(216c) The Effects of Salt on the Free Energy of Thermo-Responsive PNIPAM Transition
Thermo-responsive polymers exhibit globule-to-coil transition at their lower critical solution temperatures (LCST). The thermo-responsive poly (N-isopropylacrylamide) (PNIPAM) and its copolymers have been often grafted on membrane substrate as stimuli-responsive membranes. However, the driving force for the folding and unfolding of PNIPAM, particularly at the molecular level remains controversial and unresolved. Here classical molecular dynamics simulations coupled with metadynamics technique was used to investigate the free energies associated with globule-to-coil transition for PNIPAM in water and NaCl salt solution at their respective transition temperatures. Our results indicate that globule-to-coil transition is mainly entropically driven. Further unfolding process appears to have a higher barrier and is energetically less favored compared to the folding process at the transition temperature. The presence of salt ions alters the free energy landscape and stabilizes the folded structure.