TABLE OF CONTENTS PE is a crystallisable polymer. Since chains may be entangled or otherwise imperfect (branching, comonomers, structural defects, etc.), the structure is not completely regular, and hence PE is best described as a semi-crystalline polymer. Many of the properties of PE and most of its properties below the melting point depend largely on its crystalline content, which in turn depends essentially on the number of branches rather than their length. Crystallinity in PE is primarily a function of the number of branches along the chain. As more branches are introduced, disruption becomes greater and crystallinity decreases rapidly. At an extremely high degree of branching, PE would become an amorphous material. Long-chain branching can be ignored with respect to the properties below the melting point. LDPE has about two branches per hundred carbon atoms and a crystallinity of about 50%. Polymerisation conditions can change the degree of branching, however, and the crystallinity may be varied from about 35 to 75%. In HDPE there can be from about 0.5 to practically zero branches per hundred carbon atoms, so that the crystallinity may vary from 60 to 90%.
It is usually more convenient to refer to the density rather than to the crystallinity. These quantities are connected by the linear relationship:
where c is the weight percent crystallinity, d is the measured density, d a is the amorphous density (0.85 g/cm 3), and d c is the crystalline density at the given temperature. Density...
