The Dramatic Effect of Architecture on the Self-Assembly of Block Copolymers at Interfaces
Dramatic morphological changes are observed in the Langmuir−Blodgett (LB) film assemblies of poly(ethylene glycol)-b-(styrene-r-benzocyclobutene) block copolymer (PEG-b-(S-r-BCB)) after intramolecular cross-linking of the S-r-BCB block to form a linear-nanoparticle structure. To isolate architectural effects and allow direct comparison, the linear block copolymer precursor and the linear-nanoparticle block copolymer resulting from selective intramolecular cross-linking of the BCB units were designed to have exactly the same molecular weight and chemical composition but different architecture. It was found that the effect of architecture is pronounced with these macromolecular isomers, which self-assemble into dramatically different surface aggregates. The linear block copolymer forms disklike surface assemblies over the range of compression states, while the linear-nanoparticle block copolymer exhibits long (>10 m) wormlike aggregates whose length increases as a function of increasing cross-linking density. It is shown that the driving force behind the morphological change is a combination of the altered molecular geometry and the restricted degree of stretching of the nanoparticle block because of the intramolecular cross-linking. A modified approach to interpret the −A isotherm, which includes presence of the block copolymer aggregates, is also presented, while the surface rheological properties of the block copolymers at the air−water interface provide in-situ evidence of the aggregates' presence at the air−water interface.