Question

The PDI is the ratio of weight to number-average molecular masses, $M\;\bar w/M\;\bar n$. In natural polymer, which are generally monodispersed, PDI is ____ and in synthetic polymer which are always polydisperse, PDI is ____ because $M\;\bar w$ is always ____ than $M\;\bar n$. Fill in the blanks
A.Greater than 1, 1, higher
B.1, greater than 1, higher
C.Less than 1, 1, lower
D.1, less than 1, lower

Answer 1

We can define PDI, polydispersity index as the measure of distribution of molecular mass in a given polymer sample or we can say it is the measure of heterogeneity of a sample based on size.

Complete Step by step answer:
- So, we have seen that the polydispersity index, PDI has many definitions. Apart from this we can also define PDI as a measure of broadness of molecular distribution. Therefore, we can if the polymer is having larger PDI, then their molecular weight will be broader and vice-versa.
- So, in question itself the mathematical formula is given that PDI can be calculated by ratio of weight average by number average molecular weight.
- Monodispersed polymers are defined as uniform polymers (or all chains of same length) in which all the molecules have the same degree of polymerization or relative molecular mass. It has a polydispersity index, PDI as 1. Some of the examples of monodispersed polymers are bipolymers especially proteins. i.e., first blank would be 1.
- Now coming to polydisperse polymer, it has non-uniform and contains a polymer chain of unequal length therefore we can say that molecular weight will not be single value. It exists as a distribution of chain lengths and molecular weights. The PDI for polydisperse polymers will be in the range of 1.02 to 1.10. Some examples are many man-made polymers. i.e, second blank will be greater than 1.
- Also, we know that the average molecular weight is always greater than the average molecular weight. We can also confirm this since PDI is always greater than or equal to 1. i.e., the third blank will be higher.

Since the blanks are 1, greater than 1, higher option (B) is correct.

Note: Most of the time monodisperse polymers are referred as natural polymers and polydisperse polymers are referred as man-made or synthetic polymers. Hence, we can say that the PDI of a natural polymer is always 1 and the PDI of a synthetic polymer is greater than 1.