Analysis of Star-Branched Polymers with Triple Detection (Refractive Index, Viscometry, Light Scattering) Gel Permeation Chromatography

This paper deals with the characterization of star-branched model copolymers. The polymer branches are composed of PMMA/PtBuA di-block copolymers of poly(methyl methacrylate) and poly(tert-butylacrylate) with well-controlled chemical composition and structure (very low polydispersity). When these copolymer branches are chemically coupled, they produce star-branched copolymers with various numbers of branches. Number-average molecular weights M n of stars and branches were used to calculate the number of branches of the star-branched copolymers. In order to determine the real structure of the star-branched copolymers, we have measured the branching parameter g' and tested several hypothesis on the relationship which links the number of branches to the branching parameter g. The value of the exponent of the g/g' relationship, which characterizes the type of branching, was determined at several conditions. Values were dependent on the branching relationship, however, it was not possible to find an exponent value of 0.5, corresponding to pure star-branched molecules. We could not conclude whether or not the technique of multidetection GPC was not accurate enough to obtain this kind of information or if the studied star-branched copolymers had a complex structure, more complicated than a simple star. Excellent agreement was found between light scattering values and from viscometry with universal calibration. These results demonstrate excellent performance for universal calibration, even for highly branched polymers with unusual viscometric behavior.