Spectro-turbidimetric titration of polymer solutions

Abstract

Acco~n~G to' the classical method of turbidimetric t i t ra t ion of polymer solutions, the molecular weight distribution (MWD) is derived by determining the dependence of the turbidi ty of a precipitated polymer dispersion v on the volume fraction of the precipitant ~=V~/V-b V v (V is the solution volume without the precipitant, Vv is the volume of precipitant added). I t is assumed that the turbidi ty is proportional to the concentration of the precipitated polymer phase [1]. This assump.tion is based on the observation of two conditions [1]: the particle size of the precipitated polymer dispersion does no t change during the entire t i tration; the relative refractive index of the particles also remains unchanged, i.e. it is assumed that the immobilization of the low molecular weight component by colloidal polymer particles either does not take place at all, or the extent of immobilization remains unchanged during t i t rat ion. As a matter of fact, it is known from the theory of light scattering [2] tha t the turbidi ty of monodisperse globular particles v is a complex function of three parameters: the concentration iV (number of particles in uni t volume), the relative dimension ~ =2gr/~0/A (r is the particle radius,/~0 is the refractive index of the dispersion medium, A is the wavelength of the light in vacuo) and the relative refractive index m =l~/l~o (1~ is the refractive index of the particle) v = N R (~, m), (1)