J. Loos, X. Zheng, E.MoE van Kimmenade, J.W. Niemantsverdriet, G.W.H. Höhne, and P.C.Thüne
Eindhoven Polymer Laboratories and Dutch Polymer Institute, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands, and Schuit Institute of Catalysis, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands Email: [email protected]
Abstract
Within the wide research theme polymer crystallization a specific phenomenon is the formation of the so-called nascent morphology, namely the formation of polymer particles initiated by supported or homogeneous catalyst systems in the reactor [1 ].
If the polymerization temperature is below the melting and/or the dissolution temperature of the polyolefin, the molecules crystallize during polymerization. Fresh monomer inserts into a growing polymer chain at the catalyst active site, creating new molecules, which have to displace the already formed polymer. This interaction between the polymerization and the crystallization process results in the development of the nascent polymer morphology.
The development of the nascent state morphology of polyolefins in the reactor is understood reasonable well on the micrometer level, the overall particle morphology, as used in important industrial processes such as catalyst particle replication. However, in the first part of our study we are aiming in better understanding the formation of polymer reactor powders.
Conversely, on the molecular scale the formation of the nascent morphology is only poorly understood and, so far, nascent polyolefins have unusual and somewhat ambiguous physical properties. It is the aim of the second part of our study to introduce first results related to the chemical and physical characterization of nascent polyethylene as polymerized on fiat model catalysts.
In detail, ehylene was polymerized on the surfaces of flat models for the Phillips CrOx/SiO2/Si(100) catalyst and a homogeneous polymer film has been formed. The nascent morphology of PE samples polymerized at temperatures between 25℃ and approximately 100℃ consists of pillar- like stacked spherical entities, which are loosely connected with each others. After melting and recrystallization of these samples as well as for polymerization temperatures above 100℃ common spherulitic superstructures have been observed. Corresponding thermal analyses of the samples have shown that the melting temperatures and crystallinities of nascent samples polymerized at low temperature are significantly higher than of the same but melt-crystallized samples. Such characteristic behavior was not observed for the samples polymerized at higher temperatures.
Utilizing the fiat nature of the model catalysts used in this study (well characterized by applying surface science techniques such as XPS and SIMS [2) we intend unravel the intrinsic parameters determining the development of nascent polymer morphology. These include catalyst structure and reactivity as well as the polymerization conditions applied (temperature, monomer pressure, etc.). This is the Subject of our ongoing study.
