Research On The Viscoelastic Properties Of Polyethylene And Its Blends

The forming process of polymer included mixing,extrusion molding,fiber spinning,blow molding and so on.The rheological properties of melt played a guiding role in the process which was determined the final performance.Therefore,it was great significance to study the rheological properties of polymer.In this paper,the viscoelastic rheological behavior of low density polyethylene(LDPE),linear low density polyethylene(LLDPE),high density polyethylene(HDPE)and their blends were studied by constant shear rate capillary rheometer and rotational rheometer.It was found that in capillary rheometer,the non-newtonian exponent was calculated by Oswald-de wale power law equation through the shear stress-shear rate flow curve.The melt flow curve needed be linear in logarithmic coordinates.The flow curve of LDPE behaved linear relation well.For LLDPE and HDPE,the flow curve was discontinuity due to the stick-slip transition.So,the non-newtonian index of LLDPE and HDPE was obtained by segmental fitting in this study.Then the shear and extrusion properties of polyethylene were calculated according to Cogswell model.Segmental fitting made linear polyethylene avoid the limitation when using Cogswell model caused by stick-slip transition.According to the extrusion data obtained by Cogswell model,HDPE exhibited multi-stage strain hardening phenomenon.Based on the unified theoretical explanation of strain hardening and the flow curve of HDPE,it was proposed that the multi-stage strain hardening of HDPE might be due to two different entanglement structures of branched chain and molecular chain.In the experiment,the entrance pressure drop of melt was studied.The entrance pressure drop was calculated according to the zero-length capillary and Bagley correction methods.Then the melt flow curve,instantaneous pressure curve and extrusion morphology were compared and analyzed to determine the best method for calculating the entrance pressure drop of LDPE,LLDPE and HDPE.The viscoelastic rheological behavior of polyethylene blends was also studied.It was found that the shear viscosity of the blends accorded with Utracki ideal addition formula,but the variation of extension viscosity was more complex.When the blending ratio was 50/50,the storage modulus of LDPE/HDPE blends reached its maximum value.The viscoelastic rheological was closely related to the phase structure of the blends.The compatibility of blends was studied by direct methods(Differential Scanning Calorimetry(DSC),Phase Contrast/Polarizing Microscopy)and indirect test methods(VGP curve,Cole-cole curve,Han curve,frequency curve).It was found that the chemical structure of polyethylene were the same and the intermolecular force was weak,but the strong entanglement formed by the difference of physical structure made the different rheological properties of the blends.