ABSTRACT
Hydrophilic polymers, such as sodium polyacrylate, absorb a larger amount of water than do conventional water-absorbing materials, and the absorbed water is not removable even under some pressure. Sodium polyacrylate was set on a slide glass of a microscope connected to a videotape recorder, and distilled water or solutions were poured on the sodium polyacrylate from a hypodermic needle. The volume change of the swelling and the deswelling of sodium polyacrylate apparently follow first-order kinetics. The kinetics of the swelling and the deswelling of sodium polyacrylate were studied gravimetrically, by microscopie observation, and by calorimetric measurement. The activation energy of the swelling decreased with increased sodium chloride concentration. A large amount of water-swollen gel is free water having no interaction with the gel. In the swelling, hydration of sodium polyacrylate occurs prior to the penetration of free water into the gel; in the deswelling, the exclusion of free water from the gel occurs prior to dehydration.
