(295d) Molecular Modeling of Polymer Crystallization: Heterogeneous Nucleation

Nucleation phenomena in both small molecules and polymers pose a number of challenges for our molecular level understanding of the thermodynamic principles involved. The classical nucleation theory (CNT) offers a starting point, but does not derive from molecular principles itself. In the thermodynamic formulation of CNT, a small cluster consisting of the stable phase is separated from the surrounding, metastable phase by an interface. In the capillary approximation, the interfacial energy and the free energy difference is based on the macroscopic phases. However, for many systems of interest, the cluster itself is a multi-molecular object with potentially important fluctuations and finite size effects. In the case of crystal nucleation from the melt, transformation from the metastable phase to the stable phase via intermediate phases or structures (e.g. two-step nucleation) or reconstruction at the cluster surface, e.g. due to curvature, requires a closer examination of the molecular interactions involved. Unfortunately, due to the small spatiotemporal times scales involved, direct examination of nucleation events in the laboratory remains a significant challenge. Furthermore, for many systems of practical interest, including polymers and pharmaceuticals, nucleation occurs predominantly at the surface of foreign particles or impurities, called heterogeneous nucleation, which then plays a determining role in the final crystalline polymorph or morphology observed. There exists, therefore, the opportunity to manipulate the final structure and properties of a material through the design and selection of suitable foreign additives, called nucleating agents, that mediate the phase transition through heterogeneous nucleation.

In this talk, we discuss the development of molecular simulations that examine the phenomenon of crystal nucleation of a simple chain molecule from the melt, with particular emphasis on surface nucleation on both familiar (self-nucleation) and foreign (heterogeneous nucleation) surfaces. Through an analysis of self-nucleation [1], we obtain an atomistically detailed description with which to examine the assumptions of the classical Hoffman-Lauritzen theory of secondary nucleation for polymer crystal growth [2]. Using a multi-scale modeling approach, we then obtain a connection between molecular parameters and the observed regime behavior of polymer crystallization [3]. To study hetetrogeneous nucleation, we replace the polymer crystal surface with that of a foreign material, or nucleating agent. By systematically varying the intermolecular force field parameters that describe the foreign material, one can rapidly screen entire classes of nucleating agents to characterize both their mechanism of action and nucleation efficiency [4]. In this way, a materials genome for nucleating agents can be constructed. The approach is demonstrated with the heterogeneous nucleation of n-pentacontane, a surrogate for polyethylene, on members of the family of tetrahedrally coordinated, diamond-like materials and on the family of 2D materials like graphene with hexagonally coordinated atomic layers. From an analysis of induction times, one can extract useful heuristics regarding the design of nucleating agents for linear alkanes and polyethylene. By application of machine learning methods to this problem, we envision that high throughput computational screening of nucleating agents becomes possible.

References

[1] A.J. Bourque, C.R. Locker, G.C. Rutledge, Macromolecules 49, 3619 (2016).

[2] J.I. Lauritzen, J.D. Hoffman, J. Res. Natl. Bur. Std A 64,73 (1970).

[3] A.J. Bourque, G.C. Rutledge, Macromolecules 49, 3956 (2016).
[4] A.J. Bourque, C.R. Locker, G.C. Rutledge, J. Phys. Chem. B 121, 904 (2017).