Exploring dielectric spectra of polymer through molecular dynamics simulations

In this work, poly(<i>N</i>-isopropylacrylamide) (PNIPAM) was used as a research object to explore the feasibility of molecular dynamics (MD) simulations to study the dielectric properties of polymers. A comparison of the static dielectric constants for two different calculation schemes, the applied electric field method and the dipole moment fluctuation method, has been conducted. The dielectric properties of PNIPAM aqueous solutions with different concentrations were studied as a function of temperature. The static dielectric constants derived from dipole moment fluctuation method at different temperatures can accurately describe the phase transition process. The concentration does not affect the lower critical solution temperature, but significantly affects the relaxation process of the system. According to the frequency-dependent dielectric constants, two relaxation processes are observed, namely, the dielectric relaxation process in the region of frequency less than 30 terahertz (THz) determined by the synergistic interaction of PNIPAM and water molecules, and that in the region of frequency higher than 30 THz mainly determined by water molecules beyond the solvation shell of the polymer. The calculation results suggest that the dielectric behaviour is concentration and temperature independent, similar to Debye behaviour of bulk water. This method of calculating dielectric properties using MD simulations can be used in detecting the dynamic processes of soft condensed matter and in aiding the design of materials with excellent dielectric properties.