Screening nucleating agents for polymer crystallization by molecular simulation

Presenter: 

Alexander J Bourque

Authors: 

Alexander J Bourque, Gregory C. Rutledge, C. Rebecca Locker

Author Affiliation: 

MIT, MIT, ExxonMobil Research and Engineering

Abstract: 

Molecular dynamics simulations were performed to assess the effect of various additives on the nucleation and growth of an oligoethylene. Heterogeneous nucleation was studied for substrates belonging to a family of materials that are isomorphous with the cubic unit cell structure of diamond. This family of materials is of particular interest because they are all tetrahedrally coordinated and can be described through the systematic variation of a few parameters in the Stillinger-Weber (SW) potential, thereby permitting a comprehensive genomic characterization of this additive class using a relatively small number of adjustable parameters. Crystallized silicon (S), germanium (Ge), diamond (C) and ice (H2O) all belong to this family of materials and can be identified with particular points in the parameter space. A suite of techniques developed to study nucleation and growth in oligoethylene by molecular simulation were used to quantify the impact of different materials on the resulting crystallization behavior. A scan of the dominant crystal planes of the diamond cubic lattice showed that even the arrangement of atoms in the substrate suffices to induce a change in surface nucleation. This result is consistent with an epitaxial mechanism, wherein the additive material serves as a template for the organic crystal nucleus. Within the parameter space of the SW potential, there resides a continuum of √ítheoretical√ď materials, to which molecular simulation can be applied systematically to optimize the nucleating agent activity for this family of additives.