Highly swellable ultrathin poly(4-vinylpyridine) multilayer hydrogels with pH-triggered surface wettability

We report on the synthesis of a novel type of ultrathin hydrogel with remarkably high, fast and reversible swelling/shrinkage and sharp surface wettability transitions in response to pH changes. The poly(4-vinylpyridine) (PVP) hydrogels are produced by the selective cross-linking of PVP copolymer layers in PVP/poly(methacrylic acid) (PVP/PMAA) layer-by-layer (LbL) films assembled by a spin-assisted method on surfaces. These multilayer hydrogels exhibit drastic and reversible 10-fold swelling when pH is switched from neutral to acidic. Importantly, hydrogels at pH > 5 collapse to the thickness of dry films, evincing sharp hydrophilic-to-hydrophobic transitions. The swelling amplitude of these multilayer hydrogel coatings is controlled by varying cross-link densities within the hydrogel. Further, the pH-triggered swelling properties of PVP hydrogels are significantly affected by the multilayer assembly method. PVP films produced using the spin-assisted method experienced almost a 4-fold greater swelling enhancement over their conventionally "dipped" counterparts. Finally, PVP hydrogels dehydrated from basic and acidic solutions show drastically different surface morphology and wettability properties. The hydrogel contact angles vary from 70 degree to 20 degree for the films dried from neutral and acidic solutions, respectively. Our study provides better understanding of regulated volume and surface wettability transitions in polymer networks which is critical for developing novel materials for a wide variety of applications including sensing, transport regulation, and self-cleaning coatings.