Another possibility for deviation from eq. (5)
could be the change of the mechanical properties of
the rubber phase as a consequence of the addition of
the resin. This explanation is most probable than the
morphology inversion mentioned earlier and is completely
consistent with swelling characteristic and
dynamic mechanical behavior of the blends. As mentioned,
addition of the resin to the rubber phase
increases the crosslinking density of the rubber vulcanizates,
resulting in higher modulus of the rubber
phase than that of pure rubber vulcanizates. On the
other hand, presence of PH in the rubber phase due
to its partial solubility, as examined by dynamic mechanical
tests, increases the rigidity of the molecular
chains, leading to higher stiffness of the rubber
phase in the blends. These evidences indicate that
the modulus of the rubber phase in the blend is different
than that of the pure rubber vulcanizates used
in the Halpin-Tsai equation. Consequently, the misuse
of the modulus of the rubber in Halpin-Tsai
equation could be responsible for the severe deviation
from the model prediction.
From Figure 10, it is shown that effect of PH on
enhancement of modulus of SBR/PH blend at high
volume fraction of PH is much more pronounced
than that of NBR/PH blend. This can be attributed
to slightly higher solubility of the PH in SBR, as
illustrated in dynamic mechanical tests, resulting in
higher enhancement of modulus of SBR. Although
the difference between solubility of PH in SBR and
NBR is too small, but even very small amount of PH
can have observable influence on the modulus of the
rubber phase due to very high modulus of PH and
its curative effect.