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 H 2 O. 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, ξ, and the susceptibility, I (0), were evaluated as a function of pressure. The critical exponents were obtained to be 1.1 ≤ γ p ≤ 1.23 and 0.5 ≤ ν p ≤ 0.6, for the solution, similar to the case of the temperature dependence of ξ and I (0). The spinodal temperature, T sp, seems to merge with the binodal curve ( T clould ) 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. These structure changes by pressure will be discussed in conjunction with cold denaturation of proteins.