X-ray Reflectivity Study of Anionic Amphiphilic Carbosilane Block Copolymer Monolayers on a Water Surface

Abstract

Anionic amphiphilic diblock copolymers, poly(1,1-diethylsilacyclobutane)- block -poly(methacrylic acid) (poly(Et 2 SB- b -MAA)) and poly(α-methylstyrene)- block -poly(methacrylic acid) (poly(MeSt- b -MAA)), were synthesized, and the nanostructure of their spread monolayer on the water surface was investigated by direct in situ X-ray reflectivity (XR) measurement. XR profiles for the poly(Et 2 SB- b -MAA) monolayer showed many Kiessig fringes even though on a water surface, indicating the very smooth and flat surface and interfaces in the monolayer. On the other hand, the poly(MeSt- b -MAA) monolayer was suggested to have a larger surface roughness since no Kiessig fringe was observed in the XR profile. The XR profiles for the poly(Et 2 SB- b -MAA) monolayer clearly indicated that the polymers form monolayers consisting of two layers: a hydrophobic Et 2 SB upper layer and the hydrophilic MAA lower layer in the water subphase. The Kiessig fringe due to the overall layer including the MAA brush in the subphase was also observed for the copolymer with a longer MAA chain in addition to those for the Et 2 SB layer. The thickness and surface/interface roughness for the poly(Et 2 SB- b -MAA) monolayer at various surface pressures were evaluated by model fitting of XR curves. The thickness of the Et 2 SB layer increased continuously with increasing surface pressure, reflecting the very flexible nature of the Et 2 SB chain. The nanostructure of the poly(Et 2 SB- b -MAA) monolayer at the air/water interface was examined as a function of pH and MAA block length. Almost no change was found for monolayer nanostructures on HCl aq (pH = 2) and pure water as a subphase. The MAA length dependence of the thickness of the Et 2 SB layer showed an interesting tendency: it showed a minimum as a function of MAA length. This unexpected and interesting observation reflected the difference of the MAA “brush” structure under the water subphase.