Direct Visualization of Contrasting Hydration Structures on Poly(2-methoxyethyl acrylate) and Poly(n-butyl...

Interfacial hydration governs the biological response of polymeric biomaterials, yet directly visualizing solvation structures on soft, fluctuating polymer surfaces has remained experimentally challenging. Here we employ ultralow-noise frequency modulation atomic force microscopy (FM-AFM) to resolve the nanoscale hydration structures of blood-compatible poly(2-methoxyethyl acrylate) (PMEA) and protein-adsorbing poly(n-butyl methacrylate) (PBMA) in phosphate-buffered saline. Subnanometer frequency shift mapping combined with nanomechanical analysis reveals two distinct water architectures. PBMA shows a static multilayered structure with 0.30 nm periodicity, typical of hydrophobic surfaces such as graphite. In contrast, PMEA is exceptionally soft (apparent modulus E* ≈ 100 MPa) and, despite being fully hydrated, shows no detectable layering. We assign this absence of layering to a dynamically fluctuating, water-penetrated side-chain ("intermediate water") region, which has been proposed to act as a physical barrier against irreversible protein adsorption. Direct FM-AFM access to hydration structures at polymer-water interfaces should guide the design of blood-compatible biomaterials.