Crystallization of polyolefins from rheological measurements - Relation between the transformed fraction and the dynamic moduli

Experimental results from the literature and from this work show the reliability of the dynamic mechanical spectroscopy as a complementary tool to follow the crystallization of polymers from the melt. However the problem of the interrelation between the transformed fraction and the mechanical data is not simple and remains a topic open to discussion. To get a better understanding of these relations, the method was applied to the study of two polyolefins which show very different morphologies during their crystallization from the melt. Their morphological study has Shown that, though they both crystallize in a well-defined spherulitic structure, because of the differences of size of the crystalline entities, one can be considered as a suspension of spherical particles in a liquid matrix whereas the other behaves as a colloid of small particles. The study of the rheological behavior of the suspension-like material shows the existence of two critical values of the volume fraction. In agreement with the percolation theory, the first value is related to the appearance of a yield effect and the second indicates the maximum packing. Moreover, in this case, throughout the crystallization, the relaxation times depend on the filler content and the zero-shear viscosity varies upon the -3/2 exponent of the volume fraction. The colloid-like material behaves in a completely different way since a yield effect appears in the earliest stage of the crystallization. For both materials, the use of an equilibrium modulus is able to characterize the yield effect, and in both cases, it is described by the same type of expression with a universal exponent equal to 3 in agreement with theories for physical gels. Unfortunately, these results show that a unique expression can hardly be used to relate the transformed fraction to the rheological data and that such a derivation always requires an additional investigation of the morphology.