Measurement of Molecular Weight and Size

The molecular weight of the polymers plays a crucial role in determining the overall stability of the structure.

There are many methods to measure the Molecular weight of the polymer. They are a measurement of the colligative properties, light scattering, and ultracentrifugation; or by dilute-solution viscosity.

All the methods require solubility of the polymer.

End group Analysis:

The end group analysis depends on the nature of end groups attached to the polymers. This process varies between addition and condensation polymers.

In the case of condensation, polymers involve chemical methods of analysis with functional groups like carboxyl groups, amino acids or hydroxyl groups.

On the other hand, additional polymers the analysis is done on initiators, elements and radioactive atoms.

Colligative property measurement relation with molecular weight for dilute solutions is when solute equal to mole fraction.

The first method is osmometry, which determines osmotic pressure wherein pure solvent and dilutes solution of the polymer in the same solvent are placed on opposite sides of the semi-permeable membrane.

Due to the difference in concentrations, the solvent passes through the membrane and raise the head of the solution reservoir.

Gel-permeation chromatography is the most commonly used method for routine determination of molecular weight.

This method is based on the principle of size-exclusion chromatography to separate samples of polydisperse polymers into fractions of narrower molecular weight distribution.

Ultracentrifugation method is the most commonly used method in determining the molecular weight of high polymers like proteins.

The solution being centrifuged is held in a small cell within the centrifugal instrument. The concentration is determined by optical methods.

Also, light scattering methods used to determine molecular weight based on scattering angles.

Similar to molecular weight, molecular size affects the rheological properties which in turn impact performance.

Three methods to determine molecular size: dynamic light scattering(DLS); MALS and SLS in combination with intrinsic viscosity measurement.

We will not go depth into discussing these methods as it requires prerequisite knowledge in advance chemical science and kinetics.

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