Phase separation in poly(N-isopropyl acrylamide)/water solutions I. Cloud point curves and microgelation

Abstract

Cloud point curve was determined on binary and ternary aqueous solutions of several fractionated poly(N-isopropyl acrylamide) (PNIPA) samples with the molecular weight Mη=11.3 × 104 - 210 × 104 and the molecular weight distribution Mw/Mn=1.21 - 1.62. In both the binary and ternary solutions, the cloud point temperature Tclo slightly increases with the increase in molecular weight of PNIPA. The concentration dependence for the binary solution is weak while it becomes obvious for the ternary solution containing two different PNIPA samples in water; i.e., the molecular weight distribution alters the shape of the cloud point curve. The storage and loss moduli G′ and G″ for the PNIPA sample of Mη=210 × 104 in water at concentrations 2.2 - 8.5 wt% were measured at several temperatures encompassing the Tclo. The results indicate that the solution changes from a homogeneous fluid into viscoelastic microgels suspended in a liquid phase during the phase separation. This physical network is considered to be jointed via the intermolecular hydrophobic bonding between the N-isopropyl groups, which may provide a model system for the structure change of proteins induced in the denaturation.