Direct Phase Equilibrium Simulations of NIPAM Oligomers in Water

NIPAM
(N-isopropylacrylamide)-based polymers in
water show many interesting properties in experiments, including a
lower critical solution temperature (LCST) at 305 K and a conformational
transition of single chains at the same temperature. The results of
many simulation studies suggest that standard force fields are able
to describe the conformational transition and the phase equilibrium
well. We show by performing long molecular dynamics simulations of
the direct liquid–liquid phase equilibrium of NIPAM trimers
in water that there is no LCST in the expected temperature range for
any of the force fields under study. The results show further that
the relaxation times of single-chain simulations are considerably
longer than anticipated. Conformational transitions of single polymers
can therefore not necessarily be used as surrogates for a real phase
transition.