The structure-property relationships of polyolefins
Date
2009-03
Authors
Harding, Gareth
Journal Title
Journal ISSN
Volume Title
Publisher
Stellenbosch : University of Stellenbosch
Abstract
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.
Description
Thesis (PhD (Chemistry and Polymer Science))--University of Stellenbosch, 2009
Keywords
Fractionation, Dissertations -- Polymer science, Theses -- Polymer science, Polymerization, Polypropylene, Copolymers