Thermodynamic Prediction of Phase Separation in Polymer Blends at the Air-Water Interface

This study presents a thermodynamic modeling approach to predict the miscibility of polymer blends within Langmuir films. The model, adapted from a previously established framework, evaluates miscibility based solely on the molecular interaction energy between distinct repeat units. It was applied to three polymer blends at the air-water interface: methacryloxypropyl-terminated polydimethylsiloxane/poly(methyl methacrylate) (PDMS800/PMMA), cellulose acetate butyrate/poly(methyl methacrylate) (CAB/PMMA), and methacryloxypropyl-terminated polydimethylsiloxane/poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (PDMS800/PEO₂₆-PPO₆₀-PEO₂₆). For each system, phase diagrams were constructed, identifying spinodal and binodal boundaries that delineate miscible and phase separation regions. Unlike classical models, where temperature is the primary variable, our approach accounts for the unique conditions of Langmuir films, in which surface pressure varies, while temperature remains constant. This introduces the Lower Critical Solution Pressure (LCSP), analogous to the LCST in bulk systems. The model successfully predicts the three key outcomes (full miscibility, complete immiscibility, and LCSP-type phase behavior) in agreement with experimental observations. Additionally, it reveals how monolayer thickness influences both the width and position of the spinodal region.