Study On Molecular Weight Change Mechanism And Structure-activity Relationship Of Ultra-high Molecular Weight Polyethylene Under Different Flow Fields

Ultra-High Molecular Weight Polyethylene?UHMWPE?is a thermoplastic with excellent properties and has a broad application prospect.In this paper,the viscosity average method is used to test the viscosity average molecular weight of UHMWPE.The effects of the dissolution process of UHMWPE on its molecular weight were discussed,and accordingly,a relatively complete molecular weight test system is constructed.Combined with the infrared spectroscopy,X-ray photoelectron spectroscopy and dynamic rheometer,the changes of viscosity average molecular weight of UHMWPE and its structure-activity relationship were studied under different flow fields.Firstly,the influence of the dissolution process on the change of UHMWPE molecular weight was investigated,In the process,UHMWPE powder with a viscosity average molecular weight of 5.7 million was dissolved at 150°C for 80 min under a continuous N₂ flow,followed by holding at 135°C for 10 mins.The measurement results were compared with the standard molecular weight.The closest,the N₂ flow can effectively promote the dispersion of UHMWPE in the decalin solvent,and at the same time it can discharge the oxygen in the flask to prevent oxidative degradation of UHMWPE;the measurement results of UHMWPE samples with different viscosity average molecular weights are compared with the standard molecular weight.The fitting curve equation is:y=-0.1807x²+2.1434x-0.6688,the fitting degree R²is 0.9966,and the fitting result is ideal.Secondly,under the guidance of shear flow field,UHMWPE is subjected to high temperature and shear force during processing,and the viscosity average molecular weight is continuously reduced.Both high processing temperature and long heating time lead to decreasing in the molecular weight of UHMWPE due to contaction of UHMWPE with more air during processing.The carbonyl index and viscosity molecular weight exhibits a nonlinear relationship similar to a power function.The fitting curve equation is:y=0.235z^-0.45,and the fitness R² is 0.9938.It still has some reference value for estimating the molecular weight change trend.Finally,the change of UHMWPE molecular weight under various flow fields were studied.The results show that,at same processing temperature,the molecular weight retention rate of the product under the tensile flow field is higher than that of the shear flow field.However,the storage modulus,the loss modulus and the complex viscosity are larger,and the crystallinity is smaller.The stronger the shear flow field is,the more obvious the molecular weight reduction is.The reduction of the shear flow field ratio can increase the molecular weight retention rate;the continuous mixing extruder feeds the method of“starvation feeding”,and the material is always in contact with the air during the processing.Under the action of high temperature and strong shear,significant thermal oxidative degradation occurs,and the carbonyl index is large.