Crystallinity and Melting Behaviour

In HDPE, chains fold to form the lamellae and propagate outwards three dimensionally, creating a sphere-like formation. This sphere-like formation is referred to as a spherulite. The spherulites are very small anisotropic 

The rate of crystal growth is dependent on two factors temperature and molecular weight. Temperature affects the growth and formation of crystals due to the Gibbs free energy. Gibbs free energy can be defined as 

Where T is the thermodynamic temperature. The system is in equilibrium when G is a minimum. 

A polymer melt consists of randomly coiled and entangled chains. This gives much higher entropy than if the molecules are the form of extended chains since there are many more conformations available to a coil than for a fully extended chain. The higher value of S leads to lower value of G. Now if the melt is cooled to a temperature below the melting point of polymer, Tm, crystallisation may occur. There is a high degree of order in 

For a given polymer, the extent of crystallization attained during melt processing depends upon the rate of crystallization and the time during which melt temperatures are maintained. Below Tm (melting temperature), some polymers have high rates of crystallization, such as polyethylene, so it cannot be quenched quickly enough to prevent crystallization [6]. 

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The greatest impact of the techniques has been seen in the study of polymeric materials with crystallinity and melting behaviour, glass transitions, curing processes and polymerisation representing the different types of thermal behaviour under investigation. A measure of the importance of this area of activity is reflected in the considerable number of publications and conference presentations it has generated. 

The second chapter presents a structure of ultra high molecular weight polyethylene (UHMWPE) for orthopaedic devices structure/property relationships. This chapter explains the many subtopics such as HOPE and UHMWPE, chemical structure, crystallinity and melting behaviour, molecular weight, mechanical properties, and. sterilization by gamma rays. 

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