STRUCTURE AND PROPERTIES OF POLYETHYLENE • A long chain aliphatic hydrocarbon. • The flexibility of the C-C bonds lead to low values for the glass transition temperature. • There is no solvent at room temperature. • Excellent high-frequency insulator because of its non-polar nature.
Different grades of PE are due to • Variation in the degree of short chain branching in the polymer. • Variation in the degree of long chain branching. • Variation in the average molecular weight. • Variation in the molecular weight distribution (which may in part depend on the long chain branching). • Presence of a small amount of comonomer residues. • The presence of impurities or polymerisation residues, some of which may the long chain branching be combined with the polymer.
Effect of Branching ‘back-biting’ mechanism during polymerisation
The presence of these branch points is bound to interfere with the ease of crystallisation. • The branched high-pressure polymers have the lowest density. • the least opacity (since the growth of large crystalline structures is impeded) • A lower melting point, yield point, surface hardness Young’s modulus in tension. • The more the branching and the lower the crystallinity, the greater will be the permeability to gases and vapours.
Effect of long chain branches is on flow properties. • Unbranched polymers have higher melt viscosities than long-branched polymers of similar weight average molecular weight.