A reevaluation of forces measured across thin polymer films: Nonequilibrium and pinning effects

Abstract

We have measured forces between molecularly smooth solid surfaces separated by thin films of molten polydimethylsiloxane. We show that a long-range repulsion reported in earlier work is not an eqUilibrium force, but can be attributed to viscous drag effects. Consistent with previous results, the viscosity of the film can be modeled by assuming that a layer of polymer molecules is immobilized or "pinned" at each surface for a time longer than the time scale of the measurements. We propose that this pinning is a result of entanglement-like effects in the vicinity of a wall.

Tribology of Confined Fomblin-Z Perfluoropolyalkyl Ethers: Role of Chain-End Chemical Functionality

“…This "viscous drag" has been calculated by Horn et al 54 for systems of different bulk viscosity. For our spring stiffness of 930 N/m, radius of curvature R ) 1.2-1.8 cm, and viscosity of ap- proximately 1.5 P (estimated for 25°C from the data in Table 1) for the samples in Figure 1A-C, the equilibration time after an inward step of 100 Å should be equal to or less than 6 min (depending on the radius) at a separation of 100 Å from the "hard-wall" (immobilized surface layer) and 20 min at 20 Å from the hard-wall.…”

“…Experimental section) for these three polymers should thus be sufficient to avoid effects from viscous drag. For the extremely viscous p-phenoxyanilinium-terminated sample (Figure 1D), the calculation according to ref 54 suggests that the equilibration times already at larger separations would be on the order of hours. Interestingly, such long times were not necessary in order to observe experimentally that the surfaces had come to a steadystate position for this system, and the range of the repulsion was not longer than for the three less viscous samples.…”

“…For our spring stiffness of 930 N/m, radius of curvature R ) 1.2-1.8 cm, and viscosity of ap- proximately 1.5 P (estimated for 25°C from the data in Table 1) for the samples in Figure 1A-C, the equilibration time after an inward step of 100 Å should be equal to or less than 6 min (depending on the radius) at a separation of 100 Å from the "hard-wall" (immobilized surface layer) and 20 min at 20 Å from the hard-wall. 54 The equilibration times chosen (cf. Experimental section) for these three polymers should thus be sufficient to avoid effects from viscous drag.…”

“…The first three samples in Table 1 were slightly viscous, which might cause time effects during approach and separation of the surfaces (compression of the polymer film). 54 The p-phenoxyanilinium-terminated sample was extremely viscous, which made measurements particularly difficult. To facilitate the measurements in these systems, the stiffness of the force-measuring spring was chosen to be 930 N/m, which is an order of magnitude stiffer than commonly used for force measurements on simple liquids.…”

Force measurements with the atomic force microscope: Technique, interpretation and applications

“…First direct quantitative measurements were made with the SFA (e.g. [820][821][822][823]). Reviews are given in Refs.…”