NMR Study On Phase Transition Of PNIPAAM In Water/Methanol Mixtures

Poly(N-isopropylacrylamide) (PNIPAAM) has been widely used in physics, chemistry andmedicine. PNIPAAM in water/methanol mixtures exhibits the reentrant phase transition, e.g.,soluble-to-insoluble-to-soluble phase transition. In this dissertation, the behaviors of phaseseparation of PNIPAAM in water/methanol mixtures are studied by nuclear magnetic resonance(NMR).It is found that when the mole fraction of methanol is low, the chemical shifts of water andmethanol increase with the fraction of methanol. A clathrate-like structure, e.g., methyl ofmethanol is surrounded by water molecules with strengthening hydrogen bonds, is used tointerpret the present result.By comparing the various NMR parameters obtained in water/methanol mixtures with thoseobtained in water/methanol/PNIPAAM mixtures, it is found that when the phase transition (at 6mol % methanol) and reentrant transition (～36 mol % methanol) occurs, the related NMRparameters also change dramatically. This indicates that the NMR parameters can be used tocharacterize the phase transition sensitively. Also in the present investigation, it is found that whenthe mole fraction of methanol is less than 6 mol %, proton T₁ of HOD increases with increasingfraction of methanol, while pronton T₁ of CD₂HOD decreases with increasing fraction of methanol.This may be attributed to the competition between water-methanol hydrogen bonds andwater-PNIPAAM hydrogen bonds.