文摘
The two-dimensional self-assembly at the air/water (A/W) interface of two dendrimer-like copolymers based onpolystyrene and poly(tert-butyl acrylate) (PS-b-PtBA) or poly(acrylic acid) (PS-b-PAA) was investigated throughsurface pressure measurements (isotherms, isochores, and compression-expansion hysteresis experiments) and atomicforce microscopy (AFM) imaging. The two dendrimer-like block copolymers have an 8-arm PS core (Mn = 10 000g/mol, ~12 styrene repeat units per arm) with a 16-arm PtBA (Mn = 230 000 g/mol, ~112 tert-butyl acrylate repeatunits per arm) or PAA (Mn = 129 000 g/mol, ~112 acrylic acid repeat units per arm) corona. The PS-b-PtBA sampleforms stable Langmuir monolayers and aggregates into circular surface micelles up to a plateau observed in thecorresponding isotherm around 24 mN/m. Beyond this threshold, the monolayers collapse above the interface, resultingin the formation of large and irregular desorbed aggregates. The PS-b-PAA sample has ionizable carboxylic acidgroups, and its A/W interfacial self-assembly was therefore investigated for various subphase pH values. Under basicconditions (pH = 11), the carboxylic acid groups are deprotonated, and the PS-b-PAA sample is therefore highlywater-soluble and does not form stable monolayers, instead irreversibly dissolving in the aqueous subphase. Underacidic conditions (pH = 2.5), the PS-b-PAA sample is less water-soluble and becomes surface-active. The pseudoplateauobserved in the isotherm around 5 mN/m corresponds to a pancake-to-brush transition with the PAA chains dissolvingin the water subphase and stretching underneath the anchoring PS cores. AFM imaging revealed the presence of circularsurface micelles for low surface pressures, whereas the biphasic nature of the pseudoplateau region was confirmedwith the gradual aggregation of the micellar PS cores above the PAA chains. The aggregation numbers for both sampleswere estimated around 3-5 dendrimer-like copolymers per circular surface micelle. These rather low values confirmedthe tremendous influence of molecular architecture on the two-dimensional self-assembly of block copolymers.
