Computational steering in Monte Carlo simulations of thin film polystyrene

Abstract

High molecular weight polymer systems show very long relaxation times, of the order of milliseconds or more. This time-scale proves practically inaccessible for atomic-scale dynamical simulation such as molecular dynamics. Even with a Monte Carlo (MC) simulation, the generation of statistically independent configurations is non-trivial. Many moves have been proposed to enhance the efficiency of MC simulation of polymers. Each is described by a proposal density Q(x'; x): the probability of selecting the trial state x' given that the system is in the current state x. This proposal density must be parametrized for a particular chain length, chemistry and temperature. Choosing the correct set of parameters can greatly increase the rate at which the system explores its configuration space. Computational steering (CS) provides a new methodology for a systematic search to optimize the proposal densities for individual moves, and to combine groups of moves to greatly improve the equilibration of a model polymer system. We show that monitoring the correlation time of the system is an ideal single parameter for characterizing the efficiency of a proposal density function, and that this is best evaluated by a distributed network of replicas of the system, with the operator making decisions based on the averages generated over these replicas. We have developed an MC code for simulating an anisotropic atomistic bead model which implements the CS paradigm. We report simulations of thin film polystyrene.

Hierarchical modelling of polystyrene surfaces

“…The experimental data, 14,15 measured close to the T g and computational 28 findings, obtained at 500 K, show that the rings have a specific orientation at the surface: the angle the phenyl rings form with respect to the normal to the polystyrene (the definition is shown in Fig. 11) is approximately 57 .…”

“…In the last years both experimental studies 9,13-17 and computational studies 26,28 have been performed to investigate the structure of polymer surfaces at an atomistic level. The characteristics of a surface can influence the performance of the devices to which they belong and a knowledge of the superficial properties and how these properties change in the presence of different media 9,13,16,17 can help the engineering of the devices.…”

“…Molecular dynamics simulations have further been used to investigate the ring orientation at the surface of an amorphous atactic polystyrene thin film 26 and to investigate the effect of Ar + and Ar + /radical chemistries on a model polystyrene surface, 27 while Monte Carlo simulations, with an improved methodology to equilibrate the polymer chains, have also been performed on atactic polystyrene. 28 Recently, experimental analysis combined with atomistic simulations have shown the effect of water on the roughness and the ordering of an oxidized polystyrene surface. 29 Beadspring models have also been employed to study polymer surfaces and they have recently been reviewed in Ref.…”

Scientific Grid computing

“…At the time of writing, more than ten codes, from within and outside the RealityGrid project, have been made steerable in this way and the steering system has been adopted by other projects, including the EPSRC's Integrative Biology. 3 There are several papers in the issue which describe steering and its wide-ranging scientific applications (for example, Kenny et al 2005;Mason & Sutton 2005). Other steering systems exist and Eickermann et al (2005) describe one of these.…”

Mechanical spectroscopy of polymers with reduced dimensions and increasing cross-linking degree

“…For supported PS films, the critical thickness is about 100 nm and it is independent of the substrate . Furthermore, the theoretical calculations Mason and Sutton (2005) and experiments (Gautam et al, 2000) on polystyrene showed that at both interfaces the preferred orientation of aromatic rings are out of the interface plane, at the film/substrate interface though, the tilt angle is higher than at the free surface.…”