Friction and relaxation dynamics of highly extended polymer brush melts under compression and shear

The forces between two adhering surfaces bearing highly extended polymer melt brushes as they are sheared past each other, and between a single melt-brush-bearing substrate sheared across an adhering bare solid surface, were studied using a mica surface force balance with a high shear-force resolution. The melt brushes were created by Langmuir-Blodgett deposition of zwitterion-terminated polyisoprene chains on the mica substrates. Shear of the single melt brush by the bare surface revealed little sliding, deformation, or relaxation of the confined melt brush, under all shear regimes applied in this study. In contrast, shearing of the two melt brushes past each other under the same shear conditions showed a marked shear-rate-dependent, multistage deformation of the sheared brushes. On stopping the applied lateral motion, a logarithmically slow relaxation of the stored stress was observed, which could be quantitatively interpreted in terms of mutual retraction of the entangled tails of the two brushes. The low friction and characteristic relaxation behavior following initial adhesive contact of the brushes developed with time to a solidlike response on shear of the confined chains and was attributed to bridging of chains adsorbed on the opposing surfaces as squeeze-out of the polymer occurred.