Polyethylenes, the most widely used class of plastics in the world, include several copolymers of ethylene in addition to the homopolymer.5 The polyethylene homopolymer has the simplest chemical structure of any polymer.
The commercially available resins, however, have far more complicated structures with branched chains and semicrystalline morphologies not indicated in this simple representation. Depending on their copolymer composition and the polymerization process used, commercial polyethylenes display a wide range of average molecular weights, molecular weight distributions (polydispersity), and chain branching in the resin. These molecular parameters affect the ability of the macromolecules to pack closely into a dense matrix and also control the extent of crystallinity in the material. Because of their semicrystalline nature, polyethylenes do not display their theoretical density of 1.00 g/cm3 (or the theoretically expected melting point of about 135°C) but show a surprisingly wide range of physical properties, Based on these, particularly the bulk density, the resins are divided into three basic types.
• Low-density polyethylenes (LDPE)
• High-density polyethylenes (HDPE)
• Linear low-density polyethylene (LLDPE)
5 The first synthesis of polyethylene was from diazomethane, but its commercial preparation in the 1930s was via the polymerization of the ethylene monomer.
Figure 2.3. Market share of polyolefins in 2000.
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