Dynamics and microstructures in aqueous solutions of polyacrylamide and acrylamide monomers assessed by molecular dynamics simulations combined with optical Kerr effect spectroscopy.

Understanding the origin of the optical Kerr effect (OKE) signals in aqueous polymer systems is essential for interpreting the spectroscopic measurements of molecular dynamics in soft materials. Here, we employ molecular dynamics simulations combined with a dipole-induced dipole polarizability model to investigate aqueous acrylamide (AAm) and polyacrylamide (PAAm) solutions. Through the decomposition of the polarizability anisotropy time correlation function, we demonstrate that the OKE signals in monomer solutions arise primarily from solute reorientation, while in polymer solutions, they originate almost exclusively from water dynamics confined by the polymer network. The fast and intermediate relaxation components are nearly identical between the two systems, but the slowest component in the PAAm solutions is significantly delayed due to the much slower structural relaxation of the polymer chain, which modulates the water dynamics through hydrogen bonding. These results establish a quantitative framework for interpreting the OKE spectra of complex macromolecular systems.