(663c) Replica Exchange Molecular Simulations of Thermal-Responsive Polymers Poly (N-isopropylacrylamide)

                                                Replica Exchange Molecular simulations of Thermal-responsive polymers Poly (N-isopropylacrylamide)

Lixin Liu, Henry S. Ashbaugh*
Department of Chemical and Biomolecular Engineering, Tulane University, New Orleans, LA 70118

Abstract

Poly (N-isopropylacrylamide) (PNIPAAM) is one of the most extensively studied temperature-responsive polymers. At room temperature it exists adopts a coil-like conformation that dramatically collapses into a globule at lower critical solution temperature. We report MD simulations of PNIPAAM in aqueous solution and to investigate the molecular origin of this counter-intuitive collapse transition behavior. Replica Exchange Molecular Dynamic (REMD) has been implemented over a broad range of temperatures to obtain improved sampling of polymer conformations. Radius of gyration of single chain PNIPAAM with various lengths obtained from simulation data validate the temperature induced coil-to-globule transition. A two-state model was applied to fit our simulation data and extract the thermodynamic driving forces for chain collapse. At the same time, the distances distributions between the isopropyl groups demonstrated more intensive interactions between those groups with increasing temperatures. Results obtained from REMD simulation of single chain polyacrylamide did not exhibit collapse behavior as PNIPPM’ Our simulation reproduced PNIPAAM’s thermal-responsive first-order phase transition of structure. Based on multiple data analysis approaches, we suggest hydrophobic group (isopropyl) makes main contribution for PNIPAAM’s thermal-responsive collapse behavior.