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/β-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.