Molecular Dynamics Investigation of the Effects of Ionic Strength On LCST of Poly (N-isopropylacrylamide)

Thermal responsive membranes can be used as anti-fouling membranes for water treatment and for self-cleaning purposes. Poly(N-isopropylacrylamide) (p(NIPAAm)) is a thermo-responsive polymer which has a corresponding low-critical solution temperature (LCST) of 32oC in water. Ionic strength, pH and the chain length have a significant effect on its LCST value. Molecular dynamics (MD) simulations were carried out for a 50 monomer unit p(NIPAAm) to investigate the effects of ionic strength on its LCST using NAMD simulation package with Amber force field. The effect of ionic strength on LCST was studied by investigating the polymer conformational change in 1 M NaCl, KCl and MgCl2 aqueous solutions at different temperatures around their corresponding LCST values. Good agreement with experimental LCST data was achieved. Moreover, the contributions to the stability of the polymer structures due to electrostatic, hydrogen bonding and van der Waals interactions were analyzed.