| Thickness of AGM's crosslinked shell (crosslinking
density) can be determined by using a Scanning Transmission X-ray
Microscope (STXM), the technique of a Near-Edge X-ray Absorption Fine
Structure (NEXAFS) spectromicroscopy. |
| |
| Currently AGMs in the market are all surface crosslinked
type. Surface crosslinking means to form 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. In general, crosslinking has a deep impact on
the absorbent capacity of AGM. Absorbent capacity or "gel volume"
has inverse power-law dependence on the level of crosslinking. In
order to obtain an AGM with high gel strength (less gel blocking and
higher Performance Under Pressure, PUP) while maintaining gel volume
as high as possible, chemical reactions on the surfaces of the AGM
particles are often used to form thin shells of the so-called surface
crosslinking (see the illustration below). |
AGM's performance depends on microscopic details
of the shell structure, such as variations in the crosslinking through
and around the shell. However the detailed information about AGM's
crosslinked shell is not easy to obtain until very recently. Dr Mitchell
et al. at Dow Chemical Co. by collaborating with researchers at North
Carolina State University and McMaster University have successfully
applied Near-Edge X-ray Absorption Fine Structure (NEXAFS) spectromicroscopy
to map the variation in crosslink density through shells formed in
different ways, thereby providing a way to adjust the effect of shell-formation
processes at the microscopic level and to improve the production process
of AGMs.
Because the x-ray energy which the carbons in the AGM absorb and the
water is almost transparent can be tuned to value, Dr Mitchell et
al have successfully mapped the shell areas where crosslinking was
higher by observing the increased carbon content in these regions
using swollen AGMs as the specimen. |
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