Size exclusion chromatography and rheology of AB/AB2 hyperbranched polymers

Abstract

We report size exclusion chromatography (SEC) and linear shear rheology results for a series of AB/AB₂ hyperbranched polyesters. For these samples, the weight average molecular weight M_w was systematically varied from 3000 to 250000 but the spacer length between branch points was kept constant (by fixing the mole fraction of AB₂ monomers at 43%). The number density for all the investigated polyesters showed a power law distribution with an exponential cut-off at a characteristic upper cut-off mass M_char, n(M) ∼ M^−τ × exp(−M/M_char). When rescaled with respect to M_w, all the individual distributions collapsed onto a universal scaling curve, indicating that the molecular weight distribution for short chain branched hyperbranched polymers obeys static scaling with τ and the ratio M_char/M_w, independent of the extent of reaction. We find τ = 1.53 ± 0.05 which surprisingly is the same as the mean field value of 1.5 within experimental error. The dynamic rheological data was modelled using a dynamic scaling theory for unentangled polymeric fractals (based on the Rouse model) and excellent agreement was found. This indicates that these hyperbranched polymers are essentially unentangled. We also demonstrate that rheology is sensitive to rather subtle features of the molecular weight distribution. This means that it can be a useful tool for characterising the molecular weight distribution of branched polymers when used in conjunction with size exclusion chromatography.