Light scattering from polymerizing and coagulating systems

Abstract

The intensity of light scattered as a function of time from a system undergoing polymerization is calculated by combining Rayleigh's theory with the size distribution functions for various polymerization processes. For linear condensation polymerization the light scattering increases linearly in time with a slope proportional to the rate constant of the elementary step reaction. For monomer addition polymerization the light scattering is quadratic in time with a curvature determined by the rate constant. Light scattered from depolymerizing linear molecules decreases with time and is proportional to the hyperbolic cotangent of the time.

Rapid coagulation of colloids, which may be regarded as a linear con- densation polymerization with a rate constant determined by diffusion process, shows by calculation light scattering linearly increasing in time with a slope proportional to the square of the concentration of the colloidal substance. All rapidly coagulating systems should show the same behavior when the weight concentration, refractive index, and density of the colloid is taken into account.

When acid and sodium thiosulfate are mixed, colloidal sulfur is produced after a short induction period of from 1–3 minutes. In the first few minutes following the production of sulfur particles, they are small enough to show Rayleigh scattering. For this period of time, the intensity of scattered light depends on time and initial concentration in agreement with the theory developed in this paper. Solutions of purified tomato bushy stunt virus exhibit a similar dependence of light scattering on time and concentration when salted out with ammonium sulfate.