Thin film rheology and tribology of confined polymer melts: Contrasts with bulk properties

A modified Surface Forces Apparatus was used for making rheological and tribological measurements of thin fluid films, and polybutadiene (PBD) of M-w approximate to 7000 was studied in detail. At low shear rates, PBD exhibits bulklike properties in films thicker than about 200 nm. Ln thinner films (200-20 nm), the shear viscosity eta(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 eta(eff) GI, G '', and the friction force F as a function of frequency, sliding speed, and other system parameters.