Is Osmotic Pressure Relevant in the Mechanical Confinement of a Polymer Brush?

The structures of polymer brushes under confinement were measured using a combination of neutron reflectivity and a surface force type apparatus. The samples were either poly(ethylene oxide), PEO, used to investigate the effect of the grafting density or poly(acrylic acid), PAA, used to determine the effect of charge on the structure of a polymer brushes under confinement. Without confinement both PEO and PAA brushes are found to be highly swollen with water, >50% v/v, with the expected parabolic brush structure. Compression of the PEO brushes with as little as 0.5 bar of confinement is found experimentally to reduce the brush to a polymer block of uniform density that is significantly dehydrated, <12% v/v. Further subsequent increases in the confinement pressure only marginally decrease the hydration and thickness of the polymer block. The grafting density of the brush does not significantly influence this behavior. PAA polymer brushes with little (pH 3) or an intermediate level of charging (pH 5.5) are also found to be compressed into a single uniform density polymer block with a confinement of 5 bar. However, with a high level of charge (pH 9) the brush structure is believed to be partially retained due to the repulsion between the internal charges. These experimental results are compared against a theoretical model based on numerical self-consistent field (nSCF) theory as well as to osmotic, SFA, and AFM data. While the nSCF model correctly predicts the observed transition from a brush to a block profile, experimentally it occurs at a pressure 2 orders of magnitude lower than the simulations would suggest. The results acquired through simulation are consistent with available osmotic pressure data; however, SFA and AFM measurements are consistent with the neutron reflection experimental data presented here. This significant disagreement between the two data sets indicates that in a confined system the effective osmotic counter pressure to an applied mechanical pressure is much less than the osmotic pressure expected from the local polymer concentration.