Controlling Internal Organization of Multilayer Poly(methacrylic acid) Hydrogels with Polymer Molecular Weight

We report on tailoring the internal architecture of multilayer-derived poly(methacrylic acid) (PMAA) hydrogels by controlling the molecular weight of poly(N-vinylpyrrolidone) (PVPON) in hydrogen-bonded (PMAA/PVPON) layer-by-layer precursor films. The hydrogels are produced by cross-linking PMAA in the spin-assisted multilayers followed by PVPON release. We found that the thickness, morphology, and architecture of hydrogen-bonded films and the corresponding hydrogels are significantly affected by PVPON chain length. For all systems, an increase in PVPON molecular weight from M_w = 2.5 to 1300 kDa resulted in increased total film thickness. We also show that increasing polymer M_w smooths the hydrogen-bonded film surfaces but roughens those of the hydrogels. Using deuterated dPMAA marker layers in neutron reflectometry measurements, we found that hydrogen-bonded films reveal a high degree of stratification which is preserved in the cross-linked films. We observed dPMAA to be distributed more widely in the hydrogen-bonded films prepared with small M_w PVPON due to the greater mobility of short-chain PVPON. These variations in the distribution of PMAA are erased after cross-linking, resulting in a distribution of dPMAA over about two bilayers for all M_w but being somewhat more widely distributed in the films templated with higher M_w PVPON. Our results yield new insights into controlling the organization of nanostructured polymer networks using polymer molecular weight and open opportunities for fabrication of thin films with well-organized architecture and controllable function.