Melt rheology of aliphatic hyperbranched polyesters with various molecularweights

Rheological behaviour of a series of aliphatic hyperbranched polyesters, Bo ltorn polymers, with different molecular weights (generations) is studied i n the molten state. The onset of a nonlinear region occurs at much lower st rains for Boltorn polymers of lower generations (generation 2 and 3) compar ed with those of higher generations (generation 3 and 5) in the dynamic str ain 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 g eneration materials are Newtonian, in both oscillatory and steady shears wi thin 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. F low 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 Row. The generation 2 and 3 polymers exhibit greater Row activation energie s than the generation 4 and 5 materials due to stronger polar interactions. (C) 2000 Elsevier Science Ltd. All rights reserved.