Molecular Mass of Polymers
The growth of the polymer chain depends upon the availability of the monomers in the reaction. Thus, the polymer sample contains chain of varying lengths and hence, its molecular mass is always expressed as an average molecular mass.
Number-Average Molecular Mass Mn
If N1 molecules have molecular mass M1 each, N2 molecules have molecular mass M2 each, N3molecules have molecular mass M3 each and so on,
then, Mn = Σ Ni Mi / Σ Ni It is determined by osmotic pressure method.
It is determined by light scattering and ultracentrifugation method.
Polydispersity Index
It is the ratio of the mass average molecular mass to the number average molecular mass
PDI = Mw / Mn
For natural polymers, PDI is usually equal to one which means that they are monodisperse. In other words, such polymers are more homogeneous. On the contrary, synthetic polymers generally have PDI > 1 which means that they are less homogeneous.