Question

If $\overline{ M _{ W }}$ is the weight average molecular weight and $\overline{ M _{ n }}$ is the number average molecular weight of a polymer, the poly dispersity index (PDI) of the polymer is given by:

• A. $\frac{\overline{ M _{ n }}}{\overline{ M _{ w }}}$
• B. $\frac{\overline{ M _{ w }}}{\overline{ M _{ n }}}$
• C. $\overline{ M _{w}} \times \overline{ M _{n}}$
• D. $\frac{1}{\overline{ M _{ w }} \times \overline{ M _{ n }}}$

Answer 1

Answer: (B)

The ratio of weight average molecular weight and the number average molecular weight is called polydispersity index (PDI).
$PDI =\frac{\bar{M}_{w}}{\bar{M}_{n}}$
where,
$\overline{ M }_{ w }=$ weight average molecular weight
$\overline{ M }_{ n }=$ number average molecular weight
PDI is unity for natural monodiapersed polymer but for synthetic polymers it is always greater than unity.