Sol-gel transition characterization of thermosensitive hydrogels based on water mobility variation provided by low field NMR

Sol-gel transition properties play a key role in various applications of thermosensitive hydrogels, but conventional methods for studying the sol-gel transition have some limitations. For extensive characterization of the water-polymer interaction and microstructure change during sol-gel transition, we propose a rapid and nondestructive method based on monitoring water mobility through low field NMR (LF-NMR), and this was applied to chitosan/beta-glycerophosphate (CS/GP) hydrogels. The spin-spin relaxation time (T-2) that depicted water mobility was measured by LF-NMR within 90 s. The T-2 component corresponding to water protons trapped in polymer networks (T-21) was very sensitive to sol-gel transition. A remarkable decrease of T-21 value indicated obvious variations of water mobility when CS/GP was heated, and a turning point was observed on the T-21-time curve. The gel point associated with this turning point could be easily determined by fitting the T-21-time curves to a bilinear regression model, and the results showed good accuracy and repeatability owing to the nondestructive nature of LF-NMR. Variations in water components and microstructure of CS/GP caused by water migration after solidification were also analyzed by monitoring dynamic changes of T-2. This rapid, nondestructive method provides a powerful tool for studying the sol-gel transition of hydrogels.