Research On Basic Physics And Application Of UHMWPE Microporous Materials

Because of its large specific surface area,excellent mechanical properties,weathering resistance and high rate of quantity and cost,ultra-high molecular weight polyethylene(UHMWPE)microporous materials are widely used in energy,environment and other fields.The processing of UHMWPE microporous membranes and porous fibers involves gel extrusion,uniaxially or biaxially stretching,solvent extraction,drying and so on.It is a dynamic and continuous process of multi-scale structure evolution under the action of multi-dimensional processing field(tensile rate,tensile temperature,tensile mode,etc.).Understanding the mechanism of microstructure evolution and establishing process-structure-property relationship during tensile process is the key to achieve precise control of structure and properties of microporous membranes and fibers.As two-dimensional(2D)materials,microporous membranes are prepared by biaxially stretching,which needs the coupling between longitudinal and transverse two-dimensional stressand temperature field during processing.However,most of the researches on microstructure evolution during UHMWPE tensile processing is under the action of one-dimensional stress field,while the crystal evolution of UHMWPE under the action of 2D stress field and temperature field is less studied.In this paper,based on the physical multi-scale structure research platform of thin film tensile processing,the structure evolution mechanism of UHMWPE gel membrane under the action of 2D stress field is studied combined with off-line characterization techniques such as scanning electron microscopy(SEM).Meanwhile,the morphological phase diagram in the space of2D strain is constructed.Moreover,the structure evolution mechanism and mechanical behavior of UHMWPE microporous membrane under the action of temperature field,the application of porous membrane and porous fibers in the field of separation and ion adsorption are studied.The main findings and conclusions are as follows:(1)The crystal structure evolution of UHMWPE gel films containing paraffin oil are studied with wide-angle X-ray diffraction(WAXD)during sequentially biaxially stretching process.Also,the surface morphologies of microporousmembranes after biaxially stretching,extraction of paraffin oil and drying are characterized by scanning electron microscope(SEM).During machine direction(MD)stretching process,the crystal structure is transformed from lamellar stacks to fibrillar crystals through melting recrystallization.While for transverse direction(TD)stretching,there exist three kinds of deformation mechanisms determined by draw ratio in MD(?MD),i.e.,transformation of weakly oriented crystals into fibrillar crystals by melting recrystallization at ?MD smaller than 3;with moderate ?MD from 4 to 6,the fibrillar crystals in MD are tilted towards TD;at larger ?MD from 7 to 8,the microfibrils are teared into thinner ones or fibrillar crystals,inclining to TD.Based on the understanding the structure evolution mechanism during biaxially stretching,the crystal morphology diagram is summarized in 2D strain space,providingthe guidance for the selection of process windows for UHMWPE microporous membranes.(2)Based on UHMWPE microporous membranes,the mechanical behavior and microstructure deformation mechanisms under the action of biaxially stretching and temperature field were studied.The surface morphology of microporous membranes under different stretching processes was characterized by SEM.The fiber diameter,pore size and surface porosity were counted based on image processing software,meanwhile the crystal structure evolution of UHMWPE porous membranes was studied by WAXD.It shows that:(1)with higher stretching temperature,the membrane occurs yielding ealier;(2)with the increase of streching temperature,the microstructure evolution and micropore expansion of UHMWPE microporous membranes have different mechanisms,that is,the pore size increasing,the fiber refining,the diameter decreasing,the number of pores increasing,and the slip separation between fiber layers.This work provides the basic data for the structure and performance regulation ofcrystal morphology,pore structure,porosity of UHMWPE microporous membranes,etc.(3)Based on the understanding of structure evolution mechanism of UHMWPE under biaxially stretching field,UHMWPE nanofibrous membranes are fabricated via sequentially biaxially stretching,which are featured by excellent morphology with homogeneously distributed pores as well as good mechanical properties.The orientation of fibers along draw ratios and morphologies of the samples are inspected by WAXD and SEM.It shows that the membranes are composed of nanofibrous scaffold with fiber diameters of 30 nm-40 nm and fully interconnected pore structures with mean pore size between 19 nm and 26 nm,respectively.Meanwhile,the maximum Young's modulus and tensile strength of UHMWPE nanofibrous membranes are 1186 MPa and 154 MPa,respectively,about 10 times that of conventional electrospun nanofibrous membranes.The separation test exhibits that the membranes can reject the molecules with molecular weight larger than 100 kDa efficiently with rejection lager than 90%under the water permeance of?100 L/(m2h).The as prepared membranes show great potential in microfiltration applications.(4)Polyacrylic acid grafted ultra-high-molecular-weight polyethylene(UHMWPE-g-PAA)fibers were prepared by radiation induced grafting of acrylic acid onto UHMWPE porous fibers and were examined for Cu2+removal from aqueous solutions.The structure,morphology and performance of the developed fibers were characterized by Fourier transform infrared spectroscopy(FTIR),energetic dispersive spectroscopy(EDS),scanning electron microscopy(SEM)and mechanics test.It was demonstrated that the UHMWPE fibers modified by acrylic acid(UHMWPE-g-PAA)conferred excellent hydrophilic performance.The UHMWPE-g-PAA fibers possess considerable mechanical performance with mechanical strength about?0.4GPa.The influence of radiation dose and degree of grafting of polyacrylic acid(PAA)on the adsorption kinetics of the UHMWPE-g-PAA fibers are studiedby using pseudo-first-order,pseudo-second-order and intraparticle diffusion models.The results reveal a large adsorption capacity of 63 mg g-1 for the UHMWPE-g-PAA fibers and the adsorption behavior followed perfectly a pseudo-second-order kinetics model regarding the Cu2+solution adsorption.The as-prepared UHMWPE-g-PAA fibers with high-strength,hydrophilic and porous natures show promising application as economical adsorbent in wastewater treatment.