Study On The Regulation Of Molecular Chain Entanglement And Performance Of Polyethylene Wax/ultra-high Molecular Weight Polyethylene Composite System Under Elongational Flow Field

Ultra-high molecular weight polyethylene(UHMWPE)has excellent impact resistance,wear resistance,etc.,so it is widely used in many fields.However,extremely high molecular weight and flexible long-chain structure of UHMWPE make it easy to form molecular chain entanglements,resulting in high melt viscosity,poor fluidity,and difficulty in processing,especially for UHMWPE fibers and films.Previous studies have shown that the elongational flow field has the effect of inducing flexible molecular chains orient along the melt flow direction,which reduces the entanglement points between molecules,the energy barrier of molecular chain motion and the viscosity of the polymer melt.Therefore,studying the change rule of UHMWPE molecular chain entanglement in the elongational flow field has important scientific significance and application value.As a new type of polymer molding processing equipment,the elongational rheology mixer realizes a transformation from shear deformation to enlongation deformation during the plasticizing and mixing process.This article regards the low molecular weight polyethylene wax(PE-wax)as the "solvent",and the PE-wax/UHMWPE blends are plasticized and mixed with the elongational rheology mixer and torque rheometer.Under certain UHMWPE and PE-wax content,the effects of different processing force fields on the regulation of molecular chain entanglement in UHMWPE blends melt and the evolution of chain entanglement over time are studied.The results show that under the shear force field,the plasticizing and mixing process can be divided into three stages: In the first stage,the system forms a suspension structure in which a soft matrix envelops hard particles.The surface of the UHMWPE particles melts and preliminary entanglement occurs between the molecular chains,which is manifested by a slow increase in torque and temperature over time.In the second stage,UHMWPE particles gradually melt from the surface to the core.Under the action of the elongational field,PE-wax gradually penetrates into the molecular chains of UHMWPE,and the molecular chains between UHMWPE particles in the system begin to entangle rapidly,which is manifested by a substantial increase in the torque and temperature over time.In the third stage,the molecular chains of UHMWPE diffuse into each other,the degree of entanglement gradually becomes uniform,the torque and temperature gradually increase and tend to be flat.Different from the evolution process of the UHMWPE molecular chain entanglement under the action of the shear flow field,the mixing efficiency of the elongational flow field is higher.The two-phase uniform distribution and entanglement homogenization are achieved in a short time,then the UHMWPE molecular chain disentangling occurs in the later stages of processing.This is because under the action of the elongational flow field,the melt is in the process of repeated shrinkage and expansion,UHMWPE molecular chains are oriented along the flow direction,which induce the molecular chains unwind the entanglement points along the orientation direction.Therefore,the entanglement density of UHMWPE decreases,the crystallinity and the crystal grain size increases.At the same time,as the entanglement density of UHMWPE decreases,the tensile properties of the sample are improved.The effects of two different flow field on the dispersion,lubrication and disentanglement of highly entangled UHMWPE macromolecular chains in blends with different PE-wax content were further studied.The results show that when the processing time is 5 minutes and the PE-wax content is 30 wt%-50 wt%,the composite system presents a relatively uniform two-phase distribution.Under the action of the shear flow field,the system go through three complete stages,the degree of entanglement of the melt gradually becomes uniform.Under the action of elongational flow field,the degree of entanglement of the UHMWPE molecular chains become uniform in a short time,then the UHMWPE molecular chains will be oriented and disentangled along the processing flow direction,making the final entanglement density lower than the sample under the action of the shear flow field,the crystallization and tensile properties under the action of elongational flow field are higher.When the processing time is5 minutes and the PE-wax content exceeds 60 wt%,the system presents an uneven two-phase distribution.As the force of the shear flow field cannot effectively act on the UHMWPE particles,the UHMWPE molecular chain cannot diffuse effectively,the composite system cannot go through the complete three stages,which makes the entanglement density of the final composite system uneven,resulting in a lower apparent entanglement density.Under the action of elongational flow field,the diffusion efficiency of UHMWPE and PE-wax molecular chains is higher,the welding strength between particles is improved,so the apparent entanglement density of the sample under the action of the elongational flow field is higher and the performance is improved.