Session W48: New Experimental, Theoretical, and Computational Methods in Polymer and Soft Matter Physics

Coherent States Formulation of Polymer Field Theory

We developed a coherent-states formulation of equilibrium polymer field theory. Compared with the traditional Edwards' auxiliary field framework that underpins both field-theoretic simulation (FTS) and self-consistent field theory (SCFT) methods, this formulation has a number of attractive features, including much more local operators and a finite order polynomial action. The formalism is developed in the grand canonical ensemble for the Edwards model of polymers in an implicit solvent, and we show how to derive a numerically tractable scheme. We explore the efficiency and stability of the method in mean-field and~fully fluctuating simulations for a polymer solution confined to a~slit in one dimension.   

How Reliable Are Soft Potentials? Ensuring Thermodynamic Consistency Between Hierarchical Models of Polymer Melts

The use of soft effective potentials to represent macromolecular systems has become widespread in the areas of biophysics and materials science. A survey of the field reveals a vast array of various phenomenological potentials whose ability to provide quantitative information about several different properties of the same system is not evident. This talk will present a formally sound approach to obtain soft potentials for realistic models of simple linear polymer melts which reproduce the correct center of mass distribution of particles as well as the correct equation of state of the underlying system of interest. Furthermore, an analytical potential allows us to rigorously address the implications of coarse-graining on the entropy and free energies of the system and to account for the reduced degrees of freedom and smoothed energy landscape implicit to coarse-grained models. Finally, the transferability of the method to other systems and potential applications will be discussed.   

Simulation of Stress Induced Polymer-Polymer Interfacial Slip

The phenomena of stress-induced tangential slip at polymer-polymer interfaces is studied via a slip-link simulation technique. Simulations combine a slip-link model of entanglement with a self-consistent field description of the chemical potential landscape near an interface. We consider how the slip velocity depends upon shear stress, interfacial entanglement density, and polymer chain length. Our model is based on the idea that the strongly non-linear shear thinning of the interface observed in experiment is a result of stress-induced convective release (pulling-out) of entanglements across the interface.   

Tumbling dynamics of isolated polymer chains in strong shear flows and the effects of chain resolution

Using Brownian dynamics simulations, without hydrodynamic and excluded volume interactions, on polymer chain models encompassing a wide range of resolutions, we present a detailed investigation on the behavior of isolated chains in shear flow. We find a highly non-monotonic behavior for all models, with chain compression occurring at ultra-high shear rates that is consistent with the recent simulation studies. However, results obtained using highly refined models, with resolutions lower than a Kuhn step, reveal that this transition is an artifact of the level of chain discretization. Also, our results clearly indicate that, at high shear rates, the chain thickness in the shear-gradient direction is independent of the chain length, which differ from previously reported scaling law. We show that the chain thickness is fixed by the distance a sub-section of the chain can diffuse in the shear-gradient direction before convection stretches it out and suppresses further diffusion. Simple physical arguments are then used to derive the correct scaling laws for the coil width and the tumbling time at high shear rates. We believe that our findings presented here will provide the foundation for a better understanding of this basic problem in polymer dynamics.   

A Mesoscale Simulation Method for High Salt Concentrations

In mesoscale simulations of electrolyte solutions the interplay between electrostatics and hydrodynamics plays the critical role for computational efficiency and accuracy. In the past it became apparent that high salt concentrations are too costly, if every salt ion is treated explicitly as a separate particle. On the other hand, charges are highly screened at high salt concentrations and ion-ion correlations are less important than in the low-salt limit. Therefore, we have developed a dynamic mean-field treatment of charges which is expected to be perfectly sufficient in many cases leading to applications in controlled manipulation of polyelectrolytes and charged colloids by external electric fields.   

Molten Salt Eutectics from Atomistic Alchemical Simulations

Molten salt mixtures are gaining importance in solar thermal power applications. Unfortunately, their phase diagrams cannot be easily computed from first principles calculations. The eutectic mixture composition and temperature are located using the liquid mixture free energy and the pure component solid-liquid free energy differences. The liquid mixture free energy is obtained using thermodynamic integration of alchemical transformations of one atom to another. Numerical results for binary and ternary mixtures of alkali nitrates agree well with experimental measurements [1]. Some results involving mixtures of monovalent and divalent cations will also be presented.\\[4pt] [1] Jayaraman, Thompson, and von Lilienfeld, PRE, 84, 030201 (2011). \\[4pt] Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.   

An [N]PT Ensemble for Studying the Intricate Thermodynamics of Cluster Crystals

Exotic soft matter systems such as certain dendrimers can overlap with only a finite energy penalty. Systems governed by steep, bounded repulsive interactions, such as the penetrable sphere model (PSM), indeed form cluster crystals with multiple particles per lattice site under compression. Standard simulation approaches that keep $NVT$, $NPT$, or $\mu V T$ constant cannot directly equilibrate cluster crystals, because either $N$ or lattice constant is not free to relax. It is tempting to allow all fields to fluctuate by keeping ``$\mu P T$'' constant, but basic thermodynamics indicates that infinite fluctuations then result. We avoid this caveat by using a $[N]PT$ ensemble, in which $P$ and $T$ are fixed but $N$ is allowed to fluctuate within bounds $[N_{\mathrm{min}},N_{\mathrm{max}}]$ under a conjugate field analogous to $\mu$. The approach provides the equilibrium properties of neighboring state points through histogram reweighing. We solve the phase diagram of the PSM, and confirm that the FCC crystal lattice occupancy linearly increases with $\rho$ at high $T$. At low $T$ the occupancy plateaus at integer values, but the transitions remain continuous and the crystal does not phase separate. We also examine the critical behavior of another related model.   

From nucleus to phase: Growing dynamics of critical nucleus in polymer blends

In metastable system, nonclassical critical nucleus is not a drop of stable bulk in core wrapped with a sharp interface, but a diffuse structure depending on the metastability. Thus, forming a critical nucleus does not mean the birth of a new phase. Also, the dynamics of nucleus' growing before the phase is born is still unknown. In present work, the nonclassical growing dynamics of the critical nucleus is addressed in the metastable polymer blends by incorporating self-consistent field theory and external potential dynamics theory, and leads to an intuitionistic description for the results of scattering experiments. Our results suggest that the growth of nonclassical critical nucleus is not self-similarly, but forms shell structure, which gives the scattering peak at nonzero wavenumber in the experiments. This phenomenon comes from the spinodal-decomposition-like behaviors constrained within the critical nucleus. The growing dynamics of the critical nucleus can be considered as a spinodal-assistant nucleation process.   

An Interactive 3D Interface to Model Complex Surfaces and Simulate Grazing Incidence X-ray Scatter Patterns

Grazing Incidence Scattering is becoming critical in characterization of the ensemble statistical properties of complex layered and nano structured thin films systems over length scales of centimeters. A major bottleneck in the widespread implementation of these techniques is the quantitative interpretation of the complicated grazing incidence scatter. To fill this gap, we present the development of a new interactive program to model complex nano-structured and layered systems for efficient grazing incidence scattering calculation.   

Numerical Computation of Diffusion Properties in Molecular Systems on a Topology-Conforming Grid

Multiscale problems involving diffusion in molecular systems are a mainstay of computational biophysics. Given a molecular system, the local diffusion coefficient $D(\bf{r})$ as well as the equilibrium distribution function $P(\bf{r})$ that characterizes the local free energy are computed to describe the kinetics of diffusing particles at each point in space through the Smoluchowski equation (SE). An irregular grid of space-varying fineness conforming to $P(\bf{r})$ is generated via the method of topology-representing networks and a subsequent Voronoi tessellation. The discretized SE produces a rate matrix which describes the probabilities of particles hopping from point to point on the grid. We demonstrate the calculation of the rate matrix for ions diffusing through the balloon-like structure of the mechanosensitive channel of small conductance (MscS) and thence the determination of mean first-passage times that characterize conduction of ions through balloon and channel.   

Fluorescent excitation transfer as a tool for the phase transition studies

The use of the fluorescent resonant excitation transfer technique (FRET) to study the phase transition kinetics is demonstrated. The laser temperature jump is applied to the water/2,6-lutidine mixture and causes the demixing. Coumarin 480 and hydroxypyrene laser dyes form excitation transfer that interrogates the spatial structure of the system. Due to the differential solubility of these dyes in the components of the mixture, the excitation transfer ceases once the phase separation occurs. The spatial resolution of the method is determined by the Forster distance of the excitation transfer pair, and in this case is equal to 3 nm. The phase separation is completed within 1 microsecond. The rising edge of the fluorescence is consistent with polynomial growth of the phase separated domains, and not with Cahn-Hilliard fixed length instability. The theoretical model for the excitation transfer in a variety of systems such as separation of binary mixture, phase reorganization of membranes, formation of lamellar structure is developed.   

The Physics of Phase-Separation Fronts

When phase separation does occur in a sequential manner, e.g. due to the diffusion of a control parameter into the system, the resulting phase-separation structures have a markedly different appearance than homogeneous phase-separation patterns. The reason lies in the influence the already phase-separated material and the recent phase-separation dynamics exert on the newly phase-separating material. We call the region where phase-separation first occurs the phase-separation front. In this talk we will consider the simplest possible phase-separation front, i.e. a sharp transition in the control parameter moving through the system with a prescribed velocity inducing phase-separation. We show numerical and analytical solutions for the structures that are formed by such a front as a function of the front speed and the composition of the overtaken material.   

Protected TERS Probes for the Study of Polymer Surfaces

The chemistry of polymer surfaces can potentially be imaged with high ($\sim $20nm) lateral resolution using Tip Enhanced Raman Spectroscopy (TERS). The method's applicability can be tremendously broadened if the metallized tips central to the technique can be made more robust. Protecting the TERS probes with alumina coatings reduces chemical, mechanical, and thermal degradation, prolonging the use lifetime. Most recently we have focused on the detailed effect of the protective coating for cases in which the enhancement is particularly strong. ``Blinking'', which is characteristic of extreme enhancement, has been observed with TERS on polymer films for the first time. An alumina coating prolongs the duration of blinking from 20 min to 30 h, greatly extending the experimental window of extreme enhancement. The protective coating is also helpful for illuminating the mechanism behind blinking. Our results are consistent with thermal diffusion of molecules as the major mechanism facilitating blinking.