  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.
