Energetics of Phase Separation in Aqueous Solutions of Poly(N-isopropylacrylamide)

The energetics of phase separation in dilute aqueous solutions of poly(N-isopropylacrylamide) is studied by high-sensitivity differential scanning calorimetry. The temperature dependences of the partial heat capacity of the polymer are obtained. The effect of the heating rate on their shape is examined. The concentration dependences of thermodynamic parameters of phase transition are determined. After phase separation of the system, the partial heat capacity of the polymer is much smaller than its partial heat capacity in the state of a swollen random coil. This finding indicates the occurrence of the polymer hydrophobic structure in the concentrated phase of the system probably in the form of clusters of monomer units. The profile of transition is described by the Schroder-van Laar equation with the van't Hoff enthalpy independent of polymer concentration. The size of the cooperative unit of the ordered hydrate structure of the polymer is estimated and found to be coincident with the size of the Kuhn segment. High-velocity sedimentation measurements of the polymer are conducted at various temperatures both below and above the binodal curve of the system. It is shown that the sizes of poly(N-isopropylacrylamide) macromolecules before the phase transition temperature and in the diluted phase after phase separation of the system coincide. Thus, in the diluted phase, macromolecules retain the coil conformation.