  We study the Brownian dynamics of flexible and semiflexible polymer chains
densely grafted on a flat substrate, upon rapid quenching of the system when
the quality of solvent becomes poor and chains attempt collapse into a globular
state. The collapse process of such a polymer brush differs from individual
chains, both in its kinetics and its structural morphology. We find that the
resulting collapsed brush does not form a homogeneous dense layer, in spite of
all chain monomers equally attracting each other via a model Lennard-Jones
potential. Instead, a very distinct inhomogeneous density distribution in the
plane forms, with a characteristic length scale dependent on the quenching
depth (or equivalently, the strength of monomer attraction) and the geometric
parameters of the brush. This structure is identical to the
spinodal-decomposition structure, however, due to the grafting constraint we
find no subsequent coarsening: the established random bundling with
characteristic periodicity remains as the apparently equilibrium structure. We
compare this finding with a recent field-theoretical model of bundling in a
semiflexible polymer brush.
