Study on the Behaviors of Different Polystyrene-block-Poly(methyl methacrylate) Diblock Copolymers Adsorbed at the Air/Water Interface

Abstract

Using surface quasi-elastic light scattering (SLS), we have studied the monolayer behavior of diblock copolymers A−B, where the A block is polystyrene (PS, surface inactive and water insoluble) and the B block is poly(methyl methacrylate) (PMMA, surface active but water insoluble), at the air/water interface (two-dimensional domain) in terms of the power spectra and the surface viscoelasticity. The polystyrene blocks of the diblock copolymers have similar molecular weights, but the molecular weights of the poly(methyl methacrylate) blocks vary. Substantial changes in viscoelastic behavior accompany changes in surface concentration and can be attributed to differences in molecular packing resulting from micelle formation and surface phase behavior, which are consistent with atomic force microscope (AFM) images of monolayer films transferred from air/water interfaces to solid substrates. The state change is similar to previous results obtained using the dynamic (stepwise compression) method. At submonolayer coverage, there is a clear, molecular mass dependent deviation from pure liquid dynamics in the raw SLS data, frequency shifts, and damping coefficients, along with corroborative AFM images that are consistent with a biphasic state consisting of gaslike and liquidlike domains, where the condensed liquidlike domains are composed of surface micelles. The ill-defined PS segment sits on top of the PMMA segment without much contact with the water. The PS segment forms the core of the micelles. As the concentration goes up, these systems show characteristic surface viscoelastic behavior. Since only the PMMA segment is surface active while PS is not, the general behavior of PS−PMMA diblock copolymer films at high surface pressure (>7 mN/m) is similar to that of a pure PMMA film whereas different behavior was observed at low surface pressure (submonolayer region). Viscoelastic parameters, that is, the dynamic dilational elasticity, εd, and viscosity, κ, deduced from the SLS data indicate that the rigidity of the liquidlike domains increases smoothly showing large εd and κ values, regardless of the PMMA molecular weight.