No abstract

The dynamic mechanical shear response was measured of sharp fractions of molten siloxane oligomers, PDMS [poly(dimethylsiloxane) ] and PPMS [poly(phenylmethylsiloxane) ], confined between single crystals of muscovite mica, at film thicknesses <100 Á and a temperature of 23 °C. Five conclusions emerge.(1) A liquidlike mechanical response (in which the apparent dynamic viscosity was significantly enhanced over that of the bulk liquid) was clearly distinguished from a yield stress response (in which sliding over the experimental time scale occurred only after a critical yield stress was exceeded). These same features were observed previously for ultrathin films of smaller nonpolar molecules, and, despite quantitative differences in the present systems, the observation appears to be general. (2) The precise film thickness at the onset of the yield stress response, observed at film thicknesses <30-50 Á, did not depend on the molecular weight of the PDMS fractions but did depend markedly on details of the history of the experiment. (3) The yield stress increased with measurement time without a discernible change in separation. The times for the yield stress to reach a plateau increased with molecular weight and ranged from approximately 9 min (PDMS, Mn = 890) to approximately 400 min (PDMS, = 6330) and approximately 650 min (PPMS, Af" = 2240). ( 4) Enhanced viscous response was observed at larger film thickness than for liquids of smaller molecules. The distance dependence of the apparent dynamic viscosity at 0.875 Hz was quantified for one sample (PDMS, M" = 1670) by measuring the phase shift and amplitude attenuation in sinusoidal oscillation. The apparent dynamic viscosity appeared to diverge with diminishing film thickness. ( 5) After discussing how the act of shear may affect the structure of the liquid, we conclude that the yield stress rheological response may reflect a metastable, history-dependent state, in which relaxations of trapped chains have become slower than the experimental time scale of minutes to hours.

The design, calibration, and performance are described of an apparatus to study the shear response of ultrathin liquid films. The device, a modification of the surface forces apparatus, measures the resistance to shear of liquids confined between two atomically smooth solid surfaces. The surfaces are separated by distances on the order of the size of the liquid molecules (liquid film thickness &lt;10 nm). Shear forces with periodic time dependence are applied to one surface while the second is held fixed, and any motion so induced is analyzed to determine the behavior of the liquid film. The frequency and amplitude of the shear forces applied can be varied over a wide range (0.03–60 Hz frequency and 0.1–1000 nm amplitude) to achieve different values for the magnitude of the shear rate. The dynamic response of the device is linear in the applied force at a given frequency; nonetheless, nonlinear dependence of the liquid’s shear resistance on the shear rate, net normal pressure, and film thickness can be observed with the technique. The mechanical and electrical characteristics of the device are modeled to gain insight into its behavior and facilitate analysis of the measured data. The central results of this approach are expressions for the magnitude of the shear rate and an effective friction coefficient of the liquid film in terms of easily measured electrical quantities. For convenience the friction coefficient is restated as an apparent dynamic viscosity in analogy to continuum hydrodynamics, but the validity of the approach does not depend on a particular understanding of the structure in the liquid layer. The applications and limitations of the device are discussed, as well as other potential uses to which the apparatus may be applied by rational extension to the approach presented.

We study the solid-like shear response (the static friction) of liquid films of both compact and chain geometry, whose thickness approaches molecular dimensions. The nonpolar liquids were confined between parallel plates (step-free single crystals) of muscovite mica. The finite shear stress required to produce sliding increased with measurement time over intervals from minutes to hours, at temperatures above the bulk melting temperature or the bulk glass transition temperature. This loss of fluidity may reflect a vitrified state imposed by the liquid's confinement.