The Scientific and Technological Route to the Manufacture of High-Modulus Polyethylene
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SCIENCE
The Scientific and Technological Route to the Manufacture of High-Modulus Polyethylene Malcolm Mackley
Polyethylene Polyethylene has been manufactured commercially for over sixty years. However recently a high-modulus variant has become available. This article describes the background to the development of high-modulus polyethylene and describes the story behind the sequence of scientific discoveries and technological developments that occurred to enable this advance. Polyethylene was discovered by accident in the 1930s when two scientists working for the ICI chemical company carried out a series of experiments to study the effect of pressure on the reaction kinetics of certain organic liquids and gases.1 During these experiments, the scientists noted a waxy deposit in their small reaction vessel which marked the birth of one of the world's largest commodity polymers. The crystallographic structure of polyethylene was determined by another ICI industrial scientist2 who at the time was pioneering x-ray techniques. The crystallography of polyethylene, shown in Figure 1, is relatively simple in that it shows the long CH2-CH2 polymer chains packed in the all "trans" configuration to form an orthorhombic cell. Polyethylene rapidly developed as a commodity thermoplastic finding application in injection molding, extrusion, and packaging. Its prime material characteristics are its toughness, flexibility, and ease of processing. The stiffness of the polymer for most applications was found to be of the order of 1 GPa, which was similar to other thermoplastics but
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MRS BULLETIN/SEPTEMBER 1997
c=2.55A
b=4.93A
a=7.40A Figure 1. Schematic diagram of polyethylene unit cell of Bunn's original drawings.
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less than metals such as aluminum at 60 GPa and steel at 210 GPa. Scientists took nearly 40 years after the discovery of polyethylene to realize that the polymer could exhibit a significantly higher mechanical stiffness than normally exists. The background to this discovery was the progressive understanding of the molecular architecture and morphology of polyethylene. The concept of a "fringed micelle" model for polyethylene was initially proposed to explain the semicrystalline nature of most polyethylene samples. Subsequently the concept of spherulite morphology was included and the pioneering work of Andrew Keller at Bristol University in relation to the concept of chain folding from solution-grown polyethylene crystals was incorporated into chain folded lamella crystal growth, occurring within spherulite crystals.3 In 1969 Sir Charles Frank, working in the Physics Department of the University of Bristol, predicted the full mechanical potential of polyethylene. Frank, shown in Figure 2, was sent a preprint of a paper by Albert Pennings, a professor from Groningen University in Holland.4 In this paper,
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