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 r eliability of the dynamic mechanical spectroscopy as a complementary t ool to follow the crystallization of polymers from the melt. However t he problem of the interrelation between the transformed fraction and t he mechanical data is not simple and remains a topic open to discussio n. To get a better understanding of these relations, the method was ap plied to the study of two polyolefins which show very different morpho logies 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 cryst alline entities, one can be considered as a suspension of spherical pa rticles in a liquid matrix whereas the other behaves as a colloid of s mall particles. The study of the rheological behavior of the suspensio n-like material shows the existence of two critical values of the volu me fraction. In agreement with the percolation theory, the first value is related to the appearance of a yield effect and the second indicat es the maximum packing. Moreover, in this case, throughout the crystal lization, the relaxation times depend on the filler content and the ze ro-shear viscosity varies upon the -3/2 exponent of the volume fractio n. The colloid-like material behaves in a completely different way sin ce a yield effect appears in the earliest stage of the crystallization . For both materials, the use of an equilibrium modulus is able to cha racterize the yield effect, and in both cases, it is described by the same type of expression with a universal exponent equal to 3 in agreem ent 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 re quires an additional investigation of the morphology.