Multi-scale characterization of surface-crosslinked superabsorbent polymer hydrogel spheres

In order for super absorbent polymer (SAP) to be used in an application undergoing mechanical stress, it is necessary to regulate its shape and mechanical properties, which have been rarely studies. Here, spherical SAPs were prepared through inverse-suspension polymerization of partially neutralized acrylic acid monomers with ethylene glycol diacrylate (EGDA) crosslinkers. The surface region of SAP spheres was additionally crosslinked with ethylene glycol diglycidyl ether (EGDE) to improve the mechanical properties, producing a core-shell structure characterized by lightly crosslinked core and more densely crosslinked shell. Mechanical responses of the cross-linked SAP to global compression were obtained from experiments using a custom-made indentation device. The local mechanical properties of crosslinked surface were measured by atomic force microscopy. While the global responses of SAP were similar independent of surface crosslinking, the modulus of crosslinked surface increased as concentration of EGDE. In our experimental conditions, the surface modulus of surface crosslinked SAP was increased 2.7 folds, compared to that of SAP without surface crosslinking. The structure of SAP spheres with and without surface crosslinking was also visualized, and thickness of crosslinked surface was measured with fluorescence microscopy. We found that the thickness of crosslinked surface increased with increasing EGDE concentration but became saturated. Furthermore, a computational model was developed for core-shell SAP spheres using the measured mechanical properties, and was utilized to predict the thickness of crosslinked surface. Finally, we propose an analytical diffusion model that describes the diffusion and surface crosslinking reaction to elucidate the mechanism over which the mechanical and diffusion properties of SAP sphere are determined.