MOLECULAR DYNAMICS SIMULATION OF A MODEL OLIGOMER FOR POLY (N, N-DIETHYLACRYLAMIDE) IN WATER

Poly (N, N-diethylacrylamide) (PDEA) is a temperature sensitive polymer, whose aqueous solution separates into two phases when the temperature excess the lower critical solution temperature (LCST) accompanying with the collapse of the chains. The purpose of this work was to distinguish the different explanations of the energy change during transition and to investigate the microscopic details of the PDEA-water interaction related to single-chain conformational changes occurring across the LCST. The molecular dynamics (MD) was used to simulate the aqueous solutions of thermoresponsive PDEA molecules with 50 units at 300 K and 310 K for the compact and extended chain conformations, and the number of hydrogen bonds (HB) in the solution was analyzed in more detail. The MD simulations were performed by using the Tinker program with the force field OPLS. The simulation system consisted of 6103 TIP3P water molecules surrounding one PDEA molecule. In order to maintain simulations within the canonical ensemble (NVT), the Berendsen thermostat was used. The integration step was set to be 1 fs, and coordinates were saved every 0.3 ps. The total simulation time including the equilibration and production run, in which statistical data were gathered, was 2000 ps, where the last 300 ps of each simulation was utilized as the production run period. After the simulation, the trajectory files can be analyzed as required. Analyses of trajectories show that the water molecules mainly contact with the nonpolar groups of PDEA. The average number of HB per water molecule varies with the distance of water to polymer in the PDEA aqueous solution. The water molecules form more hydrogen bonds and are in the more ordered structure in the first shell and the second shell around the polymer than those in the bulk water. When the polymer chain transfers from the extended conformation to the compact one, the number of water molecules in the first shell, especially the number of water molecules surrounding the nonpolar groups of PDEA, reduces accompanying with the decrease of the HB number in the solution. As a result, both the enthalpy and entropy of the solution increase. It supports the assumption that the energy change mainly comes from the alternation of the number of hydrogen bonds between water molecules in the first-shell. The self-diffusion coefficients of polymer molecules and water molecules were also been discussed. The value of self-diffusion coefficient for water molecules surrounding the PDEA is smaller than that in the bulk water, and the value of self-diffusion coefficient for PDEA increases as the polymer chain is compacted.