Metrology Study On Apparent Viscosity Of Polymers

In this study,the apparent viscosity of 14 kinds of polyolefins,especially high density polyethylene(HDPE),linear low density polyethylene(LLDPE)and polystyrene(PS),were analyzed by twin-bore capillary rheometer.Apparent shear viscosity tests were based on power law model.Zero-die was used to make entrance correction.Apparent extensional viscosity was calculated by the entrance converging flow method based on Cogswell model.The results obtained in apparent shear and extensional viscosity measurements are explained from molecular structure.The structure-property relationship of polymers was discussed,such as explained the shear rate sensitivity and temperature sensitivity of apparent shear viscosity from molecular structure of polymer;explained the extensional rate sensitivity and temperature sensitivity of apparent extensional viscosity from molecular structure of polymers.The stick-slip transition affecting the apparent shear viscosity measurement was discussed in depth.The phenomenon that the critical shear stress increases with the increase of temperature at the beginning of stick-slip transition is explained by combining the entropy elasticity with the entanglement theory.In this study,the maximum error of apparent shear viscosity calculated by correcting shear rate is 23% when the stick-slip transition occurs,and the critical stress at the beginning of stick-slip transition at 220? is 23.01 k Pa higher than that at 190?.Furthermore,we found that the entrance convergence half-angle strongly affects the repeatability of the measurement when the entrance convergence method was used to measure the apparent extensional viscosity of polymers.By analyzing the results of a large number of repeated experiments of PS,it was found that increase the extrusion speed of polymer melt appropriately will reduce the entrance convergence half-angle,which will significantly improve the repeatability of the measured values of apparent extensional viscosity.Within the test range,the relative standard deviation of PS apparentextensional viscosity results decreased from 27.83% to 1.94% with the increase of extrusion speed.