Effect of Tacticity Sequence of the Poly(N-isopropylacrylamide) Oligomer on Phase Transition Behavior in Aqueous Solution

The tacticity of poly(N-isopropylacrylamide) (PNIPAM) has a strong impact on the lower critical solution temperature (LCST) in aqueous solution. The sequence of meso diads (m) and racemo diads (r) further contributes to such an effect. In this work, the phase transition behaviors of poly(N-isopropylacrylamide) pentamers with four kinds of sequences, i.e., rrmm, rmmr, mrrm, and rmrm, in water were studied applying replica exchange molecular dynamics with a modified OPLS/AA force field. The difference in local component concentration in the system was used as an order parameter to quantitatively describe the phase separation extent. It was found that the phase separation degree of rrmm and rmmr is higher than that of mrrm and rmrm at the same temperature. The LCSTs of rrmm and rmmr are lower than those of mrrm and rmrm. The radial distribution function and hydrogen bond analysis revealed that the average values of hydrogen bonds between pentamers for rrmm and rmmr are greater than those of mrrm and rmrm, whereas the average values of hydrogen bonds between pentamers and water for rrmm and rmmr are less than those of mrrm and rmrm. It was demonstrated that the isotactic triad (mm) plays an important role in the thermosensitive behaviors of the PNIPAM pentamer. The increase of isotactic triad (mm) content in the PNIPAM chain promotes the formation of intermolecular hydrogen bonds between amide and amide and leads to a higher aggregation of the pentamer with the sequence of rrmm or rmmr. Finally, the effect of the isotactic triad was qualitatively explained with the mean-field theory.