Studies on viscoelastic properties of two types of isotactic polypropylene (iPP-1 and iPP-2) and their composites filled with carbon nanotubes (CNTs) (CNTs/iPP composites) were carried out by using rheometer.The influences of viscoelastic properties on the melt extrusion behaviors of iPPs and CNTs/iPP composites were further investigated by using capillary rheometer.The rheological results demonstrate that at 200 ℃ the viscosity and storage modulus G' for iPP-1 are obviously lower than that for iPP-2.The appearance of frequency-independent plateau of G' at low frequencies and solid-like elastic behaviors for the CNTs/iPP composites containing CNT mass fraction of 5 wt% indicate the formation of entangled CNT networks in the composites.Both iPPs and CNTs/iPP composites exhibit prominent shear-thinning behaviors and the composites exhibit relatively higher viscosities and shear stresses than their iPP counterparts.The extrudate distortion and die swell for CNTs/iPP composite melts decrease compared with iPPs due to the formation of CNT networks in the composites.Although the die swell can still be observed at the high applied shear rates for the CNTs/iPP composite with the relatively high viscoelastic iPP-2 as the matrix

an appreciable phenomenon of die shrinkage rather than the common die swell at the low applied shear rates is observed for this CNTs/iPP composite.More interesting

the particular die shrinkage phenomenon is observed at all the applied shear rates for the CNTs/iPP composite with the relatively low viscoelastic iPP-1 as the matrix.The above results can be explained by that the CNT networks in the composites under shear might be able to restrict the deformation and elastic relaxation of iPP chains

which have the relatively low viscoelastic properties.