Poster: Controlling mechanical properties of poly(methacrylic acid) thin multilayer hydrogels via hydrogel internal architecture

We report on altering the mechanical behavior of ultrathin poly(methacrylic acid) (PMAA) multilayer hydrogels by changing the network's internal organization. The hydrogels were synthesized by cross-linking PMAA layers in poly(N-vinylpyrrolidone) (PVPON)/PMAA hydrogen-bonded multilayers prepared by dipped or spin-assisted (SA) layer-by-layer assembly using sacrificial PVPON with molecular weights of 40,000 or 280,000 g mol⁻¹. The effect of PVPON molecular weight on hydrogel stratification, swelling, and hydration was assessed. We found that hydrogel swelling, the number of water molecules associated with the swollen hydrogel, and water density within the SA PMAA hydrogels can be controlled by choosing the molecular weight of PVPON. With similar cross-link densities, greater swelling and hydration at pH>5 were observed for SA PMAA hydrogels synthesized using higher-Mw PVPON. The enhanced swelling of the SA hydrogels resulted in softening with a lower Young’s modulus at pH>5. A twice greater softening of the SA PMAA hydrogel than that prepared by dipping was observed, with Young’s modulus values decreasing to tens of megapascals in pH > 5 solution. Unlike simply changing bulk hydrogel cross-link density, programming polymer network architecture enabled selective modulation of the cross-link density within hydrogel strata, and polymer chain intermixing through hydrogel stratification controls the internal architecture, hydrogel swelling, and network mechanical response.