Why Volume and Dynamics Decouple in Nanocomposite Matrices: Space that Cannot Be Accessed is Not Free.

Polymer nanocomposites have important material applications and are an ongoing focus of many molecular level investigations, however, puzzling experimental results exist. For example, specific volumes for some polymer nanocomposite matrices are 2% to 4% higher than for the neat polymer; in a pure polymer melt this would correspond to a pressure change of 40 to 100 MPa, and a decrease in isothermal segmental relaxation times of 3 to 5 orders of magnitude. However, the nanocomposite segmental dynamics do not show any speed up.

Spectroscopic ellipsometry as a route to thermodynamic characterization.

Strategies for synthesizing molecularly designed materials are expanding, but methods for their thermodynamic characterization are not. This shortfall presents a challenge to the goal of connecting local molecular structure with material properties and response. Fundamental thermodynamic quantities, including the thermal expansion coefficient, , can serve as powerful inputs to models, yielding insight and predictive power for phenomena ranging from miscibility to dynamic relaxation.

Experimental and Modeling Comparison of the Dynamics of Capped and Freestanding Poly(2-chlorostyrene) Films.

Proximity to a nonrepulsive wall is commonly considered to cause slower dynamics, which should lead to greater relaxation times for capped thin polymer films than for bulk melts. To the contrary, here we demonstrate that Al-capped films of poly(2-chlorostyrene) exhibit enhanced dynamics with respect to the bulk, similar to analogous freestanding films. To quantitatively resolve the impact of interfaces on whole film dynamics, we analyzed the experimental data via the Cooperative Free Volume rate model.

The dynamics of freestanding films: predictions for poly(2-chlorostyrene) based on bulk pressure dependence and thoughtful sample averaging.

In this paper we model the segmental relaxation in poly(2-chlorostyrene) 18 nm freestanding films, using only data on bulk samples to characterize the system, and predict film relaxation times () as a function of temperature that are in semi-quantitative agreement with film data. The ability to translate bulk characterization into film predictions is a direct result of our previous work connecting the effects of free surfaces in films with those of changing pressure in the bulk. Our approach combines the Locally Correlated Lattice (LCL) equation of state for prediction of free volume values () at any given density (), which are then used in the Cooperative Free Volume (CFV) rate model to predict (, ).

A Simple New Way To Account for Free Volume in Glassy Dynamics: Model-Free Estimation of the Close-Packed Volume from Data.

In this article we focus on the important role of well-defined free volume () in dictating the structural relaxation times, τ, of glass-forming liquids and polymer melts. Our definition of = - , where is the total system volume, means the use of depends on determination of , the system's volume in the limiting closely packed state. Rejecting the historically compromised use of as a dynamics-dependent fitting function, we have successfully applied a clear thermodynamics-based route to using the locally correlated lattice (LCL) model equation of state (EOS).

To Understand Film Dynamics Look to the Bulk.

We show that shifts in dynamics of confined systems relative to that of the bulk material originate in the properties of bulk alone, and exhibit the same form of behavior as when different bulk isobars are compared. For bulk material, pressure-dependent structural relaxation times follow τ(T,V)∝exp[f(T)×g(V)]. When two states (isobars) of the material, "1" and "2", are compared at the same temperature this leads to a form τ_{2}∝τ_{1}^{c}, where c=g[V_{2}(T)]/g[V_{1}(T)].

Substrate Roughness Speeds Up Segmental Dynamics of Thin Polymer Films.

We show that the segmental mobility of thin films of poly(4-chlorostyrene) prepared under nonequilibrium conditions gets enhanced in the proximity of rough substrates. This trend is in contrast to existing treatments of roughness which conclude it is a source of slower dynamics, and to measurements of thin films of poly(2-vinylpiridine), whose dynamics is roughness invariant. Our experimental evidence indicates the faster interfacial dynamics originate from a reduction in interfacial density, due to the noncomplete filling of substrate asperities.

The cooperative free volume rate model for segmental dynamics: Application to glass-forming liquids and connections with the density scaling approach.

In this paper, we apply the cooperative free volume (CFV) rate model for pressure-dependent dynamics of glass-forming liquids and polymer melts. We analyze segmental relaxation times, [Formula: see text] , as a function of temperature (T and free volume ( [Formula: see text] , and make substantive comparisons with the density scaling approach. [Formula: see text] , the difference between the total volume (V and the volume at close-packing, is predicted independently of the dynamics for any temperature and pressure using the locally correlated lattice (LCL) equation-of-state (EOS) analysis of characteristic thermodynamic data.

Experimental Test of the Cooperative Free Volume Rate Model under 1D Confinement: The Interplay of Free Volume, Temperature, and Polymer Film Thickness in Driving Segmental Mobility.

We show that the Cooperative Free Volume (CFV) rate model, successful at modeling pressure-dependent dynamics, can be employed to describe the temperature and thickness dependence of the segmental time of polymers confined in thin films (1D confinement). The CFV model is based on an activation free energy that increases with the number of cooperating segments, which is determined by the system's free volume. Here, we apply the CFV model to new experimental results on the segmental relaxation of 1D confined poly(4-chlorostyrene), P4ClS, and find remarkable agreement over the whole temperature and thickness ranges investigated.

Explaining the T,V-dependent dynamics of glass forming liquids: The cooperative free volume model tested against new simulation results.

In this article, we derive a rate model, the "cooperative free volume" (CFV) model, to explain relaxation dynamics in terms of a system's free volume, V, and its temperature, T, over widely varied pressure dependent conditions. In the CFV model, the rate a molecule moves a distance on the order of its own size is dependent on the cooperation of surrounding molecules to open up enough free space. To test CFV, we have generated extensive T,V dependent simulation data for structural relaxation times, τ, on a Kob and Andersen type Lennard-Jones (KA-LJ) fluid.

How Free Volume Does Influence the Dynamics of Glass Forming Liquids.

In this article we show that inverse free volume is a natural variable for analyzing relaxation data on glass-forming liquids, and that systems obey the general form, log(τ/τ) = (1/) × (), where () is a function of temperature. We demonstrate for eight glass-forming liquids that when experimental relaxation times (log τ), captured over a broad pressure-volume-temperature () space, are plotted as a function of inverse free volume (1/), a fan-like set of straight line isotherms with -dependent slopes ensues. The free volume is predicted independently of the dynamic results for each state point using data and the Locally Correlated Lattice (LCL) equation of state.

Free Volume in the Melt and How It Correlates with Experimental Glass Transition Temperatures: Results for a Large Set of Polymers.

There is a continuing, strong interest in making connections between the polymeric glass transition () and bulk properties. In this Letter we apply the Locally Correlated Lattice (LCL) model to study a group of 51 polymers and demonstrate two broad correlations. In the first, we show that the theoretically determined polymeric free volume in the melt, all at a single common , (425 K, 1 atm), correlates noticeably with the experimentally determined values, and that this trend sharpens considerably when families of polymers are examined.

Thermodynamic treatment of polymer thin-film glasses.

We propose a new, simple, thermodynamically based model in order to study the effect of film thickness on the glass transition of a polymer. The model equation of state incorporates the effect of a free surface by accounting for missing interactions, and is parametrized using experimental data for bulk samples, leaving no freely adjustable parameters. To this end we focus on connecting model agreement in the bulk limit with two key physical properties: pressure-volume-temperature data and surface tension data.

A simple approach to polymer mixture miscibility.

Polymeric mixtures are important materials, but the control and understanding of mixing behaviour poses problems. The original Flory-Huggins theoretical approach, using a lattice model to compute the statistical thermodynamics, provides the basic understanding of the thermodynamic processes involved but is deficient in describing most real systems, and has little or no predictive capability. We have developed an approach using a lattice integral equation theory, and in this paper we demonstrate that this not only describes well the literature data on polymer mixtures but allows new insights into the behaviour of polymers and their mixtures.

Fluid mixtures: contrasts of theoretical and simulation approaches, and comparison with experimental alkane properties.

In this work we compare lattice and continuum versions of the same theory, as derived in the previous paper (Paper I), to predict the behavior of simple alkane mixtures. In the course of doing this we use characteristic parameters obtained for the pure alkane fluids; no fits of mixture properties are involved. Our two sets of predictions are tested against experimental data and against new Monte Carlo simulation results.

Chain fluids: contrasts of theoretical and simulation approaches, and comparison with experimental alkane properties.

In this work, we undertake a fundamental comparison of analogous lattice and continuum integral equation theories, with both held accountable to the results from Monte Carlo simulation and real experimental data on short chain molecules. Each integral equation method is applied to determine the system's microscopic intermolecular site-site distributions and the corresponding bulk thermodynamic properties. These properties and those from the MC simulations are then fitted to corresponding data on n-alkanes.

Methods for calculating the entropy and free energy and their application to problems involving protein flexibility and ligand binding.

The Helmholtz free energy, F and the entropy, S are related thermodynamic quantities with a special importance in structural biology. We describe the difficulties in calculating these quantities and review recent methodological developments. Because protein flexibility is essential for function and ligand binding, we discuss the related problems involved in the definition, simulation, and free energy calculation of microstates (such as the alpha-helical region of a peptide).

Minimalist explicit solvation models for surface loops in proteins.

We have performed molecular dynamics simulations of protein surface loops solvated by explicit water, where a prime focus of the study is the small numbers (e.g., ~100) of explicit water molecules employed.

Calculation of the Entropy of Lattice Polymer Models from Monte Carlo Trajectories.

While lattice models are used extensively for macromolecules (synthetic polymers proteins, etc), calculation of the absolute entropy, S, and the free energy, F, from a given Monte Carlo (MC) trajectory is not straightforward. Recently we have developed the hypothetical scanning MC (HSMC) method for calculating S and F of fluids. Here we extend HSMC to self-avoiding walks on a square lattice and discuss its wide applicability to complex polymer lattice models.

Free volume hypothetical scanning molecular dynamics method for the absolute free energy of liquids.

The hypothetical scanning (HS) method is a general approach for calculating the absolute entropy, S, and free energy, F, by analyzing Boltzmann samples obtained by Monte Carlo (MC) or molecular dynamics (MD) techniques. With HS applied to a fluid, each configuration i of the sample is reconstructed by gradually placing the molecules in their positions at i using transition probabilities (TPs). With our recent version of HS, called HSMC-EV, each TP is calculated from MC simulations, where the simulated particles are excluded from the volume reconstructed in previous steps.

Calculation of the entropy of random coil polymers with the hypothetical scanning Monte Carlo method.

Hypothetical scanning Monte Carlo (HSMC) is a method for calculating the absolute entropy S and free energy F from a given MC trajectory developed recently and applied to liquid argon, TIP3P water, and peptides. In this paper HSMC is extended to random coil polymers by applying it to self-avoiding walks on a square lattice--a simple but difficult model due to strong excluded volume interactions. With HSMC the probability of a given chain is obtained as a product of transition probabilities calculated for each bond by MC simulations and a counting formula.

Phase changes in Lennard-Jones mixed clusters with composition Ar(n)Xe(6-n) (n=0,1,2).

We have carried out parallel tempering Monte Carlo calculations on the binary six-atom mixed Lennard-Jones clusters, Ar(n)Xe(6-n) (n=0,1,2). We have looked at the classical configurational heat capacity C(V)(T) as a probe of phase behavior. All three clusters show a feature in the heat capacity in the region of 15-20 K.

Lower and upper bounds for the absolute free energy by the hypothetical scanning Monte Carlo method: application to liquid argon and water.

The hypothetical scanning (HS) method is a general approach for calculating the absolute entropy S and free energy F by analyzing Boltzmann samples obtained by Monte Carlo or molecular dynamics techniques. With HS applied to a fluid, each configuration i of the sample is reconstructed by gradually placing the molecules in their positions at i using transition probabilities (TPs). At each step of the process the system is divided into two parts, the already treated molecules (the "past"), which are fixed, and the as yet unspecified (mobile) "future" molecules.

A simulation method for calculating the absolute entropy and free energy of fluids: application to liquid argon and water.

The hypothetical scanning (HS) method is a general approach for calculating the absolute entropy and free energy by analyzing Boltzmann samples obtained by Monte Carlo (MC) or molecular dynamics techniques. With HS applied to a fluid, each configuration i of the sample is reconstructed by adding its atoms gradually to the initially empty volume, i.e.