Melt rheology of aliphatic hyperbranched polyesters with various molecular weights

Rheological behaviour of a series of aliphatic hyperbranched polyesters, Boltorn polymers, with different molecular weights (generations) is studied in the molten state. The onset of a nonlinear region occurs at much lower strains for Boltorn polymers of lower generations (generation 2 and 3) compared with those of higher generations (generation 4 and 5) in the dynamic strain sweep experiments. Relaxation times calculated from dynamic rheological parameters using an empirical equation indicate that the generation 2 and 3 polymers relax much slower than their higher generation counterparts. The generation 2 and 3 polymers show shear-thinning behaviour, whilst higher generation materials are Newtonian, in both oscillatory and steady shears within the deformation rates investigated. The Cox–Merz rule is found to not be obeyed by all the polymers studied. With increasing temperature, steady shear viscosity of the Boltorn polymers decreases but the features of shear rate dependence, shear thinning or Newtonian behaviour, remain the same. Flow activation energies obtained from the temperature dependence of steady shear viscosity show that Boltorn polymers have higher activation energies than most linear polymers, indicating a stronger temperature dependence of flow. The generation 2 and 3 polymers exhibit greater flow activation energies than the generation 4 and 5 materials due to stronger polar interactions.