Healing of confined polymer films following deformation at high shear rate

Recovery of equilibrated linear viscoelastic response of confined polymer m elts, following cessation of large-amplitude shear in a surface forces appa ratus, was found to be a single exponential process. The most extensive exp eriments concerned a polydimethylsiloxane of narrow molecular weight distri bution 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-amplitu de sinusoidal shear forces at 256 Hz. Surprisingly, the nonlinear and linea r shear moduli evaluated at the input frequency nearly coincided just befor e and just after cessation of large-amplitude shear. Recovery time constant s, tau(R), increased linearly with the prior shear rate at a given thicknes s (D). But at a given shear rate and variable D, tau(R) passed through a ma ximum at D/R-G approximate to 3.5; thinner films recovered more quickly. Th is contrasts with relaxation times in films that were at rest prior to shea r. Due to slip, these thinner films (D/R-G < 3.5) may have been less unifor mly deformed than thicker ones. We conjecture that chains in very thin film s were separated by large-amplitude shear into two distinct populations, ea ch moving preferentially with each of the sliding surfaces. Recovery kineti cs would then reflect interdiffusion during which chain configurations lose memory of the distinction between top and bottom surfaces. (C) 2000 The So ciety of Rheology,. [S0148-6055 (00)00305-9].