Polymer–Surface Interactions under Nanoconfinement Induce Water Resistance in Water-Soluble Polymers

Abstract Water-soluble polymers enable benign aqueous processing but are generally unsuitable for durable coatings because they rapidly swell and dissolve upon contact with water. Here, we demonstrate that extreme nanoconfinement within the interstitial pores of densely packed silica nanoparticles imparts remarkable water resistance to three water-soluble polymers with distinct chemistries: a polycation, polyethylenimine (PEI), a non-ionic polymer, polyvinylpyrrolidone (PVP), and a polyanion, poly(acrylic acid) (PAA). Whereas spectroscopic ellipsometry confirms that unconfined polymer thin films are removed within 0.5 s of water exposure, polymers confined within silica nanoparticle packings remain retained during prolonged immersion without chemical crosslinking or hydrophobic modification. To understand the origin of such water resistance, we quantify the retained polymer fraction as a function of immersion time and solution pH. At pH 5.5, all three polymers exhibit substantial water resistance, with PVP showing the greatest retention. PAA and PEI display strong but opposite pH-dependent behavior; PEI resistance decreases sharply under acidic conditions, whereas PAA resistance decreases sharply under basic conditions. In contrast, PEI under basic conditions and PAA under acidic conditions exhibit comparatively little change from their behavior at pH 5.5. PVP, which is nonionizable over the investigated pH range, exhibits consistently high resistance under acidic, near-neutral, and basic conditions, retaining approximately 91–93% of the confined polymer in 2 nm pores after 24 h of continuous water immersion. Comparison of polymer ionization states with the pH-dependent surface charge of silica shows that the observed trends are consistent with changes in polymer hydration and electrostatic interactions between polymer and silica associated with functional-group ionization. The pH-independent behavior of PVP further supports a central role for functional-group ionization in the contrasting pH responses of PAA and PEI. These findings demonstrate that nanoconfinement can stabilize water-soluble polymers against dissolution in water without chemical crosslinking or hydrophobic modification, providing a route to water-resistant nanocomposite coatings fabricated entirely through aqueous processing and without additional crosslinking chemistry.