Molecular Dynamics Calculations for the Temperature Response of Poly(alkylated tri(ethylene oxide)isocyanate) Aqueous Solution

Abstract

Aqueous solutions of conventional temperature-responsive amphiphilic polymers undergo a coil–globule conformational transition around the lower critical solution temperature (LCST) that causes the polymer surfaces to become hydrophobic and the polymers to aggregate together. Isocyanate polymers with alkylated oligo(ethylene oxide) side chains are expected to have rigid main chains and, thus, do not undergo the coil–globule structural transition, but they have recently been reported to exhibit temperature-responsive properties. In this study, molecular dynamics was used to calculate the agglomeration tendencies of two chains of poly(alkylated tri(ethylene oxide)isocyanate) (PRTEOIC, where R = methyl (Me) or ethyl (Et)) in aqueous solution to elucidate the LCST phenomenon in the absence of coil–globule conformational transition. Our MD simulations showed that aggregation also occurs in rod polymers. Furthermore, we found that both (PMeTEOIC)2 and (PEtTEOIC)2 showed parallel agglomeration of the two molecular chains with increasing temperature, but only (PMeTEOIC)2 showed a metastable T-shaped agglomeration in the middle temperature range. The crossing-point temperature (TCRP) at which the density of the first hydrophobic hydration shell around the sidechain alkyl group equals the bulk water density is a useful indicator for predicting the LCST of rod polymers with dense side chains terminated by alkyl groups.