Heterogeneous nucleation of polyethylene crystals on binary hexagonal nanoplatelets

Abstract

Crystal nucleating agents offer an effective strategy for controlling the morphology, dimensional stability and rate of solidification of polymers during processing. Molecular dynamics (MD) simulation can shed light on nucleation behavior at the nanoscopic length and time scales over which nucleation occurs. In this work, crystal nucleation of a polyethylene oligomer, n-pentacontane, on three graphene-like substrates, hexagonal boron nitride (hBN), molybdenum disulfide (MoS2), and tungsten disulfide (WS2), was simulated, and the thermodynamic efficiencies of these substrates as nucleating agents were determined. Experimental measurements of heterogeneous nucleation of a high-density polyethylene on nanoparticles of these three graphene-like materials were performed using the method of dispersed microdroplets in an immiscible polystyrene matrix. Qualitative agreement between simulations and experiments was observed for trends in nucleation rate, J, and interfacial free energy difference, Δσ, with JhBN>JMoS2>JWS2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$J_{\ ext{hBN}} > J_{\ ext{MoS}_{2}} > J_{\ ext{WS}_{2}}$$\\end{document}. The simulations are then used to gain additional insight into the mechanisms of nucleation. Epitaxy is confirmed in all systems, with small mismatches in lattice spacing being accommodated by strain in the oligomer crystal. However, epitaxy alone is insufficient to explain the observed trends. The strength of interaction between the nucleating agent and the polyethylene oligomer is found to be the strongest predictor of nucleating agent efficiency. The strength of interaction is in turn related to the density of interaction sites at the interface: hBN has the highest density, and thus the fastest nucleation rate.