Ultrathin films of simple nonpolar molecular fluids (3-8 segmental dimensions thick) show a strikingly long relaxation time in response to oscillatory shear when confined between mica plates at 27 °C. When the shear rate exceeds this inverse time, the effective viscosity decays as an apparent power law in the shear rate, implying considerable distortion of the dynamic structure. The relaxation time is orders of magnitude longer than the Brownian relaxation time in the bulk state and may reflect collective motions induced by confinement.

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The large-amplitude nonlinear shear rheology of polymer melts confined between strongly adsorbing surfaces (parallel plates of mica) was studied as a function of strain, frequency, and thickness of the polymer films. The shear strains varied from less than 0.1 (linear response) to over 30 (at which the film structure was strongly modified by the imposed shear). The measurements employed a surface forces apparatus modified for dynamic mechanical shear. The polymers were atactic poly(phenylmethylsiloxane) (PPMS), with chain lengths from 31 to 153 skeletal bonds. The nonlinear shear forces, decomposed into a Fourier series of harmonic frequencies, were always odd in the excitation frequency, as required by symmetry considerations. The in-phase and out-of-phase oscillatory shear responses at the same frequency as the excitation (the nonlinear storage and loss moduli G\ and Gi", respectively) were analyzed. Four principal conclusions emerged. First, from the frequency dependence of G\ and Gi" at constant strain, we conclude that relaxations were accelerated by large strain. Second, a marked decrease of both Gf and Gi" was observed with increasing strain at constant frequency, except at the smallest film thickness, «s40 Á, where Gi" passed through a maximum with increasing strain but Gi continued to display shearthinning. Third, the critical strain for onset of nonlinear response increased with the excitation frequency. Fourth, at sufficiently large strains (larger than 10), the shear moduli were independent of polymer molecular weight (comparisons made at fixed film thickness) and appeared to reach limiting strain-independent levels at sufficiently large strains. This final observation contrasts sharply with the linear response and is consistent with shear-induced loss of inter digitation between opposed adsorbed polymer layers, consistent with the tendency toward slippage of adsorbed polymer layers over one another.

A method is described to deposit a securely attached, self-assembled monolayer of octadecyltrichlorosilane (OTS) on the surface of freshly cleaved muscovite mica. Comparison of the infrared methylene spectra with those of closely packed Langmuir-Blodgett films implies that the surface coverage of the OTS films was a fraction 0.8–0.9 that of films formed by Langmuir-Blodgett (LB) methods. However, LB monolayers are less securely attached to the substrate. The contact angle of water on these self-assembled monolayers remained over 100° for over 24 h and it suffered no noticeable degradation after prolonged reflux in cyclohexane. The method to form an OTS monolayer on mica involves three steps; first, ion exchange of the native K+ ions of cleaved mica for H+ ions; second, control of the quantity of resulting water on the mica surface; third, adsorption and surface polymerization of octadecyltrichlorosilane (OTS) by self-assembly from dilute cyclohexane solution.

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