The control of the properties of polyethylene through its polymerisation

Abstract

In order to understand correlations between structural models of polymers and observed properties, various unusual polymerisations of ethylene were carried out. A detailed description of the polymerisation equipment and procedure is given. The effect of the major polymerisation parameters is reported and a selection of our own polymers is described.

Molecular characterisation (light scattering, G.P.C. and infrared) examination were made on our polymers as well as commercial ones. Melt viscosity was studied using a cone and plate rheometer. Mechanical property studies were made by impact and tensile experiments. Crystallisation studies were made by density determination of the degree of crystallinity and lamellar thickness by Raman Spectroscopy.

Molecular characterisation revealed that some samples prepared here have relatively high molecular weight values (about 8 to 10 times those of commercial grades used as comparisons). The melt and mechanical properties are very different from the comparisons. Some of the differences can be explained through molecular weight grounds alone but it is not sufficient. It is known that an increase in molecular weight causes mechanical properties to change because networks occur. This is due to molecular entanglements which Inevitably occur in the longest chains. It is considered that the Improved properties cannot fully be explained on this basis alone. It is suggested that the explanation comes from the original polymerisation reaction. In order to make the polymers^ polymerisation reactions were carried out such a way that the growing chains could easily entangle with each other. It is suggested that this idea should be further studied.

Some evidence was also found that some of the entanglements are destroyed during the working up of the polymers after they have been made.

In the conclusion of the thesis, it is suggested that the project has to be enlarged to maximise the presence of entanglements which might lead to the creation of new polymeric grades which could even become gels in the molten state.

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