Automatic, simultaneous determination of differential refractive index of a polymer and its corresponding monomer

Abstract

The differential refractive index (dn/dc) of a polymer/ solvent pair is an important parameter in situations involving refractometric concentration determination, light scattering determinations of absolute molar mass, and so forth. It is well known that the dn/dc can be different for a polymer and its corresponding monomer. In this Note we present a rapid and accurate means of simultaneously determining the dn/dc for a monomer and a polymer and apply it to the case of acrylamide/polyacrylamide. Our motivation for developing this technique, instead of simply making separate determinations of dn/dc for the monomer and then the polymer, was based on concerns about the hygroscopic nature of the polyacrylamide (PAAm): PAAm normally contains 10–15% water by mass, and this is difficult to remove entirely and reliably. On the other hand, AAm is far less hygroscopic, so that if one commences with an accurate concentration of AAm in a polymerization reaction the concentration of AAm plus PAAm throughout the chain-growth polymerization reaction will remain constant; thus, the problem of an unknown amount of PAAm hydration starting from a “dry” powder is avoided. The technique involves continuous measurement of the monomer and total monomer/polymer concentration while the polymerization of the monomer proceeds. This method will be useful whenever detectors can be found that can distinguish between the concentration of the monomer and polymer, and the polymer can be produced in a homogeneous monomer/solvent system. A refractometric detector (RI) detects both the monomer and polymer. In the monomers with double bonds or conjugation, such as AAm, there is a strong ultraviolet absorbance that is lost upon polymerization. This allows an ultraviolet spectrophotometer (UV) to monitor the disappearance of the monomer during a polymerization reaction. Another possible scheme for distinguishing the monomer from the polymer is via evaporative light scattering, in which the monomer and solvent can be stripped via passage through a heated drift tube from droplets containing the monomer, solvent, and polymer. Any standard chromatography pump can be used to deliver reacting liquid to the detectors. RI and UV detectors and a chromatography pump are standard devices in polymer characterization laboratories, so application of this technique will not normally require any investment in additional equipment.