Effect of degree of cross-linking on spatial inhomogeneity in charged gels. 2. Small-angle neutron scattering study

The structural inhomogeneities in a weakly charged gel consisting of (N-isopropylacrylamide-co-acrylic acid) (NIPA-AAc) have been investigated by small-angle neutron scattering (SANS) as a function of cross-linking density (CD) and temperature. The CD and temperature dependence of the scattered intensity profiles, I(q), indicated the presence of an anomalous CD dependence of the scattering intensity, where q is the magnitude of the scattering vector. At temperatures below 42 degrees C, I(q) was a monotonically decreasing function of q and increased with CD at q less than or equal to 0.08 Angstrom(-1). However, above 45 degrees C, I(q) started to have a distinct peak around q = 0.02 Angstrom(-1) and the CD dependence of I(q) became classifiable into two regimes depending on temperature; one where I(q) increases with CD, and the other where I(q) decreases with CD. Since a similar phenomenon, called "inversion", has already been observed by light scattering, it is concluded here that the inversion is a universal phenomenon observed in a wide range of the q space from 10(-4) to 10(-2) Angstrom(-1). The physical meaning of this inversion is discussed on the basis of the Rabin-Panyukov theory proposed for weakly charged gels.