Hydration and Solvent Exchange Induced Swelling and Deswelling of Homogeneous Poly(N-isopropylacrylamide) Microgel...

The investigation of the response kinetics of smart colloidal microgel films is crucial for their optimization to enable advanced applications. We study the classical thermoresponsive microgel model system N-isopropylacrylamide cross-linked with N,N'-methylenebisacrylamide. Without the typically used polyelectrolyte coating of the substrate, thin microgel films are prepared in a single spin-coating step. Atomic force microscopy measurements reveal an extremely dense packing, resulting in a homogeneous compact thin film of microgel particles. The hydration kinetics of these films in H₂O and D₂O atmospheres as well as the kinetics of the solvent exchange between both water species are investigated with in situ time-of-flight neutron reflectometry (TOF-NR) and in situ Fourier-transform infrared (FTIR) spectroscopy. With accounting for a nonconstant humid atmosphere, the intrinsic diffusion dynamics of water molecules into the thin microgel film are modeled and the specific time constant τ and the effective Flory-Huggins interaction parameter χ_eff are determined. Comparing the results in H₂O and D₂O atmospheres, TOF-NR and FTIR spectroscopy results show an increased affinity of the microgel films toward H₂O as compared to D₂O. From the FTIR spectroscopy data, we further identify different kinetics of intermolecular processes and order them according to their temporal evolution.