ON THE MEMORY EFFECT IN UHMWPE NASCENT POWDERS^*

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 10 6 g/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 TiCl 4 catalyst with EtAl as the cocatalyst had a globular structure, while those synthesized on supported TiCl 4 /Al 2 O 3 ؒSiO 2 and TiCl 3 ؒ0.3 AlCl 3 /MgCl 2 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 de-*Dedicated to Prof. Francisco J. Baltá Calleja on the occasion of his 65th 814 IVAN'KOVA ET AL.fects 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.

The straight‐chain segment length distribution in xerogels of UHMWPE, which provide high drawability of gel‐derived fibers

“…The most important factors that determine the polymer drawability are the MW 8,9 and the morphology of powder particles. [9][10][11][12][13] This work confirms the significance of the polymer MW and demonstrates that the nature of crystalline fixers in the gel network affects the efficiency of gelto-fiber transformation to a noticeable extent. In future, we hope to bind the presented here results with the morphological patterns of used reactor powders, thus covering different aspects of the problem (molecular, morphological, and structural) in a common series of experiments.…”

Investigation of Catalyst Fragmentation in Gas‐Phase Olefin Polymerisation: A Novel Short Stop Reactor

“…[1] Nascent polymerisation has been extensively studied since the 1970s. [2,3] Some works have focused upon the effects of interactions between the crystallisation and formation of the polymer chains [4][5][6][7] and the influence of monomer transport on the nascent polymer morphology. [8] Recently, morphology development has been studied through experiments carried out at very short Summary: A short stop reactor (SSR) was developed to study nascent particle morphology and reaction kinetics in the gas-phase polymerisation of olefins on supported catalysts.…”

Evaluation of the Initial Stages of Gas‐Phase Ethylene Polymerizations with a SiO2‐Supported Ziegler–Natta Catalyst

“…[4,5] The nascent phase of these polymerizations has been widely studied, but more from the point of view of exploring the relation between the crystallization, formation of the polymer chains and the impact of monomer transport upon the polymer morphology. [6][7][8][9][10][11][12][13] In spite of the importance of studies regarding the development of polymer particle morphology, the real impact of the reaction conditions on the morphogenesis of the polymer particles has not been completely explained. The morphology of the final polymer particle depends on the mechanical and structural properties of both the catalyst support and the polymer formed in the critical very early stage of the polymerization.…”