Direct measurement of crosslinked surface layer in superabsorbent poly(acrylic acid)

We propose a simple and quantitative method to evaluate thickness of crosslinked surface layer (t(x)) for superabsorbent poly(acrylic acid) (PAA) materials using a fluorescent dye of rhodamine B (RhB). The electrostatic attraction between the cationic amine group of dye and the anionic carboxylate group of PAA allows us to measure the fluorescent contrast since the surface-crosslinking causes a significant decrease in concentration of carboxylate group compared to that of inner region. Our method utilizing the distinct adsorption difference of RhB visualizes the crosslinked surface (or shell) layer and successfully demonstrates a direct characterization of t(x) in a dry state of superabsorbent materials, regardless of irregular particle shape. (C) 2018 Published by Elsevier B.V.