Thin Film Rheology and Tribology of Confined Polymer Melts:  Contrasts with Bulk Properties

Abstract

A modified Surface Forces Apparatus was used for making rheological and tribological measurements of thin fluid films, and polybutadiene (PBD) of M w ≈ 7000 was studied in detail. At low shear rates, PBD exhibits bulklike properties in films thicker than about 200 nm. In thinner films (200−20 nm), the shear viscosity η eff and moduli G ‘ and G ‘‘ become quantitatively modified from those of the bulk. However, a sinusoidal input still produces a sinusoidal output, and a modified WLF representation remains applicable to such films. On entering the tribological regime (film thickness <30 nm) polybutadiene exhibits highly nonlinear behavior and yield points, indicative of phase transitions to “glassy” or “solidlike” states. Other features are (i) a shift of the slip plane, (ii) an additional normal force component, and (iii) the emergence of new scaling relationships for η eff, G ‘, G ‘‘, and the friction force F as a function of frequency, sliding speed, and other system parameters.