Question

The P (poly) dispersity index (PDI) of a polymer is: ($\overline{{{M}_{w}}}$ = weight average molecular mass and$\overline{{{M}_{n}}}$= number average molecular mass).
A.The product of$\overline{{{M}_{w}}}$and$\overline{{{M}_{n}}}$
B. the sum of$\overline{{{M}_{w}}}$and$\overline{{{M}_{n}}}$
C. The difference between$\overline{{{M}_{w}}}$and$\overline{{{M}_{n}}}$
D. The ratio between$\overline{{{M}_{w}}}$and$\overline{{{M}_{n}}}$

Answer 1

A polymer is a compound that has a long chain of organic molecules called monomers. Polydispersity index in polymers is able to identify the distribution of the masses of the monomers throughout the long chain molecule. The numbers of monomers are represented as ‘n’.

Complete answer:
A polymer is a long chain that is made up of various small unit compounds called monomers. Each monomer contributes its molecular weight in the average weight of the polymer.
Polydispersity index of a polymer tells us about the distribution of these monomers. It tells the distribution or dispersion of the molecular mass of the various or ‘n’ number of monomers that make up a polymer.
So, polydispersity index (PDI) will be the measure of the average molecular weight of the polymer divided by the number of average molecular weights. As given, $\overline{{{M}_{w}}}$ = weight average molecular mass and$\overline{{{M}_{n}}}$= number average molecular mass. Therefore their ratio will give the polydispersity index as:
PDI =$\dfrac{\overline{{{M}_{w}}}}{\overline{{{M}_{n}}}}$

Hence, option D is correct.

Note:
The polydispersity index value tells us that the polymer is uniform and is fit to be synthesized. The PDI value of greater than 1 is considered as a parameter. It should be always greater than 1 if the polymer is to be synthesized. The natural polymer cellulose has a PDI of 1.