Perturbation of the yield-stress rheology of polymer thin films by nonlinear shear ultrasound

We investigate the nonlinear response of macromolecular thin films subjected to high-amplitude ultrasonic shear oscillation using a sphere-plane contact geometry. At a film thickness comparable to the radius of gyration, we observe the rheological properties intermediate between bulk and boundary nonlinear regimes. As the driving amplitude is increased, these films progressively exhibit oscillatory linear, microslip, and full slip regimes, which can be explained by the modified Coulomb friction law. At highest oscillation amplitudes, the interfacial adhesive failure takes place, being accompanied by a dewettinglike pattern. Moreover, the steady state sliding is investigated in thicker films with imposed shear stresses beyond the yield point. We find that applying high-amplitude shear ultrasound affects not only the yielding threshold but also the sliding velocity at a given shear load. A possible mechanism for the latter effect is discussed.

Figure 1

Experimental setup (not to scale). (a) The probe is composed of three lenses stuck on a glass plate and is deposited on a quartz coated with the polymer film. The contact area is viewed with a video camera, while the oscillation amplitude is varied. (b) The apparatus is mounted on a plane which is inclined above the threshold angle ${\theta }_c$. The movement of the probe during the steady sliding is followed with the same video system as in (a).

Figure 2

Shift of resonance frequency (a) and inverse of quality factor (b) as a function of the amplitude of oscillation for two polyisobutylene films. Black and white symbols represent several measurements for thicknesses 10 and 20 nm, respectively. Green (light gray) lines are fit to Eq. (9) (see text). When the amplitude of oscillation varies, four distinct regimes can be identified. See Fig. 1, $\theta =0$.

Figure 3

Images of the contact area under oscillations. The amplitude of oscillation is indicated on each image. $h=10$ nm; $\theta =0$.

Figure 4

(a) Shear stress as a function of sliding velocity. The solid line is a fit of experimental data with $\sigma ={\sigma }_0+kV$. (b) Contact area as a function of the velocity. Spheres and stars correspond to polymer films of 50 and 300 nm. The solid line is a fit with Eq. (12). Black symbols and red (light gray) symbols are data without and with vibrations, respectively.

Figure 5

Sliding velocity under vibrations ${\text{V}}^{vib}$ normalized by the sliding velocity without vibrations, versus oscillation amplitude. The film thickness is 50 nm for white symbols) and 300 nm for black symbols. The inset details the protocol used for the measurements (see text).

Figure 6

Hysteresis cycle for oscillation amplitudes corresponding to a shear force inferior, and equal, to the threshold friction force $F_s$ [16].

Figure 7

Microslip regime 2. (a) $k_T/k_M$ as a function of $F_T$. Straight lines are fits to Eq. (4) (b) Normalized quality factor as a function of the oscillation amplitude $U_T$. According to the text, the slope equals the coefficient of friction. $\theta =0$.

Figure 8

Proposed hysteresis cycle for oscillation amplitudes corresponding to full slip. The Mindlin cycle corresponds to light gray. Dissipation due to full slip is represented in dark gray.

Figure 9

Oscillatory shear force $F_T=k_T{U}_T$ as a function of $U_T$. For large shear amplitudes $U_T$ the shear force is constant. The legend is the same as Fig. 2. $\theta =0$.

Figure 10

Decrease of surface area plotted against oscillatory energy. $\theta =0$.

Figure 11

(a) The chain diffuses during sliding and (b) entangles with chains adsorbed on the surface. (c) Vibrations rupture entangled chains.