  Recently a novel class of responsive uncharged polymer brushes has been
proposed [Klushin et al, J. Chem. Phys. 154, 074904 (2021)] where the
brush-forming chains have an affinity to the substrate. For sufficiently strong
surface interactions, a fraction of chains condenses into a near-surface layer,
while the remaining ones form the outer brush with a reduced grafting density.
The dense layer and the more tenuous outer brush can be seen as coexisting
microphases. The effective grafting density of the outer brush is controlled by
the adsorption strength and can be changed reversibly as a response to changes
in environmental parameters.
  In this paper we use numerical self-consistent field calculations and
theoretical considerations to study this phenomenon in polydisperse brushes.
Our results reveal an unexpected effect: Although all chains are chemically
identical, shorter chains are adsorbed preferentially. Hence, with the increase
in the surface affinity parameter, a reduction in the surface grafting density
of the residual brush is accompanied by a change in the shape of its molecular
mass distribution. In particular, an originally bidisperse brush can be
effectively transformed into a nearly monodisperse one containing only the
longer chain fraction.
