A new absolute molecular weight method for linear polymers

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Abstract
The measured viscosity for concentrated solutions of linear coiling polymers decreases with increasing rate of shear. By use of a theory previously derived to explain this phenomena, it is possible to determine the absolute molecular weight of the polymer from measurements of this effect. The necessary data required for this determination are readily obtained using a simple cone viscometer. Since the theory requires only a knowledge of the variation of viscosity with shear rate together with the polymer concentration and temperature to compute the molecular weight, the method is easily applied to any polymer of this general type. This shear rate method for determining molecular weights has been checked by measuring M for a large number of samples of polystyrene and polymethyl methacrylate. Satisfactory results are obtained on these polymers. When the method is used as outlined, it gives a value for M which is close to the viscosity average. For very wide molecular weight distributions, the value found is somewhat higher than the weight average. Since the method is only strictly valid for linear polymers, it appears that it may also be used in conjunction with other molecular weight methods to obtain useful information concerning the extent of branching in the molecules.