The structure-property relationships of polyolefins

Harding, Gareth (2009-03)

Thesis (PhD (Chemistry and Polymer Science))--University of Stellenbosch, 2009


Polypropylene is an extremely versatile material and has a broad spectrum of applications due to the variations in properties which are possible with this material. The variations in the properties of the material are governed by the microstructure of the chains constituting the polymer. The microstructure varies according to the production methods, i.e. the polymerisation conditions. Varying the manner in which the polymer is produced therefore changes the properties of the material allowing the polymers’ use for different applications. The most important factor affecting the way in which the polymers are produced is the nature of the active sites on the catalyst. Changing the chemical environment of the active sites changes the way in which the polymerisation is controlled and greatly affects the types of chains produced and thus polymer properties. The study examines the structure-property relationships of polyolefins with specific focus on the polypropylene homopolymer. The temperature rising elution fractionation (TREF) technique is used extensively in order to isolate specific fractions of the polymer. The importance of specific TREF fractions is investigated via a two pronged investigative methodology. On the one hand specific TREF fractions are removed from a sample, allowing the analysis of the properties of the material without that specific fraction, thereby revealing the influence which the fraction in question has on the properties. The other branch of the study investigates the chemical modification of the active sites of a Ziegler-Natta catalyst so as to be able to modify the properties of the polymer in the reactor, in a similar manner to physically removing fractions. The techniques are related and it was discovered that the amount of the fractions of the polymer, found to be important using the one technique, also turned out to be important using the other method. Initial method development work utilised a polypropylene-1-pentene copolymer since the molecular heterogeneity of this material is such that large differences are observed upon removal of fractions. The technique was then applied to a Ziegler-Natta catalysed polypropylene homopolymer. Each TREF fraction is successively removed and the residual material analysed. Specific TREF fractions were found to play a significant role in determining the polymer properties since there was a drastic reduction in properties upon removal of these fractions. The polymerisation of propylene was also performed at a variety of conditions in order to investigate different ways in which the catalyst system could be modified. Specific reaction conditions were chosen for in-depth analysis and structure-property correlation. The chemical modification of the active sites was accomplished via the introduction of an external Lewis base (electron donor) to the polymerisation system, and also by varying the external donor/catalyst ratio used. Two different external donors were used during the study namely Diphenyl-dimethoxysilane (DPDMS) and methyl-phenyl-dimethoxysilane (MPDMS). It is observed that there are definite links between the amounts of specific fractions present in the polymer and the polymer properties, as observed via both the physical removal of fractions and the chemical modification of active sites.

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