Neutron reflectometry study of the segment-density profiles in end-grafted and irreversibly adsorbed layers of polymer in good solvents

The segment density profile of anchored layers of poly(dimethylsiloxane) in good solvents has been measured using neutron reflectometry. Two types of layers have been studied: end-grafted layers (brushes) and irreversibly adsorbed layers, on silicon/silicon dioxide. The latter can be viewed as a polydisperse brush of loops and was obtained by adsorbing the polymer from the melt or from a concentrated solution. A model-free constrained fitting procedure was developed, which gave the concentration profile with a good precision. We present this numerical method and the concentration profiles obtained for the different layers. The profiles of the two types of layers are significantly different. They are in agreement with self-consistent mean field theory for the brushes and with scaling laws descriptions for the irreversibly adsorbed layers. The reflectivity measurement enabled us to verify precisely that the concentration profile in the brush cannot be described by a step function with a Gaussian roughness but has a parabolic shape.