Small-angle neutron scattering study on weakly charged poly(N-isopropyl acrylamide-co-acrylic acid) copolymer solutions

Abstract

The scattered intensity for weakly charged polymer solutions of a poly(N‐isopropyl acrylamide‐co‐acrylic acid) copolymer (NIPA/AAc) in deuterated water, I(q), was obtained by means of small‐angle neutron scattering (SANS) as a function of temperature and polymer fraction, φ, where q is the magnitude of the scattering vector. The scattered intensity was then compared with those for the corresponding polymer gels. It was found that both structure factors were similar to each other and were well described with an Ornstein–Zernike‐type scattered intensity function when the temperature was lower than the so‐called Θ temperature, which is about 34.6 °C. The correlation length ξ for NIPA/AAc solution was given by ξ∼φ^−3/8, indicating a weakly charged polymer solution in a semidilute regime. Above this temperature, both started to have a peak at q≊0.02 Å⁻¹, which increased with temperature. However, a significant difference in I(q), due to the absence or presence of crosslinks, respectively for the solution and the gel, was observed at these temperatures. In the case of polymer solutions, a steep increase in I(q) appeared by approaching q=0, indicating a macroscopic inhomogeneity. On the other hand, I(q) for polymer gels had a finite value at q=0 due to the suppression of macroscopic inhomogeneity originated from the presence of crosslinks. A method to analyze these scattered intensity functions is proposed by comparing with the theory of Borue and Erukhimovich for polyelectrolytes in a poor solvent.