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Precise Characterization of Polymer Brushes by Quantum Beam

Atsushi Takahara Institute for Materials Chemistry and Engineering, Center for Molecular Systems (CMS), WPI-I2CNER, Research Center for Synchrotron Light Applications, Kyushu University, Japan

Quantum beams include synchrotron radiation, X-rays, neutron beams, electrons, or muons. Quantum Beam can apply to a broad range of disciplines including soft matter physics.  Polymers chemically grafted to the surface of substrates are typical soft interfaces known as polymer brushes. Surfaces covered with polyelectrolyte brushes are particularly attractive because of their potential applications including adhesion, antifouling, and water lubrication systems. In this presentation, our recent researches on characterization of soft interfaces by neutron reflectivity, SR-small angle X-ray scattering (SAXS) and SR X-ray spectroscopy are presented.
The swollen brush structures of cationic and zwitterionic polymer brushes at an aqueous interface were characterized by neutron reflectivity (NR). A series of the polyelectrolyte brushes were prepared by surface-initiated controlled radical polymerization on silicon substrates. High-graft-density cationic polymer (PMTAC) brushes in salt-free water (D2O) adopted a two-region step-like structure consisting of a shrunk region near the Si substrate and a diffuse brush region with relatively stretched chain structure at the solution interface. The diffuse region of PMTAC was reduced with increase in salt (NaCl) concentration. Sulfobetaine polymer brush (PMAPS) in D2O formed a collapsed structure due to the strong interaction between sulfobetaine groups, while significant increase in the swollen thickness was observed in salt solutions. The salt concentration dependence of polymer brush conformation on silica nanoparticle (SiNP) was characterized by SAXS. The SAXS profiles from SiNP-PMAPS solutions were well described by the core-shell model, taking into account interacting self-avoiding chains and assuming a Schulz-distributed core with two fitting parameters.
SR source generates brilliant IR beam so that we can achieve high spatial resolution mapping of surface with high SN ratio. We characterized the wetting of superhydrophilic polyelectrolyte brushes with water utilizing SR-IR. Reflection interference contrast microscopy showed that the contact angle of a water droplet on the polymer brush surface was extremely low but showed clear three phase contact line, despite the high affinity of the polyelectrolytes for water. The SR-IR demonstrated that water was present even outside the droplet. These water molecules were confined to the thin polymer brush layer and formed a highly ordered hydrogen bond network, that is, structural water.
X-ray absorption and emission spectroscopy was employed to observe the local hydrogen-bonding structure of water confined in a charged polyelectrolyte brush. Even at room temperature, a majority of the water molecules confined in the polyelectrolyte brush exhibited one type of hydrogen-bonding configuration : a slightly distorted, albeit ordered configuration.
These findings provide new insights in terms of correlation between the function and local structure of water at the interface of biological materials under physiological conditions.

Fig.1 Characterization of polymer brushes at aqueous interface by quantum beam