Third Floor- Molecular Weight

Many physical properties of a polymer depend on the molecular weight.
There are several different definitions of molecular weight for a polymer.
A desired property may improve with increasing molecular weight (the law of diminishing returns comes into play), but the processability of the polymer decreases with molecular weight, so a compromise molecular weight must be used for the product.
Molecular Weight can be determined by the following techniques:
Degree of Polymerization- If a chain of polyethylene was made by 50 ethene monomers polymerizing, this chain is said to have a Degree of Polymerization of 50. [Hiementz-3]
Polydispersity- The weight average molecular weight divided by the number average molecular weight .
```
                  M(w)
polydispersity = ------
                  M(n)
```
Because weight average molecular weight is always equal to or greater than number average molecular weight, the polydispersity is eiether equal to or greater than unity. A close examination of the
formula for weight average molecular weight and the formula for number average molecular weight shows that if the polydispersity is unity, then all the polymer chains must share the same degree of polymerization.

Last Update- July 4, 1995- wld