Visualization of Hydrogel Shrinkage Due to Ion Replacement by ²⁷Al and ²³Na Magnetic Resonance Imaging

Ionic hydrogels are polymer networks that have the ability to store large amounts of water. Due to this behavior hydrogels based on sodium acrylate (SA) are mostly used in diapers and soil conditioners. In this work diffusion and exchange processes in polyelectrolyte hydrogels were followed by ²³Na and ²⁷Al magnetic resonance imaging (MRI) techniques. We present ²³Na and ²⁷Al MRI experiments of the replacement reaction in hydrogels (classical superabsorbers, hydrogels with an inherent charge gradient and with a higher charge) as a feasibility study of monitoring the combined diffusion/exchange process as well as the shrinking and local defect growth in macroscopic model hydrogels. It is shown that the contraction of the gel induced by increasing the number of physical crosslinks through aluminum ions is of non-linear nature and takes place in several steps in contrast to high charge-density hydrogels based on acrylic acid and maleic acid.