Amphiphilic heteroarm PEO-b-PSm star polymers at the air-water interface:: Aggregation and surface morphology

We report on the surface behavior of the asymmetric heteroarm poly(ethylene oxide) (PEO)/polystyrene (PS) star polymer on the air-water interface on a solid substrate. These amphiphilic star polymers with different numbers of hydrophobic arms and a similar hydrophilic block differ by architecture (four and three arm molecules, PEO-b-PS3 and PEO-b-PS2), the length of PS chains (molecular weight from about 10 000 up to 24 000), and the number of PS arms (three and two). Detailed analysis revealed that well-developed circular domain surface morphology was formed at the air-water interface. Similar to linear diblock PEO-PS copolymers, the asymmetric heteroarm star polymers at low surface pressure formed circular nanoscale aggregates composed of PS arms. At higher surface pressure, the packing of circular domains became denser, but no clear transition to cylindrical structures was observed in condensed monolayers, contrary to linear block copolymers of similar composition. Therefore, we suggest that for star architecture the formation of highly curved interfaces is heavily favored, domain structure. This surface morphology remained stable even at very high compression close to the monolayer collapse unlike linear diblock copolymers with their tendency for structural reorganization even at very modest compressions.