Surface Nucleation and the Design of Nucleating Agents by Molecular Simulation

Alexander J. Bourque1, C. Rebecca Locker2, and Gregory C. Rutledge1

1Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge MA, USA
2ExxonMobil Research and Engineering Company, Annandale, NJ, USA

In most cases of practical interest, nucleation of polymer crystallites is believed to occur heterogeneously, through the action of additives or impurities that serve as nucleation agents, significantly reducing the activation barrier to formation of a new phase [1]. The number and activity of these nucleating agents can alter the resulting morphology of the semicrystalline polymer, for example through polymorph selection, the number and size of spherulites or the orientation of individual lamellae; the properties of the polymer can also be controlled as a consequence. The classical theories of polymer crystal growth are similarly predicated on a surface nucleation mechanism, which may proceed through one or more precursor states. We report the results of molecular simulations that examine the phenomenon of 2-dimensional nucleation on both familiar and foreign surfaces. In the first case, we obtain an atomistically detailed model for surface nucleated crystal growth that permits evaluation of some of the basic tenets of classical polymer crystal growth theories [2]. We then combine it with a continuum level model of layer-by-layer growth to create a multi-scale description of crystallization kinetics in chain molecules [3]. In the second case, we show that by systematically varying the intermolecular force field parameters that describe the foreign surface, one can rapidly screen entire classes of nucleating agents to characterize both their mechanism of action and nucleation efficiency [4]. The method is demonstrated with the crystallization of n-pentacontane, a surrogate for polyethylene, on members of the family of diamond-like materials and on those like graphene with 2D, hexagonally coordinated atomic layers. We show for the first time that, in addition to epitaxial registration and strength of adhesive interactions, the rigidity of the nucleating agent influences heterogeneous nucleation. Employing this method, high throughput computational screening of nucleating agents becomes possible.

[1] G. Strobl, The Physics of Polymers, 2nd Ed., Springer: Berlin (1997). (link)
[2] A.J. Bourque, C.R. Locker, G.C. Rutledge, Macromolecules 49, 3619 (2016). (link)
[3] A.J. Bourque, G.C. Rutledge, Macromolecules 49, 3956 (2016). (link)
[4] A.J. Bourque, C.R. Locker, G.C. Rutledge, J. Phys. Chem. (in press). (link)