Why are double-network hydrogels so tough?

1
Why are double-network hydrogels so tough?
T. Tominaga1,2, V.R. Tirumala1, P.D. Butler1,
E.K. Lin1, H. Furukawa2, J.P. Gong2, Y. Osada2,
and W.-L. Wu1
1NIST, 2Hokkaido University
DMR-0454672
Double network hydrogels (DN-gels) are prepared
by polymerizing a high molecular weight linear
polymer within a swollen polyelectrolyte network.
The linear polymer is highly viscous and readily
deformable (like putty) whereas the
polyelectrolyte network is rigid but fragile
(like jelly). Neither component is tough, yet
DN-gels have fracture toughness comparable with
joint cartilage. At the same time gt 90 by volume
is water.
Using three NCNR small angle scattering
instruments including the CHRNS 30-m SANS and
USANS, combined with selective deuteration to
control the scattering contrast, we have studied
the structures of DN-gels and those of their
components, PAMPS1 and PAAm2. The results suggest
that the heterogeneity decreases and then
increases with addition of PAAm. Structural
changes in the PAMPS gel with addition of PAAm to
form the toughest DN-gel can be conceptualized as
shown below.
1 PAMPS poly(2-acrylamide,2-methyl,1-propane
sulfonic acid) 2 PAAm polyacrylamide
Contrast-normalized scattering intensities for
DN-gels with different concentrations of PAAm,
measured over four orders of magnitude in q
(inverse length).
T. Tominaga, V.R. Tirumala, P.D. Butler, E.K.
Lin, H. Furukawa, J.P. Gong, Y. Osada, and W.-L.
Wu, paper in preparation.