Preparation And Structure-Property Relation Of Polyethylene Films Fabricated By Thermally Induced Phase Separation

In recent years,more and more attention has been paid to the research of polyethylene(PE)in separation films with the increasing demand of lithium battery and water purification However,PE is difficult to be dissolved in any organic solvent at normal temperature,so polyethylene microporous films cannot be prepared by conventional film-forming methods Generally,polyethylene film is processed by thermally induced phase separation(TIPS)method in industrial applications.Great efforts have been dedicated to the exploration of the phase separation mechanism of TIPS and the correlation between film micro structure and preparing conditions during TIPS process.However,the mechanisms of the interaction between phase separation and crystallization during the film formation of TIPS are still unclear,and the quantitative(or semi-quantitative)relationship between the film micro structure and dynamic factors(e.g.,cooling rate)is also indeterminate.In this work,the kinetics of HDPE films prepared by the TIPS method and their effects on the microporous structure and properties of the HDPE film by TIPS were intensively investigated.The phase separation mechanisms were analyzed and the best conditions for film preparation,and thermal stability of the HDPE films were greatly improved with the addition of ultra-high molecular weight polyethylene(UHMWPE)HDPE microporous films with various polymer concentrations were prepared to obtain the thermodynamic phase diagram of the HDPE/DOP system with the monotectic point determined as ?m=42%.Combination of phase diagram and film morphologies,the effect of polymer concentration on the structure of film was explored.The results showed that the average pore size decreased with the increase of ?HDPE.HDPE films prepared under various cooling conditions,and relation between structure and properties were studied.Our results show that as the quenching temperature increase,the pore size,porosity,and thermal stability of the films increase,while the water vapor transmission rate(WVTR)as well as the values of both E'max and E'"max determined from dynamic thermomechanical analysis(DMTA)increase first and then decreaseIn addition,this paper proposes a facile way of in-situ tailoring the hierarchical microstructures in TIPS films of crystalline polymers,which can be of use to the design of polymer porous films with specific properties.Morphological observations showed that the microstructures at different locations were fairly distinct and could be generally divided into three layers.The relationship between the film micro structure and cooling rate were quantitatively investigated(especially the effect of quenching conditions on the evolution of film morphology)with the aid of an enthalpy transformation method(ETM),and the three-dimensional temperature profiles were established which supplied an insight into the competition between phase separation kinetics and polymer crystallization.When ?HDPE<?m,the L-L phase separation occurred and cellular structure was finally obtained.At relatively low cooling rate(?),e.g.,?<50?/s,macroporous structure(average pore size larger than 0.45 ?m)was generally observed.A further increase in cooling rate(e.g.,90?/s<?<150?/s)would lead to mesoporous structure with an average pore size of 0.15-0.25 ?m.Interestingly,when ? was above 250?/s,a small bio-continuous pore structure were seen.On the other hand,when ?HDPE<?m,cooling rate would remarkably determine the pore size and microstructure of the films through influencing polymers'crystallization kinetics.At a relatively low cooling rate(e.g.,?<27?/s),macroporous structure with a pore size larger than 0.6 ?m was formed.When 58?/s<?<75?/s,a multi-layered platelet crystal structure was clearly observed with very small crystal size,primarily owing to the high cooling rates The cooling rate was too high to ensure the completion of the polymer crystallization at a cooling rate up to 170?/s or higher,leading to formation of lots of incomplete crystalsThe UHMWPE,HDPE and DOP were melt blended to prepare the composite films by TIPS method at a polymer concentration of 30%(i.e.,?p=?HDPE+?U=30%)under the cooling temperature of 20?.Various TIPS films were prepared by changing the amount of UHMWPE,and variation in morphologies and the properties(crystallinity,porosity,thermal stability,etc.)of the resultant films were carefully analyzed.The findings showed that with adding the UHMWPE,the pore structure still showed a typical cellular structure of L-L phase separation.When ?U<1.5%,the pore size of the blended films gradually became uniform with the increase of the amount of UHMWPE.As the amount further increased,some of the micropores began to collapse,which further induced the pore size distribution become uneven.In addition,as the amount of UHMWPE increased,the crystallinity,tensile strength,and initial thermal decomposition temperature of the blended film gradually increased,while the porosity and WVTR showed a trend of increasing first and then decreasing.According to the thermal stability analysis of the blended films,it was known that the thermal shrinkage of the blended film decreases from 13.4%to 3.7%with the increase of UHMWPE.The addition of UHMWPE reduced the initial melting temperature of the blended films,and the blended microporous film showed a lower shutdown temperature,which helped to ensure of the safety of the blended microporous film when used as a lithium battery separator.