SANS study on pressure-induced phase separation of poly(N-isopropylacrylamide) aqueous solutions and gels

Pressure-induced phase separation of poly(N-isopropylacrylamide) (PNIPA) aqueous solutions and gels were investigated by small-angle neutron scattering (SANS). The cloud point curves were constructed on the pressure-temperature (P-T) plane by visual observation for both the solutions and gels. As observed in the phase diagram of aqueous solutions of proteins, the cloud point temperatures (T-cloud) were a function of the pressure, P, and had maxima at P-0 = 51.7 MPa and P-0 = 93.2 MPa respectively for the solution and gel in H2O. The difference in T-cloud indicates that the effect of cross-linking is significant and it leads to an increase of the miscible region. The SANS intensity function for the solution was well represented by a Lorentz function, i.e., an Ornstein-Zernike (OZ) function, from which the correlation length, xi, and the susceptibility, I(0), were evaluated as a function of pressure. The critical exponents were obtained to be 1.1 less than or equal to gammap less than or equal to 1.23 and 0.5 less than or equal to nu(p) less than or equal to 0.6, for the solution, similar to the case of the temperature dependence of xi and I(0). The spinodal temperature, T-sp, seems to merge with the binodal curve (T-cloud) at P-0 for the PNIPA solution. I(q)'s for the gels, on the other hand, deviated significantly from an OZ form by approaching the spinodal and were well fitted with a sum of squared-Lorentz and Lorentz functions. This squared-Lorentz function accounts for the emergence of cross-link inhomogeneities, which become dominant near the spinodal.