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

The correct option is B $\frac{\overline{M_w}}{\overline{M_n}}$

The ratio of weight average molecular weight and the number average molecular weight is called polydispersity index (PDI).
$PDI=\frac{\overline{M_w}}{{\overline{M}}_n}$
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.