MWD—See Molecular weight distribution

Detailed modifications in the polymerisation procedure have led to continuing developments in the materials available. For example in the 1990s greater understanding of the crystalline nature of isotactic polymers gave rise to developments of enhanced flexural modulus (up to 2300 MPa). Greater control of molecular weight distribution has led to broad MWD polymers produced by use of twin-reactors, and very narrow MWD polymers by use of metallocenes (see below). There is current interest in the production of polymers with a bimodal MWD (for explanations see the Appendix to Chapter 4). 

Mechanical Properties. Mechanical characteristics of ethylene copolymers are functions of their structural characteristics, such as content and type of a-olefin, branching uniformity, molecular weight and width of molecular weight distribution (MWD), and orientation (see Table 5 for properties of films made from three grades of LLDPE). 

The molecular weight distribution (MWD), also discussed earlier, is an indication of the relative proportions of molecules of different weights/ lengths. It shows the breadth of distribution, or the ratio of large, medium, and small molecular chains in the resin. If most of the molecules have about the same MW, the MWD is classified as narrow. A wide or broad MWD implies a large variation in MW. Figure 1-5 compares wide and narrow MWDs. The MWD is independent of both density and melt index (MI) (see Chapter 10), and must be taken into account in considerations of both processing and product performance. A narrow MWD enables much better. 

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