We investigated the profiles of free energy of surface interactions in the fully thermalized crystalline PE during debonding and shearing with atomistic molecular dynamics simulations using steered molecular dynamics and umbrella sampling techniques. The stress profiles during debonding and shearing processes were also estimated and compared with those obtained from analogous deformation simulations. We estimated the vacuum surface free energies of two different crystallographic surfaces (100) and (010) of the crystalline PE from the free energy changes during the debonding process. The estimated surface free energies were insensitive to the choice of simulation protocols after combining estimates from both forward and backward processes and were in excellent agreement with those obtained from an experiment on PE single crystal aggregates, which underscores the importance of the inclusion of the entropic contribution in the free energy calculated with the fully flexible interface adopted in this study.