We study surface effects in amorphous polymer systems by means of computer simulation. In the framework of molecular dynamics, we present two different methods to prepare such surfaces. Free surfaces are stabilized solely by van-der-Waals interactions, whereas confined surfaces emerge in the presence of repelling plates. The buildup of density and pressure profiles from zero to their bulk values depends on the surface preparation method. In the case of confined surfaces, we find density and pressure oscillations next to the repelling plates. For free surfaces, we attest chain-end enrichment and present a comparison between density profile and particle coordination number.

2H NMR relaxation times of selectively deuterated polar molecules confined to a set of calibrated nanoporous silica glasses are reported. These experiments, combined with the consideration of different time scales in the theory of surface relaxation, show how confinement effects can provide detailed information on the rotational dynamics of temporarily adsorbed liquid layers in presence of biphasic fast exchange.

Here we present experimental results on adsorption of fatty acids, aliphatic alcohols, and acetone on the mesoporous silica gels with various porosity. Conclusion has been drawn that fractal resolution analysis of the silica surface may lead to apparent values of the surface dimension. These data are suggested to be analyzed by a simple model taking into account adsorbed layer thickness and steric difficulties of monolayer formation in narrow pores.

Complex liquid glass-forming systems ranging from those composed of simple molecules to polymer melts and amorphous polymers have been studied extensively as a function of temperature resulting in a basic understanding of liquid-state dynamics and glass transition phenomenology as these systems are supercooled to the vitreous state. An important aspect of this problem that remains largely unexplored, and that is relevant to the topic of this symposium, involves liquid-state dynamics and vitrification (as well as crystallization) in the regime of high pressure and high density. We describe work on “fragile” to “intermediate strength” simple organic glass-forming liquids where both temperature (T) and pressure (P) are varied. Diamond anvil cells are used to achieve pressures exceeding 10 GPa. Several optical and light scattering techniques are used to explore both static and dynamic properties of these systems. High-pressure Brillouin scattering enables us to model the longitudinal relaxation time in these systems as well as their equations of state. These can now be refined by direct measurements of the pressure dependence of the glass transition, Tg(P). Finally, we summarize depolarized light scattering studies which allow us to compare both the isobaric and isothermal evolution of structural (α) and fast (β) relaxation processes.

The orientational dynamics of organic supercooled liquids of low molecular weight confined to the geometry of porous glasses are studied by two highly related techniques, the optical method of probing the dynamics of solvation regarding a chromophoric host molecule and dielectric relaxation spectroscopy. The dielectric results display marked effects of the confinement to mesopores in terms of altered structural dynamics which appear to separate into a raster and slower responses relative to the bulk liquid. We also demonstrate that there is no trivial relation between the ε*(ω) data and the liquid dynamics in these heterogeneous samples. These effects are partially paralleled by the solvation dynamics results, but with the spatial range inherent in the optical technique being inconsistent with associating the fast and slow dynamical components to spatially distinct regimes. We conclude on the slow component being a signature of non-ergodicity which arises from the competition between the length scale of cooperativity and the pore size.

We discuss the resolution of shear force measurement in determining the effective shear viscosity of molecularly-thin films. Recent findings for the shear of confined OMCTS (octamethylcyclotetrasiloxane) are presented.

A model is presented of a particle that interacts with two periodic potentials, representing two confining plates, one of which is externally driven. The model leads to various behaviors in the motion of the top driven plate: stick-slip, intermittent regime, characterized by force fluctuations, and two types of sliding above a critical driving velocity vc. Similar behaviors are typical of a broad range of systems including thin sheared liquids. A detailed analysis of the different regimes displays a transition between the stick-slip and the kinetic regimes, ω−2 power spectra of the force over a wide range of velocities below vc, and a decrease of the force fluctuations that follows (vc – v)½ for v < vc. The velocity dependent Liapunov exponents demonstrate that the stick-slip motion is characterized by a chaotic behavior of the top plate and the embedded particle. An extension of the model to an embedded chain is introduced and preliminary results are presented and confronted with the single particle case. The role of the internal excitations of the chain in frictional dynamics is discussed.

We examine the shear flow of hexadecane and squalane confined between plates with nm separation using molecular dynamics simulations. For both molecules substantial slip occurs at the walls and the density profile exhibits strong oscillations. In contrast to surface force apparatus measurements, our calculated effective viscosities are not much greater than bulk viscosities, but the simulations shear rates are much larger than the experimental ones. The actual viscosity calculated using the shear rate measured from the observed velocity profile is almost equal to the bulk viscosity within uncertainty.

Polymers end-grafted to a surface in the presence of a shear flow are studied by molecular dynamics simulations. The solvent velocity field is observed to penetrate only a short distance into the brush consistent with predictions based on self-consistent field theory. The deformation of the brush is small except when the shear rate γ is very large. In this limit, while some of the polymer chains are stretched in the direction of flow, the brush height actually decreases slightly, in contrast to several theoretical predictions. When two surfaces bearing end-grafted chains are brought into contact, the normal force increases rapidly with decreasing plate separation, while the shear force is significantly smaller. For low relative velocity vw of the two walls, the surfaces slide pass each other with almost no change in the chain's radius of gyration or the amount of interpenetration, while for very large vw, there is significant stretching and some disentanglement of the chains. The results are in qualitatively good agreement with recent experiments using the surface force apparatus.

We have investigated simple shear flows of poly-dimethyl-siloxane (PDMS) melts on PDMS brushes with well controlled grafting density Σ, using optical techniques based on near field laser anemometry to measure the local velocity of a polymer melt at a solid interface. We have observed a slip transition from a weak slip regime to a strong one as the shear rate crosses a critical value γ*. The influence of the molecular weight of the melt Mm, of the molecular weight of the brush chains Mb and of Σ on γ*, has been investigated to elucidate the molecular mechanisms. The results are compared with an existing molecular model.

We introduce two new techniques with the sensitivity to measure spectroscopie properties of confined molecules under shear. One concerns rheo-optics of fluids in micron-sized gaps. The experimental approach consists in applying periodic shear motion to fluid films confined between parallel plates separated by controllable film thickness from 0.1 to 100 micrometers. A key aspect is the concurrent ability to perform spectroscopie experiments (infrared or dielectric spectroscopy, or x-ray or neutron spectroscopy) of these fluids during flow.

The second technique consists in flow field experiments of polymer adsorption and orientation. The experimental approach is to produce one-directional shear, at rates up to 104 sec−1, past adsorbed polymer layers of thickness less than 0.01 micrometers. The selective augmentation of vibrational modes oriented preferentially along the chain backbone is then used to probe chain orientation in the direction of flow in a laminar flow field.

Motion of a monolayer, bilayer or other surface film is affected by the presence of a surrounding fluid phase. Several recent experimental and theoretical studies have examined this viscous coupling with particular attention given to situations where the subphase provides the dominant resistance to motion. This research is summarized and the translation of a membrane-bound particle over a sublayer of finite depth is considered.

Coating of solid surfaces with uniform, thin liquid films occurs in many industrial processes. A common process consists of spraying some liquids or emulsions on a freshly created solid surface. In this context, we have investigated the impact of a single droplet of various emulsions and surface-active solutions on a solid substrate using a high frequency fluorescent visualization technique (1 picture every 0.25 ms). Whatever the materials in presence, the drop spreads and then retracts under the action of inertia and capillarity respectively. Inertia induces spreading and generates a peripheral rim which is unstable to fingering. Then contact line instabilities appear under the form of festoons with pure liquids which are damped with surface-active solutions and amplified with emulsions. When the adsorption kinetics of the surfactant is slow, the equilibrium surface tension is not restored during the duration of the experiment and the drop can bounce back.

One to three layer cyclohexane films confined between mica-like surfaces are studied to elucidate changes in the films' lattice-type. The laterally confined film is in equilibrium with the bulk fluid that is well into the liquid regime of its phase diagram. Monte Carlo simulations are conducted at constant chemical potential, temperature, and V=Ah, where A is the lateral area and h is the separation between the walls. One and two layers of fluid freeze as h increases. The one layer fluid has a triangular lattice, while the two layer fluid exhibits first a square lattice and then a triangular lattice with increasing surface separation. In contrast to previous studies, solidlike order is induced primarily by the strong fluid-solid interaction and is largely a function of pore width. A shift in the relative alignment of the surfaces perturbs the solidlike fluid structure but does not cause the sudden shear melting transition associated with epitaxial alignment of the fluid atoms with the surface. There is a correlation between the shear stress calculated in the computer experiments and that measured in Surface Forces Apparatus experiments.

We have measured the growth with time t of a wetting layer (of thickness l(t)) at the surface of a thin film of a binary liquid (polymer) mixture. Over a wide range of experimental parameters, our data is well described by a model of diffusion limited wetting which takes into account the finite film thickness. In this model, l(t) is a function of time which sensitively depends on the nature of the interfacial potential: a detailed comparison shows that long range van-der-Waals forces provide the main driving force for the build-up of the wetting layer.

Within the last few years, a surface science technique, the atomic force microscopy (AFM), has evolved to be capable of simultaneously measuring tribological (friction, wear, adhesion) and rheological (elastic moduli, viscosity, hardness) properties and topography on the nanometer scale. Particularly for thin polymeric films, the AFM can be efficiently used for studying surface mechanical properties which are of fundamental importance to help predict stress and frictional behavior of interfacially confined ultrathin films.

In this paper, the following aspects will be discussed: (a) mechanical properties of ultrathin homopolymer and copolymer films, (b) dewetting dynamics of interfacially confined phase-separated homopolymers, and (c) the influence of graft-copolymers on the wetting and dewetting characteristics of homopolymers.

The L3 phase under shear transforms to a lamellar phase L*α above some critical shear rate. We study the sequence of successive states (rheothinning, flow birefringence, transition region, lamellar) with simultaneous rheooptical methods, in situ X-ray scattering, and light microscopy observations. The transition region is biphasic, and the two phases display epitaxial relationship immediately after shearing is ceased.

We present a study of the effects of the initial distribution on the kinetic evolution of irreversible binary reactions in low dimensions. We focus on the role of initial density fluctuations and, in particular, on the role of the long wavelength components of the initial fluctuations, in the creation of the macroscopic patterns that lead to the well-known kinetic anomalies in this system. The frequently studied random initial distribution is but one of a variety of possible distributions leading to interesting anomalous behavior. Our discussion includes initial distributions that suppress and ones that enhance the initial long wavelength components.

In this paper we focus on two experimental techniques for testing predictions made by Laplace-Kelvin theory (LKT). The first technique employs the surface forces apparatus (SFA) to measure pull-off forces, the second uses a beam bending (BB) technique to determine the bending forces in a microporous film produced with a sol-gel technique. We use density functional theory (DFT) to analyze the two experiments. We find that the pull-off force in a crossed cylinder geometery contains a minimal solid-liquid contribution, which can explain why the LKT result is found at saturation. We demonstrate that a pore size distribution essential to produce a bending stress that is always compressive and has a dependence on the vapor pressure that coincides with LKT.

Hysteresis phenomena are observed in many nanoporous environments during adsorption/desorption isothermal quasiequilibrium cycles. A non-local DFT model has been developed for predicting adsorption/desorption isotherms in nanopores of different geometries in the wide range of pore sizes (0.5–10 nm) based on given intermolecular fluid/fluid and fluid/solid potentials. Depending on the confining geometry and the intermolecular potentials two types of hysteresis phenomena are occurred: capillary condensation/evaporation that implies volume filling of pores with a liquid-like matter, and layering adsorption/desorption that implies sequential step-wise formation of adsorption layers. It is shown that the DFT model qualitatively describes these phenomena and is in a reasonable quantitative agreement with some of the experiments.