Tushar S. Jain

A biased Monte Carlo technique for calculation of the density of states of polymer films

A new Monte Carlo algorithm is implemented for simulation of the density of states of free-standing polymer films. The algorithm combines the original idea of conducting a random walk in energy space with advanced trial moves such as configurational bias and end-bridging. Excellent agreement is found between the results of this new method and those from simulations in the canonical ensemble, down to temperatures in the vicinity of the apparent glass transition. The efficiency of the new algorithm is studied as a function of the types of trial moves employed. It is found that, depending on the range of energy and density, certain localized moves fail to converge to the correct distribution of states.

Role of local structure on motions on the potential energy landscape for a model supercooled polymer.

We have conducted detailed Monte Carlo and molecular dynamics simulations of a model glass forming polymeric system near its apparent glass transition temperature. We have characterized the local structure of the glass using a Voronoi-Delaunay analysis of local particle arrangements. After a perturbative face elimination, we find that a significant fraction of Voronoi polyhedra consist of 12 pentagonal faces, a sign of icosahedral ordering. Further, we have identified metabasins of particle vibrations on the potential energy landscape on the basis of persistence of particle positions and neighbors over a simulated trajectory. We find that the residence times for vibrations are correlated with a particular Voronoi volume and number of neighbors of a particle; the largest metabasins correspond to particles whose average Voronoi volume is close to the value expected on the basis of the density, and whose approximate number of neighbors is close to 12. The local distortion around a particle, measured in terms of the tetrahedricity of the Delaunay simplices, reveals that the particles with a higher degree of local distortion are likely to transition faster to a neighboring metabasin. In addition to the transition between metabasins, we have also examined the influence of vibrations at inherent structures (IS) on the local structure, and find that the the low frequency modes at the IS exhibit the greatest curvature with respect to the local structure. We believe that these results establish an important connection between the local structure of glass formers and the activated dynamics, thereby providing insights into the origins of dynamic heterogeneities.

Influence of confinement on the vibrational density of states and the Boson peak in a polymer glass

We have performed a normal-mode analysis on a glass forming polymer system for bulk and free-standing film geometries prepared under identical conditions. It is found that for free-standing film glasses, the normal-mode spectrum exhibits significant differences from the bulk glass with the appearance of an additional low-frequency peak and a higher intensity at the Boson peak frequency. A detailed eigenvector analysis shows that the low-frequency peak corresponds to a shear-horizontal mode which is predicted by continuum theory. The peak at higher frequency (Boson peak) corresponds to motions that are correlated over a length scale of approximately twice the interaction site diameter. These observations shed some light on the microscopic dynamics of glass formers, and help explain decreasing fragility that arises with decreasing thickness in thin films.

Calculation of interfacial tension from density of states

A density of states Monte Carlo formalism is used to calculate the interfacial tension of a simple fluid (3D Ising model) and of free-standing polymer films on a lattice. Good agreement is found between the results of these calculations and literature values. The proposed approach is efficient and provides the surface tension over a wide range of temperature from a single simulation. It has the additional advantage of being equally applicable to systems interacting through continuous potential energy functions, discontinuous functions, and systems on a lattice.