Microstructure Characterization of PEGDA Hydrogels Using TD-NMR

PEGDA hydrogels were synthesized at polymer concentrations from 15% to 50% and molecular weight from 575 to 3400 Da. These were characterized in a Bruker Minispec mq-20 in solution, after cross-linking, and after swelling the gel to equilibrium. A noticeable reduction in T₂ was observed for PEGDA gels upon cross-linking, indicative of the mobility constraint caused by the formation of the hydrogel network. Linear correlations between polymer concentration and the reciprocal of the transverse relaxation rates (1/T₂) were observed for all the three states, as expected from theory. Additionally, T₂ distributions were obtained via the inverse Laplace transform, and literature models were used to calculate mesh sizes from the NMR output. The calculated mesh distribution was found to decrease as polymer concentration increased, consistent with a reduction in the pore size as expected. It was hypothesized that heterogeneous networks would have multiple T₂ peaks and broader distributions than more homogeneous ones. This hypothesis was demonstrated by combining hydrogel disks formulations with different mesh sizes in a single TD-NMR test, as displayed multiple and broader T₂ peaks were obtained in this emulation of a heterogeneous network.

The results demonstrate that TD-NMR is capable of detecting water in different confinement environments, and correlated with changes introduced during hydrogel synthesis. These experiments proved that the TD-NMR technique has the potential of determining multiple mesh sizes for hydrogels. Future work will include the development of a different method of inverse Laplace transform of the NMR signal to resolve broad distributions into underlying multiple pore sizes in a hydrogel sample.