Nondestructive testing of defects in adhesive joints

compared to diffusion rate results in overshoot. Due to the thermodynamic interaction with
macromolecular chains, water enters before these chains can relax or rearrange. Later the
rearrangement of these chains result in partial exclusion of water leading to overshooting effect
[23]. In short overshooting effect depends on the NCO/OH ratio and pH of the surrounding
medium.
Conclusion
The water uptake of the hydrogel based on NR/PEG block copolymers varies sensitively with
NCO/OH ratio. The maximum Q∞ mol% uptake is shown by sample with NCO/OH ratio 1.1
followed by a decrease in the Q∞ mol% values in heavily crosslinked samples. Increase in
polymer mobility, free volume and better interactions existing in the sample with NCO/OH ratio
1.1 is the reason behind the high Q∞ mol% uptake. The sorption kinetics has been studied and the
experimental data suggests that the swelling process obeys first – order kinetics. Maximum D *
and S values are obtained in sample with NCO/OH ratio 1.1 which provide ample proof for the
better molecular interactions existing between the polymer – water system in this sample. The S
values of the block copolymers are higher compared to D * and P values resulting in a precedence
of sorption over both diffusion and permeation.
The entire block polymer samples exhibit remarkable overshoot at pH 4, pH 7 and pH 9 while in
the case of samples swollen in water, the overshooting effect is limited to samples with higher
NCO/OH ratios. The reason for the overshooting effect can be traced back to the swelling
transport mechanism. The samples exhibiting overshooting effect follow anomalous transport
with a bias to non – Fickian mode of transport. The experimental results reveal the influence of
NCO/OH ratio and pH of the surrounding medium on the overshooting effect shown by the
NR/PEG block copolymer samples.
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