Solvent quality dependent osmotic pressure of polymer solutions in two dimensions

Abstract: Confined in two dimensional planes, polymer chains comprising dense monolayer solution are segregated from each other due to topological interaction. Although the segregation is inherent in two dimensions (2D), the solution may display different properties depending on the solvent quality. Among others, it is well known in both theory and experiment that the osmotic pressure ($\Pi$) in the semi-dilute regime displays solvent quality-dependent increases with the area fraction ($\phi$) (or monomer concentration, $\rho$), that is, $\Pi\sim \phi^3$ for good solvent and $\Pi\sim \phi^8$ for $\Theta$ solvent. The osmotic pressure can be associated with the Flory exponent (or the correlation length exponent) for the chain size and the pair distribution function of monomers; however, they do not necessarily offer a detailed microscopic picture leading to the difference. To gain microscopic understanding into the different surface pressure isotherms of polymer solution under the two distinct solvent conditions, we study the chain configurations of polymer solution based on our numerical simulations that semi-quantitatively reproduce the expected scaling behaviors. Notably, at the same value of $\phi$, polymer chains in $\Theta$ solvent occupy the surface in a more \emph{inhomogeneous} manner than the chains in good solvent, yielding on average a greater and more heterogeneous interstitial void size, which is related to the fact that the polymer in $\Theta$ solvent has a greater correlation length. The polymer configurations and interstitial voids visualized and quantitatively analyzed in this study offer microscopic understanding to the origin of the solvent quality dependent osmotic pressure of 2D polymer solutions.