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

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.