Ewa Ratajski

The physics of athermal nuclei in polymer crystallization

Abstract The lower temperature limit for sporadic “thermal” nucleation, as estimated in a previous paper (Janeschitz-Kriegl (1997) Colloid Polym Sci), can be verified by a variety of earlier experiments which are compiled in this paper. Below the said limit only “athermal” nuclei are of importance in samples containing no large numbers of hetero-nuclei. The number of athermal nuclei increases tremendously (by many decades) with decreasing temperature. Why these nuclei are dormant and “awake” only at “their” temperature is explained in terms of their growth conditions, which depend on the length of the incorporated (helical) sequences. Also the sluggishness of molecular processes, occurring in the temperature range of metastable conditions, where sporadic nucleation can occur is demonstrated by some of the recalled experiments. Except for very fast industrial processes, where flow plays a dominant role, this sluggishness makes a noticeable influence of the sporadic nucleation on structure formation very improbable.

Effects of the catalyst system on the crystallization of polypropylene

In search of a better understanding of catalyst effects on final product qualities of polypropylene, an attempt was made to establish a correlation between catalyst type, polymer chain structure of homopolymers, crystallization behavior, as well as final morphology and mechanical properties. Conventional Ziegler–Natta catalyst systems as well as novel metallocene catalysts were investigated, and influences of molar mass distribution and chain regularity were investigated as separate factors. Metallocene-based isotactic polypropylene shows some special effects regarding the nucleation density and the correlation between stereoregularity and mechanics that do not fit into the general picture for Ziegler–Natta catalyst based products.

Influence of molecular structure on crystallization behaviour and mechanical properties of polypropylene

Abstract Two series of polypropylenes with different molar mass—reactor grade (RE) and peroxide-degraded (CR) types—were investigated with respect to mechanical and crystallization parameters. A significant influence of the production process on mechanical parameters was found, which can be attributed to differences in the crystallization behaviour, mainly to the number of nuclei per unit volume. At a comparable average molar mass, RE types crystallize faster and exhibit higher levels of stiffness, combined with lower levels of impact strength, than CR types.

Flow-induced crystallization in polymer melts: on the correlation between nucleation and specific work

A new counting technique for nuclei, as found after short-term shearing, has been developed with the aid of the plate-and-plate rheometer Linkam CSS450. For the purpose this apparatus was modified by the introduction of complementary polarizing optics (a polarizer and a λ-plate). With the aid of this technique the density of nuclei could be determined in a new way. When these number densities are plotted against the applied specific work, satisfactory agreement is found with results, which have been obtained previously with a sliding glass plate rheometer. Nevertheless, the limits of validity of the diagrams had to be discussed. A transition from a dynamic description with the aid of the specific work to a purely kinematic description with the aid of the frequency of successful encounters cannot be overlooked. In fact, with increasing specific work the influences of temperature and (non-Newtonian) viscosity appear to fade away.