The polydispersity index $(PDI)$ of a polymer is ($\bar{M}_w = \text{weight average molecular mass}$ and $\bar{M}_n = \text{number average molecular mass}$)

• A
  the product of $\bar{M}_n$ and $\bar{M}_w$
• B
  the sum of $\bar{M}_n$ and $\bar{M}_w$
• C
  the difference between $\bar{M}_w$ and $\bar{M}_n$
• D
  the ratio between $\bar{M}_w$ and $\bar{M}_n$