Interfacial Oriented Precursor to Secondary Nucleation of Alkane Oligomer Crystals Revealed by Molecular Dynamic Simulations

Abstract

In atomistic molecular dynamics simulations, alkane oligomers (C50 or C100) rapidly form an oriented interface when placed in contact with a crystal slab of stretched periodic polyethylene chains. The oriented atoms in this interface have a similar order parameter to those of nematic atoms. After a quench below the melting point, we show that this oriented “nematic” interface thickens from around two to three layers thick and crystalline order nucleates from this layer onto the crystal-slab surface and spreads as a two-dimensional patch. Once a crystal patch is large enough, the oriented interface above it advances by forming a stable nematic patch three layers above the crystal nucleus which grows and eventually nucleates a crystal patch within it. Simulation snapshots and mean-first-passage time (MFPT) results prior to reaching steady-state growth suggest that the nematic-to-crystal transition is rate-determining, as it is much slower than the thickening of the induced oriented interface. After steady state is established, the rate of crystallization of C100 at 360 K is determined roughly equally by the rates of nucleation and of spreading of a new crystal patch to the size large enough to propagate the nematic growth front. These findings, along with those of Bourque and Rutledge (Bourque, A. J.; Rutledge, G. C. Macromolecules 2016, 49, 3956−3964) contrast sharply with the stem-by-stem growth assumed in the Hoffman–Lauritzen theory of secondary nucleation, with the work reported here indicating the importance of the oriented “nematic” layer in the propagation of the crystalline front.

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