Melt rheology of aliphatic hyperbranched polyesters with various molecular weights T.-T. Hsieh a , C. Tiu a , G.P. Simon b, * a Department of Chemical Engineering, Monash University, Clayton, Vic. 3168, Australia b Department of Materials Engineering, Monash University, Wellington Road, Clayton, Vic. 3168, Australia Received 17 February 2000; received in revised form 2 May 2000; accepted 9 June 2000 Abstract 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.  2000 Elsevier Science Ltd. All rights reserved. Keywords: Rheology; Hyperbranched polymer; Activation energy 1. Introduction Dendritic polymers, including dendrimers and hyper- branched polymers, represent an important class of poly- meric materials that can be contrasted to conventional linear thermoplastic. Their molecular architecture is highly branched and thus the molecules are intrinsically globular (at sufficient molecular weight) with a high surface func- tionality. They are one of the most rapidly expanding fields of synthetic polymer science in recent years [1–4], despite theoretical work being reported as early as 1952 [5]. The building blocks of dendritic polymers are AB x type monomers which contain one functional group A and x functional groups B. Note that functional groups A and B react with each other. Each layer in the dendritic structure is called a generation [4]. When a B y core (a compound with y functional groups B) is used and the synthetic procedure is controlled with extreme care, dendrimers without unreacted functional group B inside the molecules can be produced. The requirements of highly purified chemicals and complicated polymerization steps mean that dendrimers are expensive to produce [2]. Although hyperbranched poly- mers can be synthesized from similar reactants as dendri- mers, the production of hyperbranched polymers is less costly as they are produced in a less controlled polymeriza- tion process and thus contain linear sequences due to unreacted B groups inside the molecules. It is the “molecular ball bearing” nature of the dendrimers which leads to their advantageous properties such as high solubility in solvents and low melt viscosity compared with the linear polymers [6]. Due to their imperfect structures, it could be expected that hyperbranched polymers would show behaviour intermediate to that of linear and dendritic polymers, yet many favourable properties, namely high solubility and low viscosity, are retained. Although many dendritic materials have been success- fully synthesized and characterized, which indicates the maturity of such materials in terms of polymer chemistry and physics [7,8], certain properties such as melt rheology have been less frequently explored. There are only limited reports even for solution rheology of dendrimers [1,8,9]. Hawker and coworkers reported melt rheology of dendri- mers that are aromatic dendritic polyethers [6]. In this brief Polymer 42 (2001) 1931–1939 0032-3861/01/$ - see front matter  2000 Elsevier Science Ltd. All rights reserved. PII: S0032-3861(00)00441-9 www.elsevier.nl/locate/polymer * Corresponding author. Fax: +61-3-9565-4940. E-mail address: george.simon@eng.monash.edu.au (G.P. Simon).