Phase Diagram of 2D Poly(Ethylene Oxide)- <i>block</i> -Poly(Propylene Oxide)- <i>block</i> -Poly(Ethylene Oxide)─Poly(Dimethylsiloxane) Blends: A Combined Neutron Reflectometry and Sum Frequency Generation Study

Polymer blends in thin films offer versatile opportunities for tailoring material properties. Among available deposition methods, the Langmuir film technique enables the formation of well-controlled nanometer-thick films at the air-water interface, revealing unique miscibility and phase behaviors not observed in bulk. In this work, we investigate 2D polymer blends composed of amphiphilic PEO₁₁-PPO₃₅-PEO₁₁ and hydrophobic PDMS using a multiscale approach that combines surface pressure-area isotherms, Brewster angle microscopy (BAM), neutron reflectometry (NR), and sum-frequency generation (SFG) spectroscopy. This methodology enabled us to construct a 2D surface pressure-composition phase diagram, identifying miscibility regions and two distinct first-order phase transitions. Across all compositions, no lateral phase separation was observed at the mesoscopic scale by BAM, even during phase transitions. Instead, NR revealed vertical segregation at specific surface pressures and compositions. In PEO₁₁-PPO₃₅-PEO₁₁-rich blends, a homogeneous monolayer undergoes a transition to a bilayer, with hydrophobic PDMS positioned atop PEO₁₁-PPO₃₅-PEO₁₁. PDMS-rich blends display a similar bilayer structure at all pressures, with the phase transition mainly involving thickening of the PDMS layer, as seen in pure PDMS films. SFG spectroscopy reveals that both polymers have distinct behavior in the blends and in pure films, due to either lateral or vertical interactions. Notably, despite their contrasting hydrophobicities, the polymers exhibit miscibility over a range of compositions and surface pressures, with evidence of attractive interactions. These findings underscore the unique behavior of confined polymer blends and the importance of 2D-specific phase diagrams for designing functional interfacial materials.