Methods for Measuring Molecular Weights of Polymers

To determine the intrinsic viscosity, both inherent and reduced viscosities are plotted against concentration (C) on the same graph paper and extrapolated to zero. If the intercepts coincide then this is taken 

The logarithms of intrinsic viscosities of fractionated samples are plotted against log or log Mn. The constants a and K of the Mark-Houwink-Sakurada equation are the intercept and the slope, respectively, of that plot. Except for the lower molecular weight samples, the plots are linear for linear polymers. Many values of K and a for different linear polymers can be found in the literature [66]. 

Actually, the Mark-Houwink-Sakurada equation applies only to narrow molecular weight distribution polymers. For low molecular weight poly disperse polymers this equation is useful, because the deviations due to chain entanglement are still negligible. On the other hand, chain entanglement in high molecular weight polydisperse polymers affects viscosity and this equation does not really apply. 

There are various physical methods available. The more prominent ones are ebullioscopy, cryoscopy, osmotic pressure measurements, light scattering, ultracentrifugation, and gel permeation chromatography (also called size exclusion chromatography). All these determinations are carried out on solutions of the polymers. Also, all, except gel permeation chromatography, require that the results 

The above two determinations, because each molecule contributes equally to the properties of the solutions, yield number average molecular weights. How much this technique is used today is hard to tell. 

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