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

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 poin...