Self-assembled layers under flow: Stabilization by chain end exchange

In previous theories of sheared grafted polymer layers, the chain configurations are stationary. Formally, the shear stress is thus always supported by the same chains. These models predict that the response to the flow (in plane tension at the wall, free end deflection) is larger than the thermodynamic fluctuations for Deborah numbers De ≪ 1. Here we take into account the exchange of the chain configurations so that an individual chain on average does not support the shear stress long enough to reach a steady-state configuration. This effect smears out the response to flow over a larger number of chains, those with ends within one coil radius from the edge. The response only reaches the magnitude of the thermal fluctuations for De = 1. This corresponds in practice to shear rates as high as several 10⁴s⁻¹. Recent experiments by Baker et al. indeed show that self-assembled copolymer layers are very stable against steady shear flow, shear-induced desorption is only detected for De ∼ 1.