Rheological properties of poly‐α‐olefin melts

In order to investigate the influence of the side‐chain‐length on the melt viscoelasticity of the hydrocarbon polymers, dynamic viscosity η′, dynamic rigidity G′ and apparent viscosity ηa of a series of six different poly‐α‐olefins have been measured with a rotating cylinder‐type rheometer. The molecular weight dependence of η′ and ηa was relatively greater at lower shear rate. At fixed shear rate and temperature, η′, G′, and ηa for each polymer sample were expressed by the form of KM . The values of K decreased with increasing side‐chain length and the values of α were approximately 3–4 for all the samples studied. η′ vs. ω G′ vs. ω and ηa vs. γ curves at different temp. (130–230°C) could be superposed according to the usual time‐temperature superposition method, and the shift factors aT obtained from η′, G′, and ηa were practically the same. The flow activation energies obtained from aT first increased with increasing side‐chain length in going from polyethylene to polyhexene‐1, and thereafter decreased in going from polyhexene‐1 to polyoctadecene‐1. They were all lower than that of high pressure polyethylene. These results appear to support the idea that the branching structure of high pressure polyethylene is much more complicated than the side‐chain structure of poly‐α‐olefins.