Polystyrene-b-Poly(tert-butyl acrylate) and Polystyrene-b-Poly(acrylic acid) Dendrimer-Like Copolymers: Two-Dimensional Self-Assembly at the Air-Water Interface

The two-dimensional self-assembly at the air/water (A/W) interface of two dendrimer-like copolymers based on polystyrene and poly(tert-butyl acrylate) (PS-b-PtBA) or poly(acrylic acid) (PS-b-PAA) was investigated through surface pressure measurements (isotherms, isochores, and compression−expansion hysteresis experiments) and atomic force microscopy (AFM) imaging. The two dendrimer-like block copolymers have an 8-arm PS core (M_n = 10000 g/mol, 12 styrene repeat units per arm) with a 16-arm PtBA (M_n = 230000 g/mol, 112 tert-butyl acrylate repeat units per arm) or PAA (M_n = 129000 g/mol, 112 acrylic acid repeat units per arm) corona. The PS-b-PtBA sample forms stable Langmuir monolayers and aggregates into circular surface micelles up to a plateau observed in the corresponding isotherm around 24 mN/m. Beyond this threshold, the monolayers collapse above the interface, resulting in the formation of large and irregular desorbed aggregates. The PS-b-PAA sample has ionizable carboxylic acid groups, and its A/W interfacial self-assembly was therefore investigated for various subphase pH values. Under basic conditions (pH = 11), the carboxylic acid groups are deprotonated, and the PS-b-PAA sample is therefore highly water-soluble and does not form stable monolayers, instead irreversibly dissolving in the aqueous subphase. Under acidic conditions (pH = 2.5), the PS-b-PAA sample is less water-soluble and becomes surface-active. The pseudoplateau observed in the isotherm around 5 mN/m corresponds to a pancake-to-brush transition with the PAA chains dissolving in the water subphase and stretching underneath the anchoring PS cores. AFM imaging revealed the presence of circular surface micelles for low surface pressures, whereas the biphasic nature of the pseudoplateau region was confirmed with the gradual aggregation of the micellar PS cores above the PAA chains. The aggregation numbers for both samples were estimated around 3−5 dendrimer-like copolymers per circular surface micelle. These rather low values confirmed the tremendous influence of molecular architecture on the two-dimensional self-assembly of block copolymers.