Monday, July 22, 2013

1307.5237 (Arno A. Veldhorst et al.)

Isomorph scaling of flexible Lennard-Jones chains    [PDF]

Arno A. Veldhorst, Jeppe C. Dyre, Thomas B. Schrøder
Short, flexible Lennard-Jones chains (LJC) are shown to agree well with the predictions of the isomorph theory. This means that the LJC liquid has curves (isomorphs) in its phase diagram along which structure and dynamics are invariant in the appropriate units. The isomorphs can be identified by a scaling exponent {\gamma} which can be obtained from fluctuations in the configurational parts of the energy and pressure. The isomorph invariance of the dynamics is seen both in segmental and center of mass dynamics, as well as in the relaxation of the rouse modes. Jumps between different state points on the same isomorph happens instantaneously without any slow relaxation. Our findings show that the isomorph theory not only applies to atomic and small molecular liquids as previously shown, but also to flexible, anisotropic molecules. Since the LJC is a simple model system for (small) polymers, our results provides an explanation to why power law density scaling is observed experimentally in many polymeric system. Furthermore, the theory provides an independent mean of determining the scaling exponent (which is usually treated as a empirical scaling parameter). At larger density changes the isomorph theory predicts that power law density scaling breaks down, and a more general isomorph scaling is needed. complement the results from density scaling, although in our case no fitting is involved in the determination of the scaling exponent {\gamma}.
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