Migration of liquid polymers on solid surfaces

In situ AFM Observation of the Movements of Isolated Isotactic Poly(methyl methacrylate) Chains in a Precursor Film of an Oligo(methyl methacrylate) Droplet Spreading on Mica

Atomic force microscopy (AFM) is a powerful tool to observe polymer chains at the molecular level. In this study, we show that the movements of isolated linear polymer chains in a precursor film of a droplet of an oligomer spreading on a substrate could be visualized in situ at the molecular level by AFM for the first time. The system was an isotactic poly(methyl methacrylate) (it-PMMA) solubilized in an oligo(MMA) matrix (it-PMMA/oligo(MMA) = 1/10,000 w/w) spreading on mica under high humidity. Because of the limited resolution of the AFM instrument, condensed linear polymer chains could not be visualized, but a small amount of it-PMMA chains that were solubilized as isolated chains in the oligo(MMA) matrix could be visualized in the precursor film, the contrast of which came from a large difference in glass transition temperature (T_g) of it-PMMA and oligo(MMA). The it-PMMA chains in the precursor film spread in the radial direction of the droplet with vigorously changing chain conformations. The spreading rate of it-PMMA chains under 72% relative humidity was ∼1/30 of the spreading rate of the oligo(MMA) matrix, which was estimated based on the decrease in the volume of the macroscopic droplet. The spreading of the it-PMMA chains and droplet strongly depended on humidity and was suppressed with the decrease in humidity, most likely because of the increase in friction with the substrate. The difference in the spreading rate of it-PMMA and oligo(MMA) further increased under low humidity. The dynamic molecular information of a precursor film by AFM should help to elucidate the wetting dynamics on a substrate.

Rouse 2D Diffusion of Polymer Chains in Low Density Precursor Films of Polybutadiene Melts

We took advantage of pseudopartial wetting to promote the spreading of precursor films whose surface density smoothly decays to zero away from a sessile droplet. By following the spreading dynamics of semidilute precursor films of polybutadiene melts on silicon wafers, we measure molecular diffusion coefficients for different molar masses and temperatures. For homopolymers, chains follow a thermally activated 2D Rouse diffusion mechanism, with an activation energy revealing polymer segment interactions with the surface. This Rouse model is generalized to chains with specific terminal groups.

Combining Ellipsometry and AFM To Probe Subnanometric Precursor Film Dynamics of Polystyrene Melts

We investigate the evolution over time of the space profiles of precursor films spreading away from a droplet of polymer in the poorly explored pseudo-partial wetting case. We use polystyrene melt droplets on oxidized silicon wafers. Interestingly, the film thicknesses measured by ellispometric microscopy are found in the 0.01 to 1 nm range. These thicknesses were validated by atomic force microscopy measurements performed on the textured film obtained after quenching at room temperature. From this, an effective thickness is obtained and compares well to the thicknesses measured by ellipsometry, validating the use of an optical method in this range of thickness. Ellipsometric microscopy provides a height resolution below the ångström with lateral resolution, image size, and framerate well adapted to spreading precursor films. From this, we demonstrate that precursor films of polystyrene consist of polymer chains with a surface density decreasing to zero away from the droplet. We further find that the polymer chains follow a simple diffusive law with the diffusion coefficient independent of density. This demonstrates that polystyrene chains spread independently in precursor films in pseudo-partial wetting condition. This behavior differs significantly from the case of chains spreading in total wetting for which the diffusion coefficient was found in the literature to depend on surface density or thickness.

Motion picture imaging of a nanometer-thick liquid film dewetting by ellipsometric microscopy with a submicrometer lateral resolution

We visualized the detwetting of a nanometer-thick unstable liquid film on a nanotextured solid surface with a high lateral spatial resolution. The dewetting was imaged as a motion picture at a submicrometer spatial resolution and a frame rate of 4 frames/s, using ellipsometric microscopy in a vertical objective configuration. The observation revealed that the dewetting process significantly depends on the sign of the disjoining pressure Pi. When Pi is negative, the film rupture due to the spinodal dewetting proceeds to droplet formation in a single step, whereas, when Pi is positive, the film rupture due to the spinodal dewetting stops when the pressure of the thicker region balances with that of the thinner region, and then the heterogeneous grooves are nucleated and grow. The dewetting process dependence on the sign of Pi can be found in systems other than that reported here because the sign of Pi changes at the local maximum of the surface energy.

Diffusive motion of molecules in submonolayer liquid films on a solid surface

The spreading of submonolayer liquid polymer films on a solid surface that consist of molecules with different mobilities was investigated. The molecular conformations of the adsorbed and free molecules were estimated from the dispersive surface energy measurement and the relationship between the diffusion coefficient of the free molecules and the coverage of the adsorbed molecules were obtained from the spreading profile measurement. A free mobile molecule lies flat on the solid surface and an adsorbed molecule lies less flat than a mobile one. The relation between the diffusion coefficient and the coverage can be explained by the percolation model at a small coverage and it can be explained by the reptation model at a large coverage.

Diffusion of hard disks and rodlike molecules on surfaces

We study the submonolayer diffusion of hard disks and rodlike molecules on smooth surfaces through numerical simulations and theoretical arguments. We concentrate on the behavior of the various diffusion coefficients as a function of the two-dimensional (2D) number density rho in the case where there are no explicit surface-particle interactions. For the hard disk case, we find that while the tracer diffusion coefficient D(T)(rho) decreases monotonically up to the freezing transition, the collective diffusion coefficient D(C)(rho) is wholly determined by the inverse compressibility which increases rapidly on approaching freezing. We also study memory effects associated with tracer diffusion, and present theoretical estimates of D(T)(rho) from the mode-mode coupling approximation. In the case of rigid rods with short-range repulsion and no orientational ordering, we find behavior very similar to the case of disks with the same repulsive interaction. Both D(T)(rho) and the angular diffusion coefficient D(R)(rho) decrease with rho. Also in this case D(C)(rho) is determined by inverse compressibility and increases rapidly close to freezing. This is in contrast to the case of flexible chainlike molecules in the lattice-gas limit, where D(C)(rho) first increases and then decreases as a function of the density due to the interplay between compressibility and mobility.

The Molecular Spreading of Nonpolar Perfluoropolyether Films on Amorphous Carbon Surfaces

The spreading mechanism of nonpolar perfluoropolyether films on carbon surfaces is examined in the mesoscopic regime, including both submonolayer and multilayer films. For the submonolayer film, adsorption-desorption is a main mechanism for spreading, and the surface diffusion coefficients increase as the film thickness increases. The driving force for the spreading in the submonolayer regime is the gradient of the disjoining pressure, which is described by the two-dimensional virial equation. For the multilayer film regime, the spreading characteristics are determined by the molecular weight and the disjoining pressure gradient, which is assumed to be purely van der Waals in nature. We adopt a partial slip boundary condition to analyze the multilayer film, which qualitatively explains the dependence of the surface diffusion coefficient on film thickness. Copyright 2000 Academic Press.

Water Adsorption on Thin Film Media

Disk drives experience a wide range of temperature and humidity during storage and operation. Humidity influences media tribology through the effects of adsorbed water on the interaction of Zdol lubricant hydroxyl end groups with the carbon overcoat. We measured the effects of Zdol 4000 thickness and chemisorption on the water adsorption isotherm and surface energy. The potential distortion model of Adamson was employed to derive the film thickness at saturation from the isotherms and contact angles. The isotherm was surprisingly independent of the Zdol chemisorption and thickness up to 1.8 nm, because, in this range of Zdol thickness, the adsorbed water resides in pores which exclude the larger Zdol end groups. The water adsorption isotherm provides a sensitive probe of overcoat structure due the ability of water to reach pores in the overcoat which are inaccessible to Zdol. Above a Zdol thickness of 1.8 nm there is a jump in the water adsorption as additional adsorption takes place on the "free" hydroxyl groups. The mechanism for the previously observed decrease in Zdol lubricated media durability at low humidity is derived from the effect of adsorbed water on Zdol surface diffusion and chemisorption. Copyright 2000 Academic Press.