Nondestructive testing of defects in adhesive joints

⎛ hθ
D = π⎜
⎜
⎝ 4Q
∞
⎞
⎟
⎠
2
………………….(5)
where h is the thickness of the sample. Due to the swelling in a short period of time, swelling
correction is necessary to get correct diffusion coefficient known as the intrinsic diffusion
coefficient (D * ) [22]
D* =
D
7 / 3
φ
……………………(6)
where φ is the volume fraction of the polymer. The variation in D * value depends on the nature of
the crosslink, penetrant size and polarity. The values of D * along with sorption and permeation
coefficients are tabulated in Table V.
The D * values varied with NCO/OH ratios, with the maximum value obtained in sample with
NCO/OH ratio 1.1. With further increase in NCO/OH ratios, D * values showed a steady decrease.
This observation can be correlated to the equilibrium swelling uptake that was found to be
maximum in sample with NCO/OH ratio 1.1. To sum up, D * depends more on the molecular
interaction between the penetrant and the polymer system. A better understanding of the
interactions and their strengths can be obtained from the sorption coefficient S, which can be
calculated as follows
S =
M
M
S …………………(7)
P
Where Ms is the mass of the solvent molecules at equilibrium swelling and Mp is the initial mass
of the polymer sample [20]. The S value is high in sample showing high equilibrium swelling
uptake values, i.e. the sample with NCO/OH ratio 1.1. The high S value in this system shows the
better accommodation of the water molecules due to the favourable interactions with the polar
region of the block copolymer. The permeability coefficient P depends on doth D and S, since
permeation is a combined effect of diffusion and sorption [20].
P = D.S ……………………(8)
The permeation values are yet again high in sample with NCO/OH ratio 1.1. The P values reflect
the net effect of diffusion and sorption. In the block copolymer studied, S values are higher
compared to D * and P, showing a larger tendency for the penetrant molecules to sorb rather than
diffuse into the polymer. Thus sorption predominates over diffusion in the systems under study.
Effect of NCO/OH ratio and pH on overshooting effect
In Figure 1 and Figure 3, the dynamic swelling curves corresponding to copolymers swollen in
water and at pH 9 respectively exhibit overshoot. Initially, the samples increase their swelling
ratio, later on they reach a maximum and finally they deswell until an equilibrium value is
reached. The sample with NCO/OH ratio 1.3, 1.4 and 1.5 exhibit overshoot in water while all the
NR/PEG samples exhibit overshooting effect at pH 4, pH 7 and pH 9. We have attributed this
feature to the anomalous transport with a bias to the non – Fickian mode (Table III). The primary
factor contributing to overshooting effect is the difference existing between the relaxation and
diffusion rates at experimental conditions. The stress generated due to the slow relaxation rate