Small-angle Scattering Study of Mesoscopic Structures in Charged Gel and Their Evolution in Dehydration

Mesoscopic structures, with length scales ∼102 (angstrom), were investigated by small-angle X-ray and neutron scattering (SAXS and SANS) in several N-isopropylacrylamide-sodium acrylate (NIPA-SA) copolymeric hydrogels with varying (NIPA)/(SA) ratios and water contents. The SAXS experiments reveal that, depending upon the (NIPA)/(SA) ratio, the dehydrated NIPA-SA gel shows two mesoscopic structures: one consists of randomly distributed SA-rich islands in NIPA matrix, while the other is a microphase-separated structure, composed of NIPA-rich and SA-rich domains. In addition, the SANS experiments reveal the mesoscopic structural features during the dehydration process. As the concentration of the network polymers increases, NIPA-rich and water-rich domains segregate in the gel. Then, an electrostatic interaction between the segregated domains induces a microphase-separated structure in the limit of the dehydrated NIPA-SA gel.