  We report on the delamination of thin ($\approx \mu$m) hydrogel films grafted
to silicon substrates under the action of swelling stresses.
Poly(dimetylacrylamide) (PDMA) films are synthesized by simultaneously
cross-linking and grafting preformed polymer chains onto the silicon substrate
using a thiol-ene reaction. The grafting density at the film/substrate
interface is tuned by varying the surface density of reactive thiol-silane
groups on the silicon substrate. Delamination of the films from well controlled
line defects with low adhesion is monitored under a humid water vapor flow
ensuring full saturation of the polymer network. A propagating delamination of
the film is observed under the action of differential swelling stresses at the
debonding front. A threshold thickness for the onset of this delamination is
evidenced which is increasing with grafting density while the debonding
velocity is also observed to decrease with an increase in grafting density.
These observations are discussed within the framework of a nonlinear fracture
mechanics model which assumes that the driving force for crack propagation is
the difference between the swelling state of the bonded and delaminated parts
of the film. Using this model, the threshold energy for crack initiation was
determined from the measured threshold thickness and discussed in relation to
the surface density of reactive thiol groups on the substrate.
