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.

The frequency-dependent shear response of an ultrathin polymer melt (polyphenylmethylsiloxane) confined between adsorbing surfaces (parallel plates of mica) is described. The sinusoidal deformations were sufficiently small to give linear response, implying that measurement did not perturb the film structure. A remarkable transition was observed with decreasing thickness. When the film thickness was less than five to six times the unperturbed radius of gyration, there emerged a strong rubber-like elasticity that was not characteristic of the bulk samples. This result indicates enhanced entanglement interactions in thin polymer films and offers a mechanism to explain the slow mobility of polymers at surfaces.

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Surface forces were measured between mica surfaces bearing polystyrene adsorbed to saturation from dilute cyclopentane solution (e temperature, T, = 19.5 "C). The temperatures were 18, 23,28, and 34 "C. Four conclusions emerge. First, for M, = 515 000, the adhesive minimum grew with the time (up to 14.5 h) that the adsorbed layers were kept together. This suggests the existence of extremely slow chain relaxations. Second, for M, = 490 000 and 515 000, the layer thickness at the point of steep shortrange repulsion was substantially less at 28 "C and higher temperature than at 23 "C and lower temperature. This shows that the mass adsorbed was remarkably sensitive to small changes in solvent quality. Third, for M, = 115 000,490 000, and 1 080 000 at fixed temperatures, the layer thickness at the point of steep short-range repulsion scaled approximately with the radius of gyration, R, , just as is expected for the layer thickness at a single surface under poor solvent conditions. Fourth, in comparisons of the longrange forces (>R,) as a function of molecular weight and temperature after forces had equilibrated for up to 2-5 min, the force-distance profiles were reversible, but their sign, strength, and range did not fit a simple pattern. The range of attraction at temperatures above T, passed through a maximum with increasing molecular weight and increasing temperature. The relative influence of kinetics and thermodynamics on the force-distance profiles is discussed. The initially distinct polymer layers appeared to interdiffuse to only a limited extent over the time scale of the experiment, which implies that the present systems were in metastable states far from thermodynamic equilibrium. A discussion of error and of experimental limitations is included.

Force-distance profiles were strikingly sensitive to small differences in temperature and solvent composition. Surfaces forces were measured at 18 and 28 °C between mica surfaces bearing polystyrene (Mw = 490000 g-mol'1) adsorbed from dilute cyclopentane solution ( temperature TQ = 19.5 °C). Forces at long range were monotonically repulsive at 28 °C and mildly attractive at 18 °C provided that a cyclopentane containing 5% linear pentanes was used. This contrasts with experiments in 99% pure cyclopentane, in which the range of detectable forces was (2-3)Rq (unperturbed radius of gyration) at 18 °C and (5-6)Rq at 28 °C, and the forces at long range were strongly attractive at 18 °C and weakly attractive even at 28 °C. A change of sign of the long-range interactions between adsorbed homopolymers, over a small range of temperature, does not appear to have been reported previously.