Crystallization of Polyolefins from Rheological MeasurementsRelation between the Transformed Fraction and the Dynamic Moduli

Abstract

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.

Using rheometry to determine nucleation density in a colored system containing a nucleating agent

“…One could think of using other approaches by applying more simple models (Boutahar et al 1996(Boutahar et al , 1998Tanner 2002Tanner , 2003van Ruth et al 2006) or empirically based scaling laws between the space filling and the storage modulus (see Coppola et al 2006;Khanna 1993;Pogodina et al 1999). However, it was demonstrated in Steenbakkers and Peters (2008) that such approaches do perform less than the suspension model as used here, and therefore, we will not apply them to our results.…”

Suspension-like hardening behavior of HDPE and time-hardening superposition

“…Perhaps a network of physical crosslinks is formed (Boutahar et al 1996(Boutahar et al , 1998Coppola et al 2006;Peters et al 2002;Pogodina et al 1999a;Zuidema et al 2001). Subsequently, spherulites grow in this altered melt, the effect of which is well described by suspension-like behavior.…”

“…A vast amount of experiments investigating the change of rheological properties can be found in the literature, cf. Acierno and Grizzuti (2003), Boutahar et al (1998), Han and Wang (1997), Lamberti et al (2007), Pantani et al (2001), Pogodina and Winter (1998), and Titomanlio et al (1997). To quantify the hardening behavior of the material, a normalized rheological function (NRF) is commonly used.…”

“…A slight difference in behavior in the two experiments might result in a large error in the hardening function. For instance, Steenbakkers and Peters (2008) found the experiments from Boutahar et al (1996Boutahar et al ( , 1998 showed suspension-like behavior but in order to get quantitative agreement between experiments and simulations, the space filling obtained from DSC measurements had to be adjusted significantly.…”

“…However, many of these models are not applicable to crystallizing polymer melts. Empirical models can also be found in abundance in literature (Boutahar et al 1998;Doufas et al 2000;Han and Wang 1997;Hieber 2002;Katayama and Yoon 1985;Khanna 1993;Shimizu et al 1985;Tanner 2003;Titomanlio et al 1997;Yarin 1992;Ziabicki 1988;Zuidema et al 2001), but there is little agreement between these models. Thus, there is need for a model that is able to accurately describe rheological properties of semi-crystalline polymers during solidification.…”