Healing of confined polymer films following deformation at high shear rate

Abstract

Recovery of equilibrated linear viscoelastic response of confined polymer melts, following cessation of large-amplitude shear in a surface forces apparatus, was found to be a single exponential process. The most extensive experiments concerned a polydimethylsiloxane of narrow molecular weight distribution and weight-average molecular weight M-w = 8330 g mol(-1), for which recovery times were in the range 2-12 h when the film thickness (D) was D/R-G = 0.5-6 (R-G is radius of gyration). Initially, to produce the deformed state, the films were sheared with effective shear rate approximate to 10(4) s(-1). Recovery was probed by the subsequent application of small-amplitude sinusoidal shear forces at 256 Hz. Surprisingly, the nonlinear and linear shear moduli evaluated at the input frequency nearly coincided just before and just after cessation of large-amplitude shear. Recovery time constants, tau(R), increased linearly with the prior shear rate at a given thickness (D). But at a given shear rate and variable D, tau(R) passed through a maximum at D/R-G approximate to 3.5; thinner films recovered more quickly. This contrasts with relaxation times in films that were at rest prior to shear. Due to slip, these thinner films (D/R-G < 3.5) may have been less uniformly deformed than thicker ones. We conjecture that chains in very thin films were separated by large-amplitude shear into two distinct populations, each moving preferentially with each of the sliding surfaces. Recovery kinetics would then reflect interdiffusion during which chain configurations lose memory of the distinction between top and bottom surfaces.