1998 MRS Fall Meeting & Exhibit

Chairs

J. M. Drake, Exxon Research & Engineering
G. S. Grest, Sandia Natl Lab
J. Klafter, Tel Aviv Univ
R. Kopelman, Univ of Michigan

Symposium Support 

• Exxon Corporate Research Laboratory
• Exxon Research & Engineering Company 

* Invited paper

SESSION W1: 
Chair: J. M. Drake 
Monday Morning, November 30, 1998 
Staffordshire (W)

8:30 AM *W1.1 X-RAY AND NEUTRON SCATTERING STUDIES OF THE STRUCTURE OF CONFINED FLUIDS. S.K. Sinha , O. Seeck, J. Wang, Advanced Photon Source, Argonne National Laboratory, Argonne, IL; M. Rafailovich, J. Sokolov, K. Shin, Y. Pu, Dept of Materials Science and Engineering, SUNY, Stony Brook, NY; M. Tolan, Dept of Experimental Physics, University of Kiel, Kiel, GERMANY. 
It is by now well-known that the structure of liquids in the vicinity of solid/liquid interfaces are in confined geometry cna be significantly different to that in the bulk. Such systems can be probed by neutron specular reflectivity or by high energy X-rays from synchrotron radiation sources, which can also be used to study off-specular scattering from the solid/liquid interfaces. We shall present some recent results on the phase separation and ordering in confined polymer films and also on the ordering of ions at liquid/solid interfaces, as studied by both neutrons and X-rays, using specular reflectivity, grazing incidence diffration and diffuse scattering. 

9:00 AM *W1.2 
DIFFUSION AND REACTIVE PROPERTIES IN DISORDERED POROUS MEDIA AND IN CONFINING GEOMETRIES. Pierre Levitz , Centre de recherche sur la matiere divisee, CNRS, Orleans, FRANCE. 

Disordered porous networks are important example of confining geometries. A challenging problem is to couple the morphology and the topology of such disordered systems with the diffusion and the reactive properties of embedded fluids (gases or liquid phase inside the porous medium). We first discuss how the geometric confinement influences molecular diffusion and how the coupling between interfacial geometry and transport evolves in space and time. Properties of the self diffusion propagator is discussed and compared to experimental data and 3D off-lattice numerical simulations (1). More especially, gas diffusion in soil and electric conductivity in diluted sponge phases are presented. In the particular case of the Knudsen diffusion, we show that the propagator can be analysed in term of a continuous time random walk formalism (C.T.W.R.). An interesting consequence is observed for some specific disordered porous media or ``low dimension'' geometries where Knudsen diffusion exhibits a non Gaussian regime and has to be considered as a Levy walk (2). In the second part of this talk, we relate some relaxation kinetics or molecular reactions to the structure and the interaction characteristics of the pore network considered as a confined geometry. Such reactive properties can be probed with either fluorescence spectroscopy or NMR relaxometry (3) and some recent experimental results on model systems will be discussed. 

9:30 AM W1.3 
EQUILIBRIUM, SPONTANEOUS TRANSITIONS AND HYSTERESIS PHENOMENA IN FLUIDS CONFINED IN POROUS MEDIA. Alexander V. Neimark , TRI/Princeton, Princeton, NJ. 

Behavior of a fluid confined by solid surfaces differ significantly from its behavior in bulk. Recent experiments with mesoporous molecular sieves and graphite surfaces provide a reliable body of experimental data that permits to make some quantitative conclusions concerning specific properties of confined fluids. At certain external conditions, there exist multiple equilibrium states which can be achieved by alternating the changes of intensive thermodynamic variables. Multiplicity of equilibrium states causes a phenomenon of permanent hysteresis which does not disappear with the slowing down of the process and is observed during arbitrary slow changes of external parameters. Hysteresis manifests itself in the course of direct and reverse processes in appreciable distinctions of sorption isotherms, heats of sorption, and solvation forces. Hysteresis phenomena in confined fluids constitute a special subject for thermodynamics. A system traces a sequence of equilibrium (possibly metastable) states while the transitions between these states are not always reversible. Irreversibility is associated with spontaneous phase transitions occurring in nanopores and imposes restrictions on the applicability of the reversible thermodynamics to confined fluid processes. Thus, macroscopic description of hysteresis processes in porous media requires a microscopic analysis of fluid behavior in pores. We study the hysteresis phenomena at two scale levels. At the microscopic level of molecular scales, we apply the density functional theory to study physical mechanisms of reversible and spontaneous transitions related to condensation and evaporation in single nanopores. A newly developed canonical ensemble version of the non-local density functional theory allows us to trace the multiple states including stable and metastable states along the hysteresis loop, and also unstable states inside the hysteresis loop. At the macroscopic level of pore networks, we apply the methods of irreversible thermodynamics to relate the experimentally measured quantities to the extensive thermodynamic characteristics of the confined fluid equilibrium states. 

10:15 AM *W1.4 
DIELECTRIC AND MECHANICAL RELAXATION OF GLASS-FORMING LIQUIDS IN NANOPORES. Hauke Wendt, Ranko Richert , Max-Planck-Institute Polymer Research, Mainz, GERMANY. 

We use solvation dynamics measurements in order to study the relaxational behaviour of glass-forming liquids under geometrical contraints in porous silica-glasses. Probing the local dielectric relaxation in these viscous liquids with dipolar chromophores, the cooperative coupling to the liquid-glass interface as well as dynamic heterogeneity can be observed. Both native and silanized glass surfaces are investigated in order to differentiate between confinement and interfacial effects. We compare these results with those obtained by non-polar solvation experiments using a probe molecule which does not alter its dipole moment upon electronic excitation. Here, the observable is the time dependent response of the probe/solute pair-potentials after a perturbation. The latter technique is an optically detected mechanical relaxation measurement in the highly local strain field of a single molecule, thereby facilitating its application to materials confined to nanopores. 

10:45 AM W1.5 
DIELECTRIC RELAXATION IN A DEEPLY SUPERCOOLED LIQUID CRYSTAL CONFINED IN RANDOM POROUS MEDIA. G.P. Sinha and F.M. Aliev, Dept. of Physics and Materials Research Center, University of Puerto Rico, San Juan, PR. 

Nematic liquid crystal--pentylcyanobiphenyl (5CB)--was confined in random porous media with narrow pores of mean size equal to 100 Å and investigated by means of dielectric spectroscopy in deeply supercooled state. In the nematic phase, bulk 5CB has two dielectrically active modes. The main mode with the relaxation time s is due to the rotation of molecules about its short axes, and the secondary mode is due to the tumbling motion of molecules with the relaxation time s. Bulk 5CB is a nonglass former and it crystallizes at C. The confinement strongly influences the dynamical behavior of 5CB and is resulted in qualitative changes in it's properties. We observed deep supercooling of 5CB in pores up to 160 degrees below the bulk crystallization temperature. The relaxation time of the process due to the molecular rotation in deeply supercooled state is slower than at the temperatures corresponding to nematic phase by a factor of 10⁶. This slowing down is accompanied by anomalous broadening of the dielectric spectra. The widths at half maximum of the dielectric spectra and the relaxation time distribution functions of deeply supercooled 5CB are 5 times wider than in nematic phase. The temperature dependence of the relaxation time is glass-like. Other new properties observed in confined 5CB are two low frequency relaxation processes absent in bulk 5CB. One of these processes is due to the molecular relaxation in the surface layers at liquid crystal-solid pore wall interface. The second process is probably a collective mode due to the relaxation of the surface induced polarization. The collective process due to surface polarization and the surface molecular mode also show features typical for glass formers. 

11:00 AM *W1.6 
ROTATIONAL DIFFUSION OF MICROCONFINED LIQUIDS. Brian J. Loughnane, Richard A. Farrer, John T. Fourkas , Boston College, Dept of Chemistry, Chestnut Hill, MA. 

Optical Kerr effect spectroscopy has been used to study the temperature-dependent reorientational dynamics of a number of liquids confined in nanoporous sol-gel glasses. Nonwetting liquids such as carbon disulfide and methyl iodide exhibit biexponential relaxation behavior that is interpreted in terms of bulk-like and surface populations of molecules. Wetting liquids such as acetonitrile display more complex dynamics, which we argue is due to molecular exchange between the surface and bulk-like populations that occurs on a time scale that is fast compared to that of surface orientational relaxation. 

11:30 AM W1.7 
PHASE TRANSITIONS OF SMALL MOLECULES AND LIQUID CRYSTALS IN POROUS SOL-GEL GLASS. T.W. Zerda and Y. Shao, Texas Christian University, Dept of Physics, Fort Worth, TX. 

Raman spectroscopy was used to investigate solid-solid phase transitions of small molecules (cyclohexane, cyclohexanone, ethanol) and liquid crystals (PAA and HOBA) in porous sol-gel glass of pore diameters varying between 2.9 nm to 22 nm. Selected porous samples had their internal surfaces modified to evaluate the effects due to surface interactions. For cyclohexanone, PAA and HOBA the surface modifications, in which the -OH groups were replaced with -Si(CH₃)₃, had a negligible small effect, but were important for cyclohexane. For all systems the phase transition temperatures were found to be depressed, and the depressions were pore size dependent. The hysteresis effects in the phase transition temperature were observed. Amorphous structures were detected near the interface and the size of the amorphous structures depends on the strength of the molecule-silica interactions. Confinement results in broadening of the phase transitions, indicating that in the pores they become weak first-order transitions. 

11:45 AM W1.8 
COMPARATIVE THERMODYNAMIC BEHAVIOR OF PHYSICALLY RESTRICTED CYCLOHEXANE AND CYCLOHEXANONE IN POROUS SILICA. Samuel Amanuel , Vivak M. Malhotra, Southern Illinois University, Department of Physics, Carbondale, IL. 

Both cyclohexane and cyclohexanone, besides having similar chair configuration, undergo two structural transitions at 160 K < T < 300 K, i.e., liquid-to-solid (freezing/melting) transition and solid-to-solid transition. We undertook comparative differential scanning calorimetry (DSC) measurements on cyclohexane and cyclohexanone, physically confined in porous silica of pore radius 4, 7.5, 15, 30, and 62.5 nm, with a view to ascertain how guest fluid-surface host interactions affect the thermodynamic properties of the confined fluids. Our results can be summarized as follows: (a) No distinct signature of freezing or melting transition was observed from the physically confined cyclohexanone, irrespective of whether the bulk was present outside the pores. However, this was not the case for cyclohexane. (b) The orthorhombic-to-cubic transition temperature of cyclohexanone inversely scaled with the pore radius of the host porous silica. Similar behavior for cyclohexane's solid-to-solid (monoclinic-to-cubic) transition has been reported by us previously. Though the inverse dependence was still valid, the presence or absence of bulk cyclohexanone outside the pores affected the degree of suppression of the transition temperature. (c) The cubic-to-orthorhombic transition of cyclohexanone was strongly influenced by whether the bulk fluid was present outside the pores. In the absence of the bulk, the transition temperature was considerably suppressed relative to the bulk transition temperature. However, in the presence of the bulk, the confined and the bulk transitions occur at the same temperature. This was not true for cyclohexane's solid-to-solid transition temperature. 

SESSION W2: 
Chair: Pierre E. Levitz 
Monday Afternoon, November 30, 1998 
Staffordshire (W)

1:30 PM *W2.1 
CONFINEMENT PROBED IN 3D ON THE NANOSCALE. Rene M. Overney , University of Washington, Dept. of Chemical Engineering, Seattle, WA. 

The mechanical and rheological properties of matter in the submicrometer vicinity of interactive solid interfaces is discussed. Matter of particular interest are polymer films and complex liquids. We will discuss in this paper several aspects of confinement: (a) confinement due to interfacial interaction, (b) confinement due to molecular complexity, and (c) confinement due to strain. The measurements which will be described were conducted by a modified scanning probe microscope (SPM) on the nanometer scale under quasi-static equilibrium conditions. The paper will discuss recent results obtained by SPMís nanorheometer and will review calibration procedure necessary for the quantification of SPM results related to rheological measurements. 

2:00 PM *W2.2 
RESPONSE OF NANOSCALE CONTACTS TO SHEAR. K.J. Wahl , W.R. Barger, S.A. Asif, Code 6170, Chemistry Division, Naval Research Laboratory, Washington, DC; W.N. Unertl, LASST, U. Maine, Orono, ME. 

Mechanical properties of contacts with nanometer dimensions are important in understanding the behavior of microscale sliding contacts, especially if viscoelastic materials are involved and as films approach two-dimensional limits. In our experiments, we investigate dynamic processes occurring during the formation and breaking of nanometer scale contacts to various materials including poly(vinylethylene) and Langmuir-Blodgett monolayers. Measurements are made using an AFM (atomic force microscope) operated in shear modulation mode, where the sample position is modulated laterally with amplitudes as small as 1 and modulation frequencies in the range 50 to 1200 Hz. The elastic/viscoelastic mechanical response of contacts to shear modulation is measured and analyzed to obtain contact stiffness and relaxation times, and data are compared and contrasted with macroscopic values. 

2:30 PM W2.3 
ADHESION FORCE MEASUREMENT OF HYDROLYZED PVA USING A FORCE MICROSCOPE. Wentao Li , Shouren Ge, Dilip Gersappe, Mariam Rafailovich, Jon Sokolov, Dept of MS&E, SUNY at Stony Brook, Stony Brook, NY; and G.H. Ko, MCRIC, Cambridge, MA. 

The adhesion force between colloidal particles plays a critical role on behaviors of dispersions and emulsions. The structure of dispersing agents(or surfactants) affects the adhesion force and hence the stability of colloidal systems. In this study, we attached hydrolyzed PVA(polyvinyl acetate) chains onto silicon wafer surfaces and AFM tips(refered as modified tips). Using a force microsope, we measured the adhesion force between PVA(in air, water and PVA solution) on silicon wafer surfaces and AFM tips(unmodified and modified type). The force-distance curves indicated that Flory-Huggins parameter and chain length of PVA chains affect the adhesion force. The simulation based on SCF calculation gave good agreement between theory and experimental results. 

3:15 PM *W2.4 
SLIDING FRICTION BY SCANNING PROBE MICROSCOPY. Virginie Pasquier and J.M. Drake, Exxon Research and Eng. Co., Annandale, NJ. 

Scanning Probe Microscopy techniques have shown a real capability in probing atomic processes that are taking place at the contact between sliding surfaces. This tool has the potential to improve our atomic scale understanding of tribological behaviors. An example of challenging problem which remains unresolved is demonstrating the origin of nanoscale friction. Using AFM for tribology studies of bare interfaces with or without molecular ìfriction modifiersî will be discussed. A particular attention will be paid to the technical aspects of AFM measurements: force calibration, tip physical and mechanical characterization, which are the key issues that must be resolved to quantify and standardize AFM friction measurements. 

3:45 PM W2.5 
ATOMIC SCALE FRICTION: WHAT CAN BE DEDUCED FROM THE RESPONSE TO A HARMONIC DRIVE? V. Zaloj , M. Urbakh and J. Klafter, School of Chemistry, Tel Aviv University, Tel Aviv, ISRAEL. 

We investigate the response of a confined chain to a harmonic driving force. A model is introduced which mimics recent measurements on friction using surface forces apparatus. The model predicts a critical driving amplitude below which the response is linear. For higher amplitudes the system exhibits a nonlinear behavior and shear thinning. A novel origin for the thinning is proposed which stems from energy dissipation due to stick-slip motion and the transition to smooth sliding. We establish relationships between the microscopic parameters of the system and phenomena observed in rheology and tribology. 
The predicted properties of the harmonically driven system should be amenable to experimental tests. In particular, we propose investigations of the shear moduli at low driving amplitudes from which one can deduce microscopic scale parameters, such as the friction constant, the effective amplitude and period of the substrate potential. These parameters characterize the system for all driving amplitudes. Our interpretation of the mechanism that leads to shear thinning, which occurs at higher amplitudes, can be verified by careful analysis of the spring force time series. This can be done, for instance, by studying the corresponding power spectrum. 

4:00 PM W2.6 
MOLECULAR MODELING OF ALKYLSILANE MONOLAYERS AS LUBRICANTS. Mark J. Stevens , Sandia National Laboratories, Albuquerque, NM. 

Self-assembled monolayers of alkylsilanes have been of great interest the last 20 years. Recently, work has focused on using these SAMs as lubricant in micromachines which involve tiny gaps demanding new types of lubricants. One of the best candidates are alkyltrichlorosilanes. These systems are thought to get added stability by cross-polymerizing. I will show that cross-polymerization is impossible due to steric effects and that it would yield monolayer densities that are too large. The effect of steric interactions on the monolayer coverage will be discussed particularly comparing alkyltrichlorosilanes and alkylmonochlorosilanes. The basic nature of the adsorption of alkylsilanes onto silica surfaces remains to be understood. Results of molecular dynamics simulations of the monolayers will be presented. The molecular tilt angle are other structural characteristics of the SAMs will be discussed. 
Simulations of two monolayers in contact to determine friction and mechanical characteristics will be presented. 

4:15 PM W2.7 
MOLECULAR DYNAMICS SIMULATION OF FRICTION FOR HYDROCARBON THIN FILMS. Hiroyuki Tamura , Zhou Hui, Ryuji Miura, Momoji Kubo, Kazuo Teraishi, Akira Miyamoto, Dept. of Materials Chemistry, Graduate School of Engineering, Tohoku Univ, Sendai, JAPAN. 

In machinery, automobile, electronics, and many other industry, control of rheological and tribological phenomena is particularly important to prevent wear and to save energy. For example, to increase the capacity of magnetic storage systems, the lubrication of the head-disk interface must be improved. To rationalize rheological and tribological mechanisms, atomic-level origins of friction have been investigated both experimentally and theoretically. It is observed that friction properties of lubricant films depend on the molecular structure and the film thickness. In this study, friction properties of cyclohexane, n-hexane and iso-hexane molecules confined between two solid surfaces were calculated by MD simulations. The interaction between atoms in organic molecules was approximated by consistent valence force field (CVFF) potential. 
In this simulation, layered structure was observed for each model, and the friction coefficients depended on the film thickness. When the interaction between solid surfaces and the molecules is stronger than that between molecular layers, the slippages occurred at the interfaces of molecular layers. In these cases, the friction coefficients of bilayers are lower than those of monolayers. As for n-hexane monolayer, the rolling of molecules around the chain axis was observed. The rolling of n-hexane molecules mitigates the frictional force. This effect results in an anisotropy of friction depending on the sliding direction. As for n-hexane bilayer, bridging molecules increase friction at interface of molecular layers. Bridging molecules are pulled along sliding direction by shearing stress. When slippage interface is separated, the frictional force decreases. Calculated friction coefficients of iso-hexane and cyclohexane molecules were larger than that of n-hexane molecules. Similar tendency is observed by experimental measurements under elastohydrodynamic lubrication. 

4:30 PM W2.8 
MOLECULAR CONFIGURATIONS AND SOLVATION FORCES IN CONFINED ALKANE FILMS. Jee-Ching Wang and Kristen A. Fichthorn , The Pennsylvania State University, Dept. of Chemical Engineering, University Park, PA. 

Experimental studies of confined alkanes with the surface force apparatus have shown that small structural differences can lead to dramatically different solvation forces in these films. Linear alkanes exhibit oscillatory solvation forces, while irregularly shaped molecules, such as isoalkanes and squalane, show a monotonic, nonoscillatory relationship between force and separation. Recent computer simulations have reproduced and explained the origin of oscillatory solvation forces for confined symmetric molecules. However, to date, no simulation study has been able to create non- oscillatory solvation-force profiles for confined, asymmetric molecules. Based on its novel, pillared layered structure in its free-standing film on a solid surface [Wang & Fichthorn, J. Chem. Phys. 108, 1653 (1998).], the doubly branched t-butyl-hexane molecule is believed to be asymmetric enough to show nonoscillatory solvation forces in simulation. By employing a new NP_||AT ensemble method, which we have developed for simulating dense, confined, molecular systems, the molecular configurations, solvation forces, and a number of other structural and dynamical properties of the confined n-decane and t­butyl-hexane films will be compared and discussed. 

4:45 PM W2.9 
WETTING OF SOLID/VAPOUR INTERFACES WITH ALIPHATIC MOLECULES OF VARIOUS CHAIN LENGTHS. Hans Riegler , Max-Planck-Institut für Kolloid- und Grenzflächenforschung, Berlin, GERMANY. 

Experimental results on the statics and kinetics of wetting of solid/vapour interfaces with alkanes of various chain lengths (between 15 and 50 C-atoms) investigated by optical microscopy, AFM, x-ray- and neutron-reflectivity, ellipsometry, and contact-angle measurements are presented and discussed. Depending on the chain length and temperature n-alkanes wet silicon oxide/air interfaces with up to three different topologies: 
(1) At temperatures below bulk melting: frozen droplets (polycrystalline lamellae) and/or pancake-like islands (lamellar ordering parallel to the interface) on a closed monolayer (condensed lamellar phase, molecules oriented normal to the interface). 
(2) In an intermediate temperature range: droplets of liquid alkane on the condensed monolayer. 
(3) At higher temperatures: films of liquid alkane with a thickness of about 200Å (maximum) or less. If there is more material available than necessary for a 200Å thick closed film it is accumulated in droplets of liquid alkane on top of the liquid film (pseudopartial wetting configuration). 
If the amount of spread alkane is less than necessary for one monolayer (molecules oriented upright) we find dendritic shaped domains (solid) of monolayer thickness in coexistence with an ultrathin film (few Å, liquid?) in temperature ranges (1) and (2). We are currently investigating the impact of the confined geometry (film thickness) on the molecular ordering and the phase transition behaviour. The observed wetting topologies are explained by the difference between bulk and interface freezing temperatures and by the sensitivity of wetting properties to the interfacial molecular ordering. In perspective the investigations aim at the elucidation of the relationship between local molecular ordering, short and long range interactions, and wetting properties. 

SESSION W3: 
Chair: Sunil K. Sinha 
Tuesday Morning, December 1, 1998 
Staffordshire (W)

8:30 AM *W3.1 
CONTROLLING FRICTIONAL FORCES. Joseph Klafter, Mikhail G. Rozman, Michael Urbakh , Veaceslav Zaloj, Tel Aviv Univ, School of Chemistry, Tel Aviv, ISRAEL. 

From a practical point of view one wishes to be able to control frictional forces so that overall friction is reduced or enhanced, the stick-slip regime is eliminated, and instead smooth sliding is achieved. Controlling frictional forces has been traditionally approached by chemical means, namely, using lubricating liquids. A different approach, proposed here, is by controlling the system mechanically. Our goal is twofold: (a) to achieve smooth sliding at low driving velocities, which otherwise correspond to the stick-slip regime; (b) to decrease the frictional forces. The possibilities to control friction are demonstrated using a model system that displays the main experimentally observed behaviors, obtained in measurements on nanoscale confined liquids and granular layers. The methods should be applicable to real systems for which time series of dynamical variables are experimentally available.The only necessary condition for the application of the proposed methods is the existence of (possibly unstable) sliding regimes of motion in the experimental systems. 

9:00 AM *W3.2 
FRICTION FROM ATOMIC TO TECTONIC SCALES. Jean Carlson , University of California, Santa Barbara, Department of Physics, Santa Barbara, CA. 

Friction and wear lead to significant losses both economically and environmentally. While recipes to minimize and control friction have been available since ancient times, both the underlying mechanisms responsible for dissipation and damage and the phenomenological descriptions which characterize the macroscopic forces in many cases remain poorly understood. However, a combination of new experimental methods, increased computational capacity, and progress in understanding nonequilibrium phenomena in complex systems are creating new and exciting opportunities for progress. Topics that will be discussed include derivation of phenomenological friction laws from experimental data, microscopic models of interfaces, and application of friction models in seismology and engineering. 

10:00 AM *W3.3 
ROLLING FRICTION: THE CASE OF FULLERENES AND INORGANIC FULLERENE-LIKE MATERIALS. R. Tenne , Dept of Materials and Interfaces, Weizmann Institute, Rehovot, ISRAEL. 

The rheological and mechanical properties of small solid particles sheared between two reciprocating metal surfaces will be discussed. The importance of rolling friction in solid lubricants made of nanoparticles (<0.1 micron) heaving close to a spherical shape will be presented. The synthesis of hollow and quasispherical nanoparticles (inorganic fullerene like IF) from inorganic compounds with layered structure, like MoS₂ was recently accomplished. Nanoparticles of such compounds are shown to be unstable in the planar form and adopt a spherical seamless topology which minimizes their surface energy.