Structure of Polyethylene
Polymeric materials are made of macromolecules and appear in a variety of products. These macromolecules are long chains of thousands of repeating units, which give polymers many of their useful properties. In polyethylene, molecules are basically long chains of carbon and hydrogen atoms. The smallest unit (monomer) is ethylene (C2H4), whereas a typical polyethylene molecule contains from approximately 900 to 215,000 of these units. It has been shown that polyethylene possesses a structure with spherulitic morphology, as illustrated in Figure 1. Within this structure, molecules are capable of adopting two distinct arrangements, the amorphous and crystalline phases. Amorphous polyethylene is characterized by those segments where molecules have, on average, no favoured directions. On the order hand, crystalline polyethylene corresponds to segments of highly ordered molecules embedded within the amorphous phase. Polyethylene crystals form * Corresponding author: M. A. Polak, E-mail: polak@uwaterloo. ca
M. Pandey et al. (eds), Advances in Engineering Structures, Mechanics & Construction, 449-458. © 2006 Springer. Printed in the Netherlands.
‘ ” ‘ c = 2.55 A
by folding the molecules alternately up and down and by arranging the straight segments between folds into a periodic array. The thickness of polyethylene crystals ranges from 50 to 250 A (A=10-10 m), and the lateral dimensions range from 1 to 50 x Ю^т Only about 40 monomeric units are included in each of the straight segments between folds (Lin & Argon, 1994). The structure of a crystal is of the orthorhombic crystal class, as illustrated in Figure 1. The parameters a, b, and c are the lattice parameters, where the c-axis is parallel to the molecules.
Figure 1. Schematic illustration of a polyethylene spherulitic structure and a crystal.
According to the American Society for Testing and Materials, polyethylene can be classified into four groups. Low-density polyethylene (LDPE) has a density ranging from 0.91 to 0.93 g/cm3 and is predominantly amorphous. Medium-density polyethylene (MDPE) typically has average densities from 0.93 to 0.94 g/cm3. High-density polyethylene (HDPE) has a density above 0.94 g/cm3 with only a few branched chains. As a result, these molecules align into more compact arrangements with up to 90% crystalline regions. Linear low-density polyethylene (LLDPE) has longer and more branches than HDPE. Consequently, it has a higher density than LDPE but still a linear structure like HDPE.