280122 Molecular Simulation of Homogeneous Crystal Nucleation From Entangled Polymer Melts

Wednesday, October 31, 2012: 3:45 PM
Butler West (Westin )
Peng Yi and Gregory C. Rutledge, Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA

One of the most important phenomena in molecular systems is homogeneous nucleation of the crystal phase from a melt.  This phenomenon is particularly interesting for polymers due to their hierarchy of multiple time and length scales. In this work we report the results of molecular simulations of homogeneous crystal nucleation from the melts of entangled C150 molecules.  We employed a realistic united atom force field which reproduces the experimental melting temperature [1, 2].  MD simulations were used to study the crystal nucleation process at super-cooling ranging from 10% to 30%.  In the C150 melts, we have observed a two-step crystal nucleation.  The super-cooled melt first transforms through a collective structural change into a denser melt phase with a higher trans state population, then crystallites appear from the denser melt by nucleation and growth mechanism.  The nucleation rate and the critical nucleus size estimated by the Mean First-Passage Time method are comparable to those in C20 melts, suggesting that crystal nucleation is a local event and insensitive to the chain length.  Detailed examination of the simulations reveals the critical nucleus to be a bundle of stretched segments of about 10 CH2 groups long, organized into a cylindrical shape with chain folding occurring in the early nucleation stage.  The thickness of the cylindrical nuclei increases during the growth stage until the system is fully crystallized or the entanglements of the chains create an un-crystallizable amorphous region between the neighboring crystallites. 

1.             Yi, P. and G.C. Rutledge, Molecular simulation of crystal nucleation in n-octane melts. The Journal of Chemical Physics, 2009. 131(13): p. 134902.

2.             Yi, P. and G.C. Rutledge, Molecular simulation of bundle-like crystal nucleation from n-eicosane melts. The Journal of Chemical Physics, 2011. 135(2): p. 024903-11.

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See more of this Session: Structure and Properties in Polymers II
See more of this Group/Topical: Materials Engineering and Sciences Division