Determination of the Molecular Weight Distribution by Diffusion Measurement

Abstract

A diffusion method is proposed for establishing molecular weight distribution of a polymer. The principle of the method makes use of the fact that the diffusion constant of monodisperse polymers is dependent on their molecular weight, and the diffusion curve of polydisperse polydisperse polymer is thought to be the summation over all curves of the components. The diffusion curve obtained from the interferometric method is expressed mathematically with Fredholm's integral equation of the first kind which provides the necessary relationship of molecular weight distribution and the diffusion constant of each component. A few errors might be inherently induced even in the most accurate measurement of the curve. Therefore, a new evaluating process is introduced for the numerical analysis of the curve. The flow chart is shown in Fig.2.
The validity for the application of the method is examined by the diffusion measurement of polystyrene in ethyl methyl ketone solution by a Mach-Zehnder interferometer at a carefully controlled temperature. The distribution is determined for the especially prepared polymers whose distributions have been preliminary known. These experimental results support the validity or The justification of the present method. The method has an advantage that it does not require any calibration curves. In addition, a limitation involved in the method is also discussed.