Masami Terada

Wettability and Antifouling Behavior on the Surfaces of Superhydrophilic Polymer Brushes

The surface wettabilities of polymer brushes with hydrophobic and hydrophilic functional groups were discussed on the basis of conventional static and dynamic contact angle measurements of water and hexadecane in air and captive bubble measurements in water. Various types of high-density polymer brushes with nonionic and ionic functional groups were prepared on a silicon wafer by surface-initiated atom-transfer radical polymerization. The surface free energies of the brushes were estimated by Owens-Wendt equation using the contact angles of various probe liquids with different polarities. The decrease in the water contact angle corresponded to the polarity of fluoroalkyl, hydroxy, ethylene oxide, amino, carboxylic acid, ammonium salt, sulfonate, carboxybetaine, sulfobetaine, and phosphobetaine functional groups. The poly(2-perfluorooctylethyl acrylate) brush had a low surface free energy of approximately 8.7 mN/m, but the polyelectrolyte brushes revealed much higher surface free energies of 70-74 mN/m, close to the value for water. Polyelectrolyte brushes repelled both air bubbles and hexadecane in water. Even when the silicone oil was spread on the polyelectrolyte brush surfaces in air, once they were immersed in water, the oil quickly rolled up and detached from the brush surface. The oil detachment behavior observed on the superhydrophilic polyelectrolyte brush in water was explained by the low adhesion force between the brush and the oil, which could contribute to its excellent antifouling and self-cleaning properties.

Polyelectrolyte brushes: a novel stable lubrication system in aqueous conditions

Surface-initiated controlled radical copolymerizations of 2-dimethylaminoethyl methacrylate (DMAEMA), 2-(methacryloyloxy)ethyl phosphorylcholine (MPC), 2-(methacryloyloxy)ethyltrimethylammonium chloride) (MTAC), and 3-sulfopropyl methacrylate potassium salt (SPMK) were carried out on a silicon wafer and glass ball to prepare polyelectrolyte brushes with excellent water wettability. The frictional coefficient of the polymer brushes was recorded on a ball-on-plate type tribometer by linear reciprocating motion of the brush specimen at a selected velocity of 1.5 x 10(-3) m s-1 under a normal load of 0.49 N applied to the stationary glass ball (d = 10 mm) at 298 K. The poly(DMAEMA-co-MPC) brush partially cross-linked by bis(2-iodoethoxy)ethane maintained a relatively low friction coefficient around 0.13 under humid air (RH > 75%) even after 200 friction cycles. The poly(SPMK) brush revealed an extremely low friction coefficient around 0.01 even after 450 friction cycles. We supposed that the abrasion of the brush was prevented owing to the good affinity of the poly(SPMK) brush for water forming a water lubrication layer, and electrostatic repulsive interactions among the brushes bearing sulfonic acid groups. Furthermore, the poly(SPMK-co-MTAC) brush with a chemically cross-linked structure showed a stable low friction coefficient in water even after 1400 friction cycles under a normal load of 139 MPa, indicating that the cross-linking structure improved the wear resistance of the brush layer.

Reversible adhesive-free nanoscale adhesion utilizing oppositely charged polyelectrolyte brushes

Two prepared silicon wafers bearing cationic and anionic polyelectrolyte brushes were joined with 2 μL of water under 0.098 MPa of pressure at room temperature. The bonded area was fixed at 5 × 10 mm2. A lap shear strength of 1.52 MPa was achieved through the adhesion of poly[2-(methacryloyloxy)ethyl trimethylammonium chloride] and poly(3-sulfopropyl methacrylate potassium salt) brush substrates due to electrostatic attractive interactions between the positively and negatively charged polymers. In contrast, the lap shear strength of bonded polymer brushes with like charges was only 0.027–0.072 MPa. The polyelectrolyte brushes remained on the substrates even after their separation in the lap shear test; and the brush substrates readhered in the presence of a small amount of water. Furthermore, the adhering substrates were smoothly debonded in aqueous NaCl solution due to the electrostatic interaction of the hydrated salt ions. However, the substrates did not separate in deionized water. In summary, reversible nanoscale adhesion was achieved using oppositely charged polyelectrolyte brushes combined with aqueous solution.

Characterization of Swollen States of Polyelectrolyte Brushes in Salt Solution by Neutron Reflectivity

Cationic and zwitterionic polyelectrolyte brushes on quartz substrate were synthesized by surface-initiated atom transfer radical polymerization of 2-(methacryloyloxy)-ethyltrimethylammonium chloride (MTAC) and 2-(methacryloyloxy)ethyl phosphorylcholine (MPC). The effects of ionic strength on brush structure are investigated by neutron reflectivity (NR) in NaCl deuterium oxide (D2O) solutions. We observed that poly(MTAC) chains were drastically shrunk at concentrations above 0.1 M NaCl/D2O, which may be the change in charge-screening effect against ions on poly(MTAC). On the other hand, effect of salt concentration on a swollen state of poly(MPC) brush was negligible, even at the high concentration (5.0 M) close to saturation. The behaviour of poly(MPC) in salt aqueous solution is completely different from that of poly(MTAC), which may arise from the unique interaction properties, neutral nature, and hydrated water structure of phosphorylcholine units.

Applications of polymer brushes to structural nano-coatings

Hydrophilic polymers show unique characteristics at various interfaces. High-density hydrophilic polymer brushes nano-coatings were prepared on Si-wafer and various substrates by surface-initiated controlled radical polymerization. Chain conformation at liquid/solid liquid/solid interface was characterized by neutron reflectivity. Applications of polymer brushes as structural nano-coatings such as super hydrophilicity, antifouling behavior, low friction behavior, and intelligent adhesion are presented.