Molecular weight of polymers and polydispersity Index

Molecular weight of polymers

The molecular weight of simple molecules are fixed.

For example molecular weight of ethylene is 28 gm.

The molecular weight of polymers are not fixed.

For example in Polyethylene

Since the molecular weight of polymers are not fixed they are expressed in terms of average molecular weight.

The average molecular weight is expressed as

1) Number average molecular weight.

2) Weight average molecular weight.

3) Z- average molecular weight.

4) Viscosity average molecular weight.

1) Number average molecular weight.

Let n₁ be the number of molecules having molecular weight M₁, n₂ be the number of molecules having molecular weight M₂, and so on.

The weight of fraction n₁ i.e W₁= n₁M₁

The weight of fraction n₂  i.e W₂ = n₂ M₂

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Therefore number average molecular weight = Weight of all molecules/ Number of molecules

       = n₁M₁ + n₂ M₂+........./ n₁+n₂+..........

2) Weight average molecular weight.

The weight average molecular weight is given as

3) Z- average molecular weight.

The z average molecular weight is given as

4) Viscosity average molecular weight.

The Viscosity average molecular weight is given as

Where a is Mark - Hawking constant.

Polydispersity Index (PDI).   

It is the ratio of weight average molecular weight to the number average molecular weight. Next

Polydispersity Index.