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 .