Question

Your company processes several polymer resins that are purchased from an outside supplier. The melt viscosities of the resins are changing from batch to batch, presumably from differences in the molecular weight and PDI. Your boss asks you to set up a method for determining the molecular weights and molecular weight distributions for all incoming batches so that processing equipment temperatures can be appropriately adjusted. The method needs to provide actual molecular weight values and should require a relatively little amount of labor. What would you recommend to her?

Answer 1

For a polymer, there are three types of molecular weights based on the number of polymers of the same type and based on the number of monomer units attached. These three molecular weights are:

1. Number average molecular weight (Mn): It is defined as the average molecular weight of all the polymer chains in the sample.

2. Weight average molecular weight (Mw): This molecular weight depends on the weight of the polymer chain and not the number of monomer units making the polymer. Heavier the polymer, more will be the Mw. Light scattering techniques are used to differentiate the polymers based on this weight.

3. Higher average molecular weight (Mz, Mz+1):This molecular weight is more sensitive to the polymers of high weight and also precisely measuring this molecular weight. Techniques such as sedimentation and diffusion are used to determine this type of molecular weight.

Based on the polydispersity index, the increasing order of molecular weights can be written as:

Mn < Mw < Mz < Mz+1

Polydispersity index (PDI) is defined as below:

PDI = Mw/Mn

Larger the PDI, broader the molecular weight. A monodisperse polymer will have a PDI equal to 1.

Based on this understanding of molecular weight distributions, you will be able to set the correct tempertaure range for separating the polymers within a set molecular weight range. Alternatively, you can also segretate based on PDI which also helps in separating the polymers being processed.