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The average molecular weight between chain entanglements, Me, of a thermoplastic polymer is of great importance in determining both its melt rheology and its mechanical properties.

The melt viscosity, which determines the ease of processing via fabrication methods such as extrusion and injection molding, increases with increasing Mw/Mcr ratio, where Mw denotes the weight-average molecular weight and Mcr≈2Me is the critical molecular weight.  Increasing melt viscosity means more difficult melt processing.

The mechanical properties, and most importantly the predominant mode of failure and the energy required to cause a specimen to fail, also depend on the ratio Mw/Me.  The mechanical properties improve as Mw/Me increases, although the improvements level off and hence are generally not worth the cost of the increasing difficulty of melt processing as the ratio becomes very large.

For polymers within the “typical” polydispersity range of 2.0≤(Mw/Mn)≤2.5 (where Mn is the number-average molecular weight) often obtained by traditional condensation polymerization methods, an "optimum" Mw of 10Me to 15Me often results in the best balance of ease of processing and good mechanical properties.  Polymers in this optimum range of Mw usually have chains which are long enough to approach the "high polymer" limit for the mechanical properties, but short enough for the melt viscosity to be sufficiently low not to cause great difficulty in melt processing. 

It is important to recognize, however, that while Mw plays the key role in determining the melt viscosity, the mechanical properties depend more directly on Mn.  Consequently, methods such as anionic polymerization, which produce nearly monodisperse polymers with Mw/Mn ranging from 1.0 to 1.1, can be used to synthesize polymers of much lower optimum Mw (ranging from 5Me to 7.5Me) and thus of much lower melt viscosity while having good mechanical properties.

Polydispersity Range

Good Melt Processability,
Poor Mechanical Properties

Optimum Mw Range

Good Mechanical Properties,
Poor Melt Processability









Finally, it is important to note that, while the optimum molecular weight range considerations provided above are relevant to most thermoplastic applications, there are many important exceptions. For example, adequate processability in some applications requiring the injection molding of articles with very intricate and finely detailed features necessitates the use of a polymer of Mw below the generally recommended range to lower the melt viscosity, even if the use of such a very low Mw will produce a brittle product.  At the other extreme, the demands of some applications for mechanical properties approaching the upper limits achievable with a given type of polymer necessitate the use of a polymer of Mw above the generally recommended range, even if the use of such a very high Mw will create challenges during melt processing.

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