Synthetic Polymers

Abstract

A differential refractive index detector (RI refractometer) and an ultraviolet (UV) absorption detector are the most commonly used detectors for SEC. Molecular weight (MW) averages of a polymer are calculated by SEC on the premise that the magnitude of the ordinate (the response of the detector) on the chromatogram is proportional to the amount of the polymer which appears at each slice. The relationship between the refractive index n of a species and its MW is:

$$\frac{{dn}}{{dc}} = A + \frac{B}{M}$$

(9.1)

where c is the concentration of the species, M is the MW of the species, and A and B are constants. Although the refractive index of a polymer with MW above 10000 is assumed to be independent of the MW, an increase in the refractive index for polystyrene (PS) with MW over 10000 has been reported [1]. The value of dn/dc of PS having MW 1800000 increases 9% to that of PS having MW 2000 in toluene and 4% in methyl ethyl ketone (MEK). The refractive index of toluene (1.496) is closer to that of PS (1.55–1.60) than that of MEK (1.3814). A solute and solvent pair having a smaller difference in refractive index has a larger MW dependency.