Quantitative Characterization of Microscopic Variations in the Cross-Link Density of Gels

We report the visualization and quantitative analysis of the cross-link structure in model core/shell hydrogel polymers on the microscopic scale, in a fully swollen state, using soft X-ray microscopy. The cross-link density in these materials and their microscopic or even nanoscopic variation critically influence materials characteristics, yet the cross-link density is difficult to characterize by conventional methods. By the use of soft X-ray microscopy, one can investigate these materials in a fully swollen state and thus directly visualize and quantitatively determine the cross-link structure on a microscopic scale. Materials that were cross-linked by different methods were shown to give rise to differently shaped profiles. Abrupt and gradient cross-link density profiles have been investigated, and the spatial variation in their cross-link density has been determined quantitatively.