Polymer Brushes in Restricted Geometries
The structure of polymer brushes has been the subject of considerable theoretical and experimental activity. The equilibrium properties of uncompressed low and medium density polymer brush layers predicted by self-consistent field theory and simulations have been confirmed by a number of experiments primarily utilizing scattering techniques. Much less well understood is the structure such brushes adopt during compression and confinement. Reported here are the structural determination results of two different cases of confinement. In the first case, moderate grafting density PS brushes are confined between two surfaces. Although force spectroscopy techniques such as the Surface Force Apparatus and various Scanning Probe Microscopies have been very useful in quantifying the interaction forces between polymer layers, these techniques are indirect in that interaction forces are measured and structure is inferred. Presented are measurements of polymer brush density distributions in good solvent as a function of compression. The extension and interpenetration of the brushes are unambiguously determined using neutron reflectivity and contrast matching techniques. The measurements reveal a significant increase in brush concentration at the grafting surface with compression. This is in contrast to theoretical predictions of uniform density distributions as polymer brushes are compressed. The low interpenetration and brush compaction at the anchor surface provides new insight into why such layers are so effective at reducing friction and wear between surfaces. Secondly, ultra-high grafting density polystyrene (PS) brushes available through new, "grafting from" synthetic schemes based on atom transfer radical polymerization (ATRP) allow confinement studies due to lateral crowding of the chains. Most literature work on ATRP grafted polymer films has employed ellipsometry for structural characterization. Using neutron reflectivity which provides higher resolution information of the film's thickness, roughness, and uniformity, the structure of 0.44 chains/nm2 and 0.71 chains/nm2 polystyrene brushes is given a function of molecular weight. The dry film thickness scaled linearly with chain molecular weight. Under good solvent conditions, strongly stretched brushes of moderate molecular weight were found to maintain a parabolic density distribution consistent with theoretical predictions. Anomalous behavior was observed for higher molecular weights and higher grafting density, suggesting that entanglements are much more pronounced in "grafted from" systems.