Influence Of Ultrahigh Molecular Weight Polyethylene On The Crystallization Of Linear Low Density Polyethylene

Polyethylene is one of the most widely used polymer materials in the world.The study on the crystallization behavior of polyethylene is beneficial to understanding the evolution of the aggregation structure of semi-crystalline polymers.The crystallization behavior of polyethylene is influenced by internal entanglement and Van der Waals forces on the molecular chain diffusion and regular arrangement,but the external temperature field,force field,external field application time and additional substances have greater influence on the crystallization behavior.Each of these factors has a great influence on the crystallization behavior of polyethylene,and it is usually that many factors affect the actual processing process simultaneously.Therefore,the crystallization process of polyethylene is complex,and the study on crystallization behavior of polyethylene under external field is of great significance to guide the polymer processing.In this paper,linear low-density polyethylene?LLDPE?was used as substrate,the influence of addition of ultra-high molecular weight polyethylene?UHMWPE?which had the same segment structure but different chain length with LLDPE on the LLDPE crystallization was studied.Differential scanning calorimetry,rheometry,and scanning electron microscopy were used to study the crystallization kinetics and morphology of blends under quiescent crystallization and shear-induced crystallization.Effect of UHMWPE on the isothermal crystallization behavior of LLDPE was investigated by DSC.The research on the quiescent isothermal crystallization of LLDPE/UHMWPE blends at 120°C indicated that the crystallization rate of the blends increased with the increase of UHMWPE content.However,the simulation results of Avrami equation showed that the growth dimension of crystals decreased with the addition of UHMWPE.If the blend is pre-crystallized at 124°C?only the UHMWPE part of the blends could be crystallized?for 30 minutes before isothermal crystallization at 120°C.,the half-crystallization time of the blend is further reduced,which means that the pre-crystallization increases the crystallization rate of the common component.The crystallization behavior of blends under shear condition was investigated by rheometer.The steady shear,direct isothermal crystallization,pre-shear before isothermal crystallization and pre-isothermal treatment and pre-shear before isothermal crystallization were intercompared by rheometer.The result indicated that the addition of UHMWPE would make LLDPE crystallization happen earlier.This meant that UHMWPE acted as a nucleating agent to accelerate the nucleation rate of LLDPE.A comparison of crystalline structure of blends treated with above methods indicated that when the blends were pre-sheared with a low shear rate at0.05 s^-1,only the blends with pre-formed UHMWPE crystals served as nucleating agent could form cylindrite.Blends without pre-formed crystals could not form new crystals except spherulite.A further study on the influence of shear after pre-crystallization on the crystallization of blends was performed.The experiments of changes of shear time and shear rate after pre-isothermal treatment by rheometer indicated that the proportion of cylindrite increased with the increase of pre-shear time but without shish-kebab structure formed during this period.However,the increase of pre-shear rate would transfer the cylindrite into shish-kebab structure.