Self-Assembly of Polystyrene-block-Poly(Ethylene Oxide) Copolymers at the Air−Water Interface:  Is Dewetting the Genesis of Surface Aggregate Formation?

Abstract

Block copolymer self-assembly at the air-water interface is commonly regarded as a two-dimensional counterpart of equilibrium block copolymer self-assembly in solution and in the bulk; however, the present analysis of atomic force microscopy (AFM) and isotherm data at different spreading concentrations suggests a nonequilibrium mechanism for the formation of various polystyrene-b-poly(ethylene oxide) (PS-b-PEO) aggregates (spaghetti, dots, rings, and chainlike aggregates) at the air-water interface starting with an initial dewetting of the copolymer spreading solution from the water surface. We show that different spreading concentrations provide kinetic snapshots of various stages of self-assembly at the air-water interface as a result of different degrees of PS chain entanglements in the spreading solution. Two block copolymers are investigated: MW = 141k (11.4 wt % PEO) and MW = 185k (18.9 wt % PEO). Langmuir compression isotherms for the 185k sample deposited from a range of spreading concentrations (0.1-2.0 mg/mL) indicate less dense packing of copolymer chains within aggregate cores formed at lower spreading concentrations due to a competition between the interfacial adsorption of PEO blocks and the kinetic restrictions of PS chain entanglements. From AFM analysis of the transferred Langmuir-Blodgett films, it is clear that PS chain entanglements in the spreading solution also affect the morphological evolution of surface aggregates for both samples, with earlier structures being trapped at higher concentrations. At the highest spreading concentration for the 141k copolymer, the coexistence of long spaghetti aggregates with cellular arrays of holes, along with various transition structures, indicates that various surface aggregates evolve from networks of rims formed as a result of dewetting of the evaporating spreading solution from the water surface.

Air–water interfacial behavior of linear-dendritic block copolymers containing PEG and azobenzene chromophores

“…The shape of the PEG-AZO2 isotherm is similar to other previously reported isotherms of PEG-containing BCs [40][41][42][43]. A characteristic pseudoplateau, assigned to the PEG chains desorption from the air-water interface into the water subphase, can be clearly observed at 10 mN/m.…”

Interfacial Organization of Y‐Shaped Rod–Coil Molecules Packed into Cylindrical Nanoarchitectures

“…Similar effects were observed for linear polystyrene (PS)-b-PEO chains as a function of spreading solution concentration due to differences in PS packing. [10] Following the pseudoplateau, the surface pressure sharply increases at low molecular areas (above 40-60 mN m À1 ), which indicates that the film is more rigid toward compression. At this point, a pure PEO monolayer of such chains would dissolve in water.…”

“…The nanosized structures can be easily controlled by varying different parameters, such as the relative chain length of the hydrophilic and hydrophobic blocks, [5a, 7, 8] the surface pressure, [9] and the concentration of the spreading solution. [8,10] Asymmetric diblock copolymers, such as rod-coil molecules consisting of a rigid rodlike segment and a flexible coil segment, give rise to novel ordered structures. [12,13] However, there are only a few reports on the interfacial assembly of rod-coil molecules at the air/water interface.…”

Surface-induced assembly of sophorolipids

“…Block copolymers have been intensively explored as they are known to assemble spontaneously on planar surfaces or when constrained as thin films. [17][18][19][20] Modification of hydrophobic/hydrophilic block ratio, 21 or the chemical nature of one block, 22 are well-known methods to change the wettability of a surface and influence the surface pattern. Even though these approaches are well-established, drawbacks exist in terms of tedious process involved in controlling homogeneity 23 and the necessity of solvent and temperature annealing.…”