Effect of the Diblock Content on Adhesive and Deformation Properties

Effect of the Diblock Content on Adhesive and Deformation Properties 

Generally PSAs are well known for their very viscoelastic behavior, which is necessary for them to function properly. It was therefore important to characterize first the effect of the presence of diblocks on the linear viscoelastic behavior. Since a comprehensive study on the effect of the triblock/diblock ratio on the linear viscoelastic properties of block copolymer blends has recently been reported [46], we characterized the linear viscoelastic properties of our PSA only at room temperature and down to frequencies of about 0.01 Hz. Within this frequency range all adhesives have a very similar behavior in terms of elasticity, as can be seen in Fig. 22.10. The differences appear at low frequency, a regime where the free iso-prene end of the diblock chain is able to relax. This relaxation process is analogous to the relaxation of an arm of a star-like polymer [47], and causes G to drop to a lower plateau modulus, the level of which is only controlled by the density of triblock chains actually bridging two styrene domains [46]. 

If we now turn to Fig. 22.11a and b, which show probe test curves for the four adhesive blends at two different probe debonding velocities, the differences between the four adhesive blends are striking and although measured stresses are systematically lower for the low probe velocity, the plateau stress is clearly lowered by the presence of diblock while the maximum extension is increased. The question is, then, whether these differences can be predicted by the Hnear viscoelastic properties. 

When trying to relate the probe test curves, performed at a certain probe velocity, with the linear viscoelastic properties of the adhesives, measured at a certain pulsation, the question of the equivalence between probe velocity, Vdeb and frequency always occurs. In fact a probe test imposes a highly inhomogeneous deformation on an adhesive layer and no equivalence can be rigorously made between a non-steady state experiment, which deforms the material in a 

In our experiments, the probe test curves show that, regardless of the adhesive and of the applied Vdeb. the initial portion of the plateau region always falls at 2, i.e., or equivalently at h/bo = 3. This point on the curve corresponds to the situation where all cavities have expanded in the plane and the walls start to be extended in the tensile direction (see Fig. 22.8). Given that the initial adhesive layer thickness is 100 pm, the nearly uniaxial strain rate applied to the cavity waU at =2 is 0.005 s and 0.5 s for Vdeb = 1 h tests and Vdeb =100 1 tests, respectively. If one assumes that a strain rate of 0.5 corresponds to an equivalent frequency cojln, significant differences due to the presence of diblock are expected at low strain rates, but not at high strain rates where almost all four blends have 

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