All-atom REMD simulation of poly-N-isopropylacrylamide thermodynamics in water: a model with a distinct 2-state behavior

Abstract

Poly-N-isopropylacrylamide (PNIPAM) is a thermoresponsive polymer, an essential building block of a large family of soft smart materials. In neat water it undergoes upon heating at $\approx$305K a phase transition from swollen to collapsed state. This interesting polymer behavior has been subject to a large number of computer modelling studies, mostly described with original OPLS force field suggested in the first works, where qualitatively correct behavior was observed. Nevertheless, employing converged replica exchange molecular dynamics simulations, we have shown on PNIPAM 30mer that this widely used force-field can not describe the phase transition thermodynamics. We found that the reason is a poor balance of hydrophobic-polar character of the polymer. We have developed and tested three new force fields OPLS1.2x, QM1, and QM2, which all exhibit two state behavior, and reasonably reproduce experimental lower critical solution temperature and transition thermodynamics. The best performance was found for QM2, which is suggested for future PNIPAM studies. Finally, the effect of polymer chain length on collapse transition thermodynamics was investigated by simulations of 20, 30 and 40mer chains. Consistent with experimental data, the transition enthalpy per monomer unit is only weakly increasing with the chain length. However, the calculated transition enthalpy is around times smaller than the experimental calorimetry value.