Superabsorbent polymers are a specific example of a polymer gel. Occurring in both natural and synthetic forms, gels exhibit an intriguing combination of the properties of both liquids and solids. One feature that makes gels useful is their ability to respond strongly to very weak external stimuli, such as minute changes in pH or temperature. For example, a polymer gel might first absorb a quantity of liquid and later release it as the external conditions change. Timed release of pharmaceuticals is one example among many in which a control stimulus determines the rate of release. Crosslinking is a key feature of the polymer microstructure that governs actual performance.

Dow sells SAP in the form of small (less than 1.0 mm in size) beads of sodium polyacrylate that is lightly crosslinked to form an insoluble, hydrophilic gel. The ability to soak up great quantities of fluid makes SAPs attractive for use in diapers. But, like a sponge when compressed, some of the fluid is squeezed back out when the baby moves, negating part of the benefit. The strategy for preventing leakage under weight-bearing load is the formation of a thin shell of more tightly crosslinked polymer. The effectiveness of the shell depends in part on the density profile of the crosslinking through the shell, a distance of several microns. Several different methods have been developed to make surface crosslinked SAP gels, but there has been no good way to visualize and assess the resulting core-shell structure and the crosslink density profile.

To obtain the desired information the researchers turned to near-edge x-ray absorption fine structure (NEXAFS) spectromicroscopy, using the scanning transmission x-ray microscope (STXM) on Beamline 7.0.1 to make images of the polymers in the fully hydrated state (in excess water). Because the x-ray energy could be tuned to a value where the carbon in the polymer absorbs and the water is almost transparent, they could map the areas where crosslinking was higher by observing the increased carbon content in these regions.

Baby Diapers Drive Polymer Improvements

The global market for disposable diapers is $20 billion annually, but manufacturers face a twofold challenge. To keep the baby dry, the diaper must be able to take up a large quantity of liquid and then hold it "under load" (i.e., as the baby moves around). At the same time, customers demand thin diapers, thereby ruling out the extensive use of the bulky cellulose fluff that was once used. Superabsorbent polymers (SAPs), materials comprising long chains of intertwining molecules with the happy ability to soak up lots of liquid, now dominate the disposable diaper market.

By means of chemical reactions on the surfaces of submillimeter-sized SAP beads, polymer makers form thin shells of so-called tightly crosslinked polymer in which the strands are connected at the cross links. The shell makes it more difficult for liquid to leak out, but actual performance depends on microscopic details of the shell structure, such as variations in the crosslinking through and around the shell. This kind of detail has been hard to come by, but Mitchell et al. have now successfully applied an x-ray microscopy technique (NEXAFS spectromicroscopy) to map the variation in crosslink density through shells formed in different ways, thereby providing a way to gauge the effect of shell-formation processes at the microscopic level and to improve the production process.

Crosslinking was stimulated by treating the surface of SAP beads with varying amounts of either ethylene glycol diglycidyl ether or glycerol. Sectioned beads were then exposed to 0.9% saline solution to put them in the fully swollen state for imaging. Analysis of the images yielded two extreme cases for the crosslink profile through the shell. In one, the crosslink density decreased smoothly over a distance of 18 microns from a maximum at the outer surface. In the other, the density was uniform over a distance of 5 microns and then dropped abruptly. These differences reflect a complicated interplay between the dynamics of the swelling of the bead in water, the diffusion rate of the crosslinker in the water phase, and the rate of the crosslinking reaction. Dow was able to use this kind of information in designing a new SAP-fabrication plant.

NEXAFS spectromicroscopy images of shells formed around SAP beads by two different methods. (A) Images of a bead crosslinked with ethylene glycol diglycidyl ether at three photon energies. The shell is the arc of graded density with the outer surface to the right. (B) Map of the polymer concentration obtained from analysis of the three images. (C) Similar polymer concentration map for a bead crosslinked with glycerol showing a sharply delineated density profile.

Crosslink density across the shells from the outer surface (position 0) to the interior of SAP beads treated in two different ways. Comparison shows strikingly different profiles that reflect the complicated kinetics of the SAP swelling when exposed to liquid and the different shell-formation processes.

Research conducted by G.E. Mitchell, L.R. Wilson, M.T. Dineen, F. Hayes, and E.G. Rightor (The Dow Chemical Company); S.G. Urquhart and H.W. Ade (North Carolina State University); and A.P. Hitchcock (McMaster University).

Research funding: The Dow Chemical Company, National Science Foundation, and Natural Sciences and Engineering Research Council of Canada. Operation of the ALS is supported by the Office of Basic Energy Sciences, U.S. Department of Energy.

Publication about this research: G.E Mitchell, L.R. Wilson, M.T. Dineen, S.G. Urquhart, F. Hayes, E.G. Rightor, A.P. Hitchcock, and H.W. Ade, "Quantitative Characterization of Microscopic Variations in the Crosslink Density of Gel" (submitted 2001).

ALSNews Vol. 172, March 14, 2001