2018-11-28
10:15 at HCP F 43.4

Atomistic simulation of structural glasses: Towards realistic time-scales

Markus Hütter

Department of Mechanical Engineering Eindhoven University of Technology, The Netherlands

Physical aging, which originates from slow ongoing changes in the microstructure, is a hallmark of structural glasses, and affects many physical properties, e.g. the yield stress. This presentation addresses the modeling of such materials in terms of the energy-landscape picture. More specifically, we strive to unravel the atomistic origins of physical aging and related features of the glass. The results of a Molecular Dynamics (MD) simulation of atactic polystyrene show that, in the course of time, the system visits several basins in the energy landscape, which are envisioned as discrete states [1, 2] connected to a network by infrequent transitions. It can be shown that this network has scale-free and small-world characteristics [3]. In order to overcome time-scale limitations in MD simulations, we have implemented a computational technique that focuses on generating the network of discrete states connected by infrequent transitions over saddle points, while rapid intra-basin properties are captured by the basin free energy. Using this technique, good agreement is obtained with experimental data on infrared-spectroscopy, NMR, dielectric spectroscopy, and mechanical properties. The major benefit of the simulation technique is that it allows establishing a direct link between the dynamics on atomistic scale and macroscopic observations, on realistic time scales.

Coauthors
Georgios G. Vogiatzis, Lambert C.A. van Breemen.

Acknowledgement
Stimulating discussions with Doros Theodorou are gratefully acknowledged. This work is supported by the Dutch Polymer Institute (DPI), project no. 745ft14 and project EU-FP-001 COMPNANOCOMP, respectively.

References
[1] D.J. Wales, Energy landscapes, Cambridge University Press (2003).
[2] F.H. Stillinger and T.A. Weber, Hidden structure in liquids, Phys. Rev. A, 25, 978-989 (1982).
[3] G.G. Vogiatzis, L.C.A. van Breemen, and M. Hütter, Network topology of the states probed by a glassy specimen during physical aging, submitted to J. Chem. Theory Comput. (2018).