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Atomistic simulations of spinodal phase separation preceding polymer crystallization

Nature Materials volume 5, pages 39–43 (2006)Cite this article

A Corrigendum to this article was published on 01 February 2009

An Erratum to this article was published on 25 December 2005

Abstract

Many polymeric materials crystallize when cooled below their melting temperature. Although progress has been made in our understanding of the crystallization process through both experimental1 and theoretical2,3,4,5,6 efforts, these studies have focused mainly on the crystal nucleation and growth mechanism, where critical nuclei are formed from a metastable state during the first stages of crystallization, leading ultimately to the growth of crystal domains. Attention has also been given to the structure during the precrystallization (induction period7,8,9,10,11,12,13). A pretransition state occurring before crystallization has been characterized as an unstable phase separation initiated by density and orientational fluctuations. These fluctuations are caused by an increase in the average length of rigid trans segments along the polymer backbone during the induction period. These observations are consistent with the theory proposed in ref. 14 on the isotropic-to-nematic transition of polymer liquid crystals, that is, the parallel ordering of polymers is caused by an increase in chain rigidity. Here we use large-scale computer simulations to investigate melts of polymers in the early ordering stages (induction period) before crystallization. In the ordered domains we identify growing dense regions similar to smectic liquid crystals. Our simulations reveal a ‘coexistence period’ in the ordering before crystallization, where nucleation and growth mechanisms coexist with a phase-separation mechanism.

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Figure 1: Snapshots from the molecular dynamics simulations showing the evolution of the spinodal-assisted crystallization process for the polar (orange ‘atoms’) and nonpolar (grey ‘atoms’) polymer models at a temperature of 600 and 450 K, respectively.
Figure 2: Evolution of the structure factor, S (q,t ), for the polar and nonpolar (768-bead) polymer model at 600 and 450 K, respectively.
Figure 3: The evolution of the average segment length, the integrated intensity of the induction peak and long-period peak, and the evolution of the second-order Bragg peak location for polar and nonpolar polymers.
Figure 4: Chains entering a single ordered polymer domain show a fringed-micelle-like morphology for the polar polymer model (similar results are seen for polyethylene).

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Acknowledgements

The work was performed under the auspices of the US Department of Energy by the University of California Lawrence Livermore National Laboratory under Contract W-7405-Eng-48. We thank Livermore Computing for generous amounts of CPU time on Thunder and MCR clusters.

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  1. University of California, Lawrence Livermore National Laboratory, Chemistry and Materials Science Directorate, Livermore, California, 94551, PO Box 808, L-268, USA

    Richard H. Gee, Naida Lacevic & Laurence E. Fried

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  1. Richard H. Gee
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Correspondence to Richard H. Gee.

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Gee, R., Lacevic, N. & Fried, L. Atomistic simulations of spinodal phase separation preceding polymer crystallization. Nature Mater 5, 39–43 (2006). https://doi.org/10.1038/nmat1543

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