ON THE MEMORY EFFECT IN UHMWPE NASCENT POWDERS1*

Abstract

Morphology and molecular structure of three nascent ultra high molecular weight polyethylene (UHMWPE) powders synthesized on unsupported and supported Ziegler-Natta heterogeneous catalysts were studied with the help of electron microscopy and infrared (IR) spectroscopy. A synthesis was carried out in a slurry process in n-heptane at a temperature of about 70°C, which resulted in production of UHMWPE of one and the same molecular weight, approaching 106g/mol. The morphology of the nascent particles dramatically differed, however, which is evidence of the important role of the catalytic system used. Nascent particles synthesized on unsupported TiCl4 catalyst with EtAl as the cocatalyst had a globular structure, while those synthesized on supported TiCl4/Al2O3·SiO2 and TiCl3·0.3 AlCl3/MgCl2 Ziegler-Natta catalysts with the same cocatalyst demonstrated fibrillar (cobweb) and spiral (wormlike) structures, respectively. As revealed by IR spectroscopy, there was a significant difference in the nature of the crystalline regions formed during the synthesis/crystallization process; in the amount of short trans sequences not included in crystallites (taut tie molecules); in the content of irregular conformers of various types, including typical point defects 2G1 kinks; and so on. The melt-crystallized films produced from the investigated reactor powders were markedly distinguished by their drawability and by the existence and the amount of different molecular conformations despite a sufficiently high recrystallization temperature and prolonged preheating. The difference in molecular structure of the drawn films was retained in a whole range of draw ratios. It is concluded that there is a well-pronounced “memory effect” and the morphology of the original nascent particles depending on the catalyst/synthesis conditions clearly play a role in the properties of the end product. *Dedicated to Prof. Francisco J. Baltá Calleja on the occasion of his 65th birthday.