Stratified Temperature-Responsive Multilayer Hydrogels of Poly(N-vinylpyrrolidone) and Poly(N-vinylcaprolactam): Effect of Hydrogel Architecture on Properties

We report on the effects of hydrophilicity and architecture on the temperature-responsive behavior and surface morphology of nonionic double-stack hydrogels prepared from cross-linked hydrogen-bonded layer-by-layer films. A hydrophilic poly(N-vinylpyrrolidone) (PVPON)_n multilayer hydrogel is integrated with a relatively hydrophobic temperature-sensitive poly(N-vinylcaprolactam) (PVCL)_m network as either a top or bottom stratum, where n and m represent numbers of layers for each individual stratum. Neutron reflectometry revealed that all double-stack films in the dry state are well stratified with two distinct (PVPON) and (PVCL) strata of higher and lower scattering density, respectively, unlike highly mixed alternating (PVCL/PVPON) hydrogels. We have found that the order of stacking and stack thickness significantly influence hydration of the (PVPON)_n(PVCL)_m and (PVCL)_m(PVPON)_n networks at ambient temperature and above the LCST of PVCL. The hydration of the hydrogels consistently increases with PVPON amount within the network, resulting in suppressed temperature response. This effect is more pronounced for (PVPON)_n(PVCL)_m as compared to its mirror counterpart as explained by the two adjacent aqueous interfaces in which the (PVCL)_m stack is sandwiched between the hydrophilic (PVPON)_n stack below and the bulk of water above it. Our results yield new insights into controlling the temperature response and surface properties of nanostructured polymer networks, which is relevant to both fundamental and applied research where the dynamics of hydration, thickness, and control of surface hydrophobicity are important.