Study On Microstructure And Rheological Properties Of Polyethylene/paraffin Oil Binary And Ternary Blends

During the preparation of microporous membranes by thermally-induced phase separation(TIPS),the raw material parameters and external flow fields will directly affect the pore structure and properties of the final product.Polyethylene(PE)/paraffin oil(LP)is a typical blending system for preparing microporous membranes by TIPS method,but a single type of PE shows a large degree of restriction on the structure and performance of the final microporous membrane.Therefore,the introduction of the third component into PE/LP system could effectively improve its processing rheological properties and regulate microscopic morphology.Small amplitude oscillatory shear(SAOS)can explore the linear shear rheological behavior of the blend system,but SAOS can only provide linear viscoelasticity,it is not sensitive for SAOS to characterize the changes under the processing flow field.The evaluation of nonlinear viscoelasticity by large oscillation test(LAOS)with greater structural sensitivity is more consistent with the actual processing conditions.At present,the research work about the preparation of PE microporous membrane at home and abroad mainly focuses on the materials,diluents,and molding process conditions on the pore formation by using thermally-induced phase separation,and less attention is paid to the rheological processing properties of polyethylene/paraffin oil binary and ternary blend systems.In this thesis,relying on the principle of TIPS,a variety of blending systems with polyethylene(PE)as the main material,paraffin oil(LP)as the diluent,and PE2 or ethylene-octene copolymer(POE)as the third component were prepared.This thesis uses SAOS to study the linear viscoelastic properties of the blend system,and uses LAOS to induce the system to form more abundant rheological behavior,to clarify the processing properties and microscopic morphology of PE/LP,PE/PE2/LP and PE/POE/LP binary and ternary blend systems.1.The phase diagram of the PE/LP binary blend system with three different molecular weights of PE was drawn by DSC and PLM.The processing performance was tested by rheometer,and the microstructure was characterized by SEM.The results show that PE1/LP has no monotectic point?_PE1.The monotectic points of PE2/LP and PE3/LP are?_PE2=40%and?_PE3=46%,respectively,which indicates the existence of liquid-liquid phase separation.The compatibility between PE1 and LP is the best,PE2 is the second,and PE3 is the worst,indicating that the compatibility is dependent on the molecular weight of PE.The difference in the phase separation mechanism leads to different microscopic morphologies of PE1/LP,PE2/LP and PE3/LP.The crystal structure of PE1/LP is more apparent,so that subsequent addition of LP shows little effect on the number of pores,indicating that the pore-forming effect is poor.The number of holes of PE2/LP and PE3/LP systems increase significantly when the LP content is greater than 50%.PE1 has a low molecular weight,after adding LP,the processing performance is excellent.PE3 has the highest molecular weight and serious entanglement,when the LP content is greater than 50%,the pressure oscillation during the extrusion process can be effectively weakened.Meanwhile,not only the distortion of the Lissajous curve will decrease,but also the entanglement will be weakened,and the nonlinear response will be reduced with the increase of LP content.The processing performance of PE2/LP system is between PE1/LP and PE3/LP.With the increase of LP content,the nonlinear response is continuously weakened,furthermore the distortion degree of Lissajous curve is lower than that of PE3/LP,so the strain resistance is stronger.The PE1/LP system has good processing performance,but the pore-forming effect is poor.When the LP content of PE2/LP and PE3/LP system is greater than 50%,the processing rheological properties are improved,and the microporous structure is better than that of PE1/LP.2.This thesis selects PE2 and PE3 with close compatibility with LP and good pore-forming effect as the polymer matrix,fixing the LP content.SAOS,LAOS and SEM are used to study the influence of PE2:PE3 ratio on the structure and performance of PE2/PE3/LP ternary blends.It is found that when PE2:PE3 ratio changes from 1:9 to 5:5,the number of micropores continues to increase,because the continuous addition of PE2would weaken the severe chain entanglement,thus the LVE be widened,and the relaxation time is shortened,and the processability is improved.Beyond that the reaction under large strain conditions of PE2/PE3/LP blend system also becomes insensitive with variable ratio of PE2:PE3,so that the Lissajous curve gradually tends to be sinusoidal,and the relative harmonic intensity In/1 decreases continuously,which weaken the nonlinear response.When PE2:PE3 is 6:4 and 7:3,due to poor blending effect,the micropores of the two blends are collapsed,and the PE crystal aggregates are apparent.Because the degree of entanglement of the two blends increases,the Lissajous curve is distorted,In/1 increases,and the ability to resist large strains is weakened.Under the action of different strain shear fields,the entanglement density of PE2/PE3/LP system changes,which affects the crystallization rate and decreases with the increase of strain.A smoother surface with a finer size of pores can be obtained under appropriate shear field strength,but the large and fast shear field will cause irreversible damage to the final morphology of the blend system.3.This thesis selects POE elastomer as the third component to add into the PE3/LP system,changes the POE:PE3 ratio from 1:9 to 5:5,and uses SAOS,LAOS,and SEM to explore the rheological behavior and microscopic morphology of PE3/POE/LP ternary blend system.It is found that when the ratio of POE/PE3 is 1:9,the POE/PE3/LP system with low viscosity and excellent processing performance,of which LVE is 46.7%,has better pore forming effect.And its entanglement network is simple,with?of 0.1 s.In the long-term oscillating field,the decline of G'of this blend system is low,and the entanglement network is relatively stable,therefore the area of the Lissajous curve is small,and the I3/1 is the smallest,which is 0.04.Then the POE:PE3 ratio is fixed at 1:9.It is found that with the increase of LP content,the pore-forming effect increases,and the fluidity of the blend system is enhanced,the nonlinear response is continuously weakened.In addition,the stronger the strain shear field,the easier the entanglement of the PE3/POE/LP system is opened,causing not only the non-isothermal crystallization rate increases,but also the pore morphology changes under large strain conditions,leading to more obvious coarsening of pores.