Molecular Dynamics Simulations of Thermoresponsive Poly (N-isopropylacrylamide) and Its Copolymer

Fouling is a significant concern for polymeric membranes in many applications including water treatment, bioseparations and biomedical applications. Surface modification of membranes to create low-fouling or nonfouling surfaces is an active area of research. It was suggested that antifouling properties are correlated to the flexibility of the polymeric chains and the water structures around them. The flexible and hydrophilic polymers tend to impart high resistance to fouling. However, the exact nature of the mechanisms for fouling resistance at the atomic and molecular level remains elusive and controversial.  In addition to static properties such as hydrophilicity, responsive polymers with dynamics changes in conformation are also being developed as antifouling surfaces. Poly(N-isopropylacrylamide) (PNIPAM) and PNIPAM-co-PEGMA, a copolymer of PNIPAM and poly (ethylene glycol) methacrylate (PEGMA) are thermo-responsive polymers which are being widely investigated for applications in controlled release and as antifouling surface coatings.

Thermo-responsive polymers exhibit hydrophobic−hydrophilic phase transition at their lower critical solution temperatures (LCST). For PNIPAM, the LCST phase transition is due mainly to the presence of a hydrophilic amide group on the side chain and a hydrophobic isopropyl group in each NIPAM unit as well as a hydrophobic C-C backbone. The LCST of PEG is much higher than that of PNIPAM due to its much higher hydrophilicity. It has been shown that the presence of salt in solution has a significant impact on the LCSTs of PNIPAM and PEG. In order to understand the conformational changes of PNIPAM and PNIPAM-co-PEGMA at their LCSTs and the water structures associated with the conformational change, classical molecular dynamics (MD) simulations were conducted in aqueous solutions. Since protein separations are normally carried out in a buffer solution containing salt, the interaction between the salt ions and the polymers as well as their effects on the polymer water structures were also investigated.