Abstract
The interaction between surfaces displaying end-grafted hydrophilic polymer brushes plays important roles in biology and in many wet-technological applications. In this context, the conformation of the brushes upon their mutual approach is crucial, because it affects interaction forces and the brushes' shear-tribological properties. While this aspect has been addressed by theory, experimental data on polymer conformations under confinement are difficult to obtain. Here, we study interacting planar brushes of hydrophilic polymers with defined length and grafting density. Via ellipsometry and neutron reflectometry we obtain pressure-distance curves and determine distance-dependent polymer conformations in terms of brush compression and reciprocative interpenetration. While the pressure-distance curves are satisfactorily described by the Alexander-de-Gennes model, the pronounced brush interpenetration as seen by neutron reflectometry motivates detailed simulation-based studies capable of treating brush interpenetration on a quantitative level.
Highlights
Insight into structures ‘‘buried’’ between two surfaces or interfaces, such as the conformation of polymer brushes under confinement, is generally difficult to obtain experimentally
Neither of the two surfaces is accessible by scanning near-field techniques like atomic force microscopy (AFM), because this is prevented by the presence of the respective counterpart
The relevant structures are of the order of 0.1–100 nm; these small length scales cannot be resolved by optical microscopy and at the same time impede the use of fluorescent labels, excluding complementary techniques like fluorescence resonance energy transfer (FRET)
Summary
Insight into structures ‘‘buried’’ between two surfaces or interfaces, such as the conformation of polymer brushes under confinement, is generally difficult to obtain experimentally. Neither of the two surfaces is accessible by scanning near-field techniques like AFM, because this is prevented by the presence of the respective counterpart. The relevant structures are of the order of 0.1–100 nm; these small length scales cannot be resolved by optical microscopy and at the same time impede the use of fluorescent labels, excluding complementary techniques like fluorescence resonance energy transfer (FRET). X-ray and neutron scattering are among the very few techniques that can probe such structures with the required subnanometer spatial resolution. They can be used in a wide pressure and temperature range and provide sample-averaged structural information.
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