Nondestructive testing of defects in adhesive joints

Swelling experiment
Circular shaped samples (diameter ~ 13 mm) were cut from the block copolymer sheet by means
of a sharp edged steel die and thickness of the sample was measured with an accuracy of ±
0.01nm. Dry weight of the cut samples were taken before immersion into distilled water at room
temperature. The samples were periodically removed from test bottles. Then the samples were
weighed on an electronic balance (Shimadzu, Libor AEU-210, Japan) and immediately replaced
into the test bottles. This procedure was continued until equilibrium swelling was attained in the
case of each sample. The time taken for each weighing was kept constant to a minimum of 20-30s
in order to avoid errors due to the escape of solvent from the samples. The results of these
experiments were expressed as moles of solvent uptake by 100 g of polymer (Qt mol %).
Results and Discussion
The swelling behaviour of the hydrogels depends on the nature of the polymer and the
environmental conditions. The polymer’s nature involves crosslink density, ionic content etc. The
environmental conditions include pH and temperature [6,7]. A series of NR/PEG copolymeric
hydrogels was investigated in terms of their swelling characteristics. The effect of NCO/OH ratio
and pH on the swelling behaviour was studied.
Sorption dynamics and effect of NCO/OH ratio
The data from the sorption studies are plotted in Figure 1 with the percentage uptake of the
penetrant against t 1/2 , where t is the time taken to attain equilibrium. Since PEG is hydrophilic
polymer and NR is hydrophobic in nature, the present block copolymer possesses these dual
characteristics. In the case of a typical polar solvent like water, the interaction is restricted to only
the matching hydrophilic domain in the block copolymer while the hydrophobic domain
conveniently excludes it.
A remarkable feature that is observed in the sorption curves of the swelling ratio versus t 1/2 is that
they are slightly sigmoidal initially and later level off. The sigmoidal shape is related to the time
taken by the polymer chains to react to the swelling stress and realign themselves to
accommodate the solvent molecules. Initially, due to the large concentration gradient, the
swelling rate is too high. The end result of this behaviour is solvent induced stress in the polymer
sample. However, as the concentration gradient decreases, the swelling ratio decreases and at
equilibrium swelling the concentration differences almost vanishes [17].
In the NR/PEG hydrogels, Q∞ mol% values varied with the variation in NCO/OH ratios (Table
II). The maximum uptake was shown by the sample with NCO/OH ratio 1.1 followed by a
decrease in the uptake values with the increase in NCO/OH ratios. This sort of behaviour maybe
the outcome of increase in polymer mobility, free volume and better interactions existing in the
sample with NCO/OH ratio 1.1.
Mechanism of sorption
The sorption data of the polymer – solvent systems for a circular geometry of the sample before
the attainment of 55% equilibrium sorption have been fitted in the following empirical formula
[18,19]
Q t
=
Q
∞
kt
n
………………… (1)