Interfacial behavior of anionically synthesized amphiphilic star block copolymers based on polybutadiene and poly(ethylene oxide) at the air/water interface

We describe the preparation and the surface properties of monolayers of a new set of (PBb-PEO)(4) amphiphilic four-arm star block copolymers. A divergent anionic polymerization method yielded copolymers with well-defined molecular weights, block volume fractions, and architecture. Initially, tri-and tetracarbanionic initiators were used to initiate the polymerization of butadiene followed by end-capping of living chains with ethylene oxide. The tri- and tetrafunctionalities of the resulting hydroxyl-terminated star polydienes were confirmed by H-1 NMR spectroscopy. The four-arm star polymeric alcohols were then titrated with diphenylmethylpotassium to yield the analogous potassium alkoxides. The resulting macroinitiators were used to polymerize corona blocks of ethylene oxide. Different samples of well-defined (PB-b-PEO)(4) amphiphilic star block copolymers exhibiting narrow molar mass distribution were prepared with poly(ethylene oxide) over a range of volume fractions. Isotherm experiments at the air/water interface showed the following three characteristic regions: a compact brush region, a pseudoplateau at a pressure of ca. 10 mN/m, and a pancake region where the observed surface area depended on the amount of PEO present. The elasticity properties of the monolayer were examined by repetative compression/expansion cycles and resulted in reproducible hysteresis at different pressures. The monolayers were also transferred as Langinuir-Blodgett films on mica at various surface pressures and analyzed by atomic force microscopy (AFM), demonstrating different morphologies from analogous (PS-b-PEO)(4) star copolymers.