Kawak Pierre, Banks Dakota S, Tree Douglas R
Department of Chemical Engineering, Brigham Young University, Provo, Utah 84602, USA.
J Chem Phys. 2021 Dec 7;155(21):214902. doi: 10.1063/5.0067788.
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.
半结晶聚合物无处不在，然而尽管它们具有基础和工业重要性，但从熔体中均匀成核的理论仍然是一个有争议的话题。争议的一个关键因素是聚合物结晶是一个非平衡过程，这使得难以区分纯粹动力学效应和由潜在热力学产生的效应。由于计算成本的限制，聚合物结晶的模拟通常采用具有大过冷度的非平衡分子动力学技术，这进一步加剧了热力学和动力学之间的耦合。与这种方法不同，在本研究中，我们分离了短的半柔性低聚物熔体简单模型的近平衡成核行为。我们采用了几种蒙特卡罗方法，并计算了温度 - 密度平面中的相图以及表征成核行为的二维自由能景观（FEL）。相图显示除了无序熔体相之外还存在有序向列相和结晶相。熔体 - 晶体转变的FEL中的最小自由能路径显示出协同转变，即随着系统结晶，向列序和单体位置序同步移动。这种近平衡相变机制与最近关于聚合物刚度在结晶中起重要作用的证据大致一致，但在机制细节上与最近的几种理论不同。我们得出结论，对于更大且更精细的模型计算多维FEL对于评估和完善聚合物结晶均匀成核理论将是重要的。
