Formation and Collapse of Single-Monomer-Thick Monolayers of Poly(n-butyl acrylate) at the Air−Water Interface

Abstract

The behavior of poly(n-butyl acrylate) (PnBA) spread at the air−water interface has been studied for a full range of surface coverages and several molecular weights. At low and intermediate surface coverages, the surface pressure−area isotherm behavior of the polymer is found to follow the expected scaling laws. In the dilute regime the pressure is an increasing function of surface coverage and a decreasing function of molecular weight. In the semidilute regime the surface pressure becomes independent of molecular weight, and a Flory exponent for the two-dimensional radius of gyration is found to be ν = 0.57 ± 0.02. Beginning in the high coverage concentrated regime, at a surface pressure of around 15 mN/m, and through the full coverage regime (where the water in the subphase is fully covered and not exposed to air), X-ray reflectivity (XR) measurements show the formation of a continuous water-free monolayer (i.e., one monomer thick) film of the polymer. At surface concentrations above the transition point to the full coverage regime (alternatively called the “collapsed” regime hereafter for the reason that will become apparent below), Brewster angle microscopy (BAM) shows that the excess polymer material does not distribute uniformly in the polymer film layer but instead leads to formation of micrometer-scale isolated globular domains of roughly uniform size. Further, it was observed that the number of such domains increases as the surface polymer concentration is increased, whereas the size of the globular domains is largely unaffected by the concentration variation. X-ray grazing incidence diffraction (GID) indicates that these domains are regions of bulklike (amorphous) polymer. These and other observations, including the invariant nature of the monolayer throughout the compression (confirmed by XR), the plateau nature of surface pressure−area isotherm throughout the collapsed regime, and the reversible nature of the domain formation (evidenced by BAM), suggest that the globular domains formed at high surface concentrations of PnBA are in a type of coexistence with the uniform monolayer. A simple thermodynamic model considering the entropic penalty of confining the polymer chains to monolayer, the translational entropy of the domains, and the surface energy of the interface is made in order to understand the behavior of the polymer as it becomes excluded from the monolayer. This argument suggests that the excess polymer should form a single large domain in order to minimize the large surface energy at the water−polymer interface. The presence of many small domains suggests the domains are kinetically trapped in a local, rather than global, equilibrium.