(661a) The Effects of Salt On Lower Critical Solution Temperature Transition of Thermo-Responsive Pnipam-Co-PEGMA Copolymer

Classical molecular dynamics (MD) simulations were carried out to investigate the lower critical solution temperature (LCST) transition of PNIPAM−co−PEGMA, a thermo-responsive copolymer of poly (N-isopropylacrylamide) (PNIPAM) and poly (ethylene glycol) methacrylate (PEGMA). The simulations were carried out in 1 M LiCl, NaCl, KCl, CsCl and NaI aqueous solutions below and above their respective experimental LCSTs. The copolymer consists of 38 monomer units of NIPAM with 2 PEGMA chains attached to the PNIAPM backbone. The PNIPAM−co−PEGMA was observed to go through the hydrophilic-hydrophobic conformational change for simulations carried out above its LCST. Cations were found to bind strongly and directly with amide O, and more strongly with the O atoms on PEGMA chains, whereas anions only exhibit weak interaction with the polymer. A unique caged stable metal-organic complex involving Na⁺ coordinated by 6 O atoms from the copolymer in NaCl solution was observed after a hydrophobic folded structure of the copolymer was formed. Interestingly, the binding affinity between the cation and the copolymer appears to depend not only on the size and charge of the cation, but also on the anion present. The cation−polymer interaction is found to be modulated by the cation−anion interaction.