Light Scattering Studies on Coiling Polyelectrolytes

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Abstract
The molecular weight and size of polyacrylic acid in solution have been determined by measurement of light scattering. Use was made of Debye's 90° light scattering method employing two different wavelengths of light. When the reciprocal specific scattering is plotted against concentration for homopolar polymers, a straight line is obtained which is readily extrapolated to zero concentration. Polyelectrolytes, however, are capable of ionizing in aqueous solution, thus becoming electrically charged by amounts depending upon the concentration. The electrostatic repulsion of the charges very probably affects the molecular size and gives rise to peculiar light scattering curves. By utilizing buffer solutions as solvents, the molecular size is held constant and the difficulties involved in the interpretation of scattering data are largely resolved. The weight‐average molecular weight of a certain sample of polyacrylic acid at pH values of 1.2, 3, 4, and 5 was found to be 206,000±10 percent. Molecular weight values at a particular pH using a specific buffer were reproducible to about ±3 percent. The remainder of the error was attributed to association of buffer ions with the charged polymer. The size of the particles in solution was found to be less than 550A in the low pH regions, with a definite tendency to increase at higher pH values. The slope of the straight line obtained when the reciprocal specific scattering was plotted against concentration was shown to be dependent upon pH. This dependence of slope may be related to particle size and shape. In a non‐ionizing solvent such as ethanol, the polymer exhibits scattering behavior typical of non‐ionized polymers. The molecular weight of polyacrylic acid under these conditions was found to be 206,000, in agreement with values obtained in buffer solvents.

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