PHASE-SEPARATION IN POLY(N-ISOPROPYL ACRYLAMIDE) WATER SOLUTIONS I - CLOUD POINT CURVES AND MICROGELATION

Cloud point curve was determined on binary and ternary aqueous solutio ns of several fractionated poly(N-isopropyl acrylamide) (PNIPA) sample s with the molecular weight M-eta = 11.3 x 10(4)-210 x 10(4) and the m olecular weight distribution M-w/M-n = 1.21-1.62. In both the binary a nd ternary solutions, the cloud point temperature T-clo slightly incre ases viith the increase in molecular weight of PNIPA. The concentratio n dependence for the binary solution is weak while it becomes obvious for the ternary solution containing two different PNIPA samples in wat er; i.e., the molecular weight distribution alters the shape of the cl oud point currie. The storage and loss moduli G' and G '' for the PNIP A sample of M-eta = 210 x 10(4) in water at concentrations 2.2-8.5 wt% were measured at several temperatures encompassing the T-clo. The res ults indicate that the solution changes from a homogeneous fluid into viscoelastic microgels suspended in a liquid phase during the phase se paration. This physical network is considered to be jointed via the in termolecular hydrophobic bonding between the N-isopropyl groups, which may provide a model system for the structure change or proteins induc ed in the denaturation.