Scientific American

SODIUM POLYACRYLATE has sodium carboxylate groups hanging off the main chain. In contact with water the sodium detaches, leaving only carboxyl ions. Being negatively charged, these ions repel one another so that the polymer unwinds and absorbs water, which is attracted to the sodium atoms. The polymer also has weak cross-links, which effectively leads to a three-dimensional structure. In addition, it has molecular weights of more than a million; thus, it cannot dissolve but instead solidifies into a gel.read more in scientific american