Study On Structural Regulation And Application Exploration Of Uhmwpe Microporous Composite Membrane

As one of the new membrane materials,ultra-high molecular weight polyethylene（UHMWPE）has excellent chemical resistance,wear resistance,anti-aging resistance,biocompatibility,outstanding mechanical strength,and low price.Its microporous membrane can be widely used in many industrial fields such as water treatment,chemical industry,food,medicine,and biological engineering.The conventional methods for preparing UHMWPE porous membranes include thermally induced phase separation（TIPS）method,biaxial stretching method,sintering and salt leaching method.Among them,TIPS method is a mature method for preparing UHMWPE membranes,and the prepared membranes have the advantages of less defects,good reproducibility,and high accuracy in controlling pore architecture.However,the UHMWPE membrane prepared by TIPS method will shrink due to the capillary pressure during the drying process,and the poor permeation of membrane also caused by the closed/semi-closed disconnected sponge or honeycomb structure.In this thesis,the technology of reducing the shrinkage of UHMWPE microporous membrane was firstly explored,and then the methods of utilizing the different phases separation process of UHMWPE/white oil,UHMWPE/decalin,and UHMWPE/good solvent/poor solvent systems to improve the fluxes were discussed.The structure of UHMWPE porous membranes was controlled by changing the molecular weight of UHMWPE,solution concentration,solvent and extractant composition,annealing process,solvent removal method and other factors,in order to improve the air and water vapor permeability.Finally,the suitable modification methods,such as physical blending,chemical grafting,etc.,to give UHMWPE microporous membranes different functions,and the membranes were used in the fields of air and water vapor permeable X-ray shielding garment,membrane distillation（MD）and other fields.The specific research contents and results are as follows:（1）Reducing the shrinkage of UHMWPE microfiltration membranes during drying is a prerequisite for the preparation of high permeable UHMWPE membranes.In UHMWPE/white oil system,there is a Liquid-Liquid（L-L）phase separation zone in the process of cooling down from a homogeneous solution to a gel,and the gel membranes were annealed at an intermediate temperature（110℃）between the melting point and crystallization temperature（within the L-L phase separation zone）to increase the membrane pores and thus reduce the capillary pressure during drying,which leads to a reduction in the shrinkage of the membrane.Annealing also improves the crystallinity and grain size of UHMWPE,which could increase the pore wall strength to resist shrinkage caused by capillary pressure during the drying process.In addition,the effect of extractants with different surface tension on the shrinkage of UHMWPE membranes by atmospheric drying was also investigated,based on which the multiple stage extractant exchange drying（MSEED）technique was proposed,which the white oil is exchange to water（white oil-CH₂Cl₂-ethanol-water）and followed by ambient drying.The UHMWPE membranes obtained by this drying method have the lowest shrinkage rate compared to the use of other low surface tension extractants（e.g.ethanol and acetone）.This drying method results in a decrease in volume shrinkage from 53.9%to 16.5%for the membrane after annealing 25 min,and a more than 3-fold increase in pure water flux compared to the non-annealing membrane.Also,the membrane has a high rejection ratio for 0.5 wt%carbon ink（99.1%）,which is much higher than that of the supercritical CO₂ dried membrane（volume shrinkage 9.8%,rejection ration for carbon ink 25.3%）.（2）Current X-ray shielding garments are mostly prepared by mixing a large amount of toxic Pb-based inorganic fillers with a rubber matrix,which are lacking in air and water vapor permeability,results in discomfort and health risks to the wearers.The UHMWPE/white oil system prepared membrane has certain permeability,and it can maintain the original pore architecture while mixing large amounts of inorganic particles into UHMWPE matrix.Therefore,this thesis mainly explores the preparation of highly air permeable and water vapor permeable X-ray shielding garment by mixing X-ray shielding fillers into UHMWPE matrix.Using Ce O₂ as the X-ray shielding filler,the size of Ce O₂ particles（<1μm）was reduced by wet ball milling method,and the low-density polyethylene（LDPE）was coated on the surface of the particles in wet ball milling process,thus improving the dispersion uniformity and filling efficiency of Ce O₂/LDPE in the UHMWPE/white oil system.Finally,the porous X-ray shielding composite membrane（UPE/Ce/LPE15）with high air and water vapor permeability was prepared by TIPS method after hot-pressing UHMWPE/white oil/Ce O₂ mixing gels with a polyester（PET）mesh.Annealing the gel membrane significantly increases the pore size and reduces the resistance to gas passage,thus effectively improving the air permeability of the composite membrane.Air permeability of the annealing 25 min membrane is slightly lower than that of Tyvek,but the water vapor transmission rate is 129.1 g/（m² h）,higher than that of Tyvek of 99.9 g/（m² h）.Finally,the porous,flexible,air and water vapor-permeable UPE/Ce/LPE15 composite membranes presenting a good absorption of X-rays in the energy range of 40.4～50 ke V,and specially the absorptivity is 29.1%at the X-ray photon energy of 48 ke V,and could be used to prepare low-energy X-ray shielding garments.（3）UHMWPE composite membrane with spherical structure was prepared by the UHMWPE/decalin system to further improve the air permeability of UHMWPE-based X-ray shielding composite membrane.Bi（Z=83）has higher atomic number and less toxicity than Pb（Z=82）,so it was used as the filler to prepare high air-permeable X-ray shielding composite membrane.Due to the better shielding performance of nanoscale fillers against X-rays,the TEMPO-oxidized cellulose nanofiber（CNF）as the template,then Bi³⁺could bind to carboxyl groups on the surface of CNF,after in situ reduction by Na BH₄,Bi singlet nanoparticles（4.4 nm）attached to CNF were prepared as X-ray shielding fillers（CNF/n Bi）.The cetyltrimethylammonium bromide（CTAB）was used to improve the dispersion uniformity of CNF/n Bi in the UHMWPE/decalin system.The best air-permeability of the composite membrane was prepared at a UHMWPE concentration of 10wt%,and the X-ray shielding composite membrane（UPE10/CNF/n Bi）was prepared by adding 20wt%CNF/n Bi（Bi content 40.39 wt%）into this substrate.The spherical structure of the UHMWPE/decalin system effectively shortens the path of gas molecules through the membrane and reduces the resistance,so the composite membrane has better air permeability than Tyvek,and the water vapor transmission rate of the membrane is 127.1 g/（m² h）,which is higher than that of Tyvek99.9（g/（m² h））.Moreover,the mechanical properties of the composite membrane higher than those of commercially Tyvek and PET fabrics.The filling of high Z Bi singlet nanoparticles makes the composite membrane more effective in shielding low energy X-rays,and the absorptivity is 44.0%at X-ray photon energy of 33 ke V.（4）The highly air permeable UHMWPE composite membrane prepared by UHMWPE/decalin system can further expand its application in high flux membrane distillation（MD）.The spherical structure can reduce the tortuosity of the membrane pores,and the thin nylon（PA,thickness 50μm）mesh was used as the reinforcing mesh,after hot-pressing with UHMWPE/decalin gels,the composite membrane with a thickness of about 100μm was obtained,and the prepared thin membrane can reduce the resistance to the passage of water vapor and helps to improve the distillation flux.Comparing to the M_η=1.6,4.0,6.0×10⁶ UHMWPE,membrane prepared from M_η=9.0×10⁶ UHMWPE has the smallest average pore size（d,0.30μm）,and d of the membrane could be significantly increased to 0.40μm by removing the amorphous region of the membrane after hot-washing in pure decalin at 85℃,and the largest distillation flux after hot washing is up to43.5 kg/（m² h）.Finally,tannic acid（TA）-3-aminopropyltriethoxysilane（APTES）nanoparticles were deposited on the surface of the membrane to increase the surface roughness,and then fluorosilane was grafted on the nanoparticles via vapor-phase silanization to reduce the surface energy of the membrane,and improving the anti-wetting property.The water contact angle of the composite membrane was increased from 122.4°without modification to 152.5°after modification,but the TA-APTES nanospheres would block part of the membrane pores,which reduced the distillation flux of the modified membrane to 31.0 kg/（m² h）.Furthermore,the modified membrane has a rejection ratio of over 99.9%for both cationic/anionic dyeing wastewater,and the water recovery rate is 80-90%,while the permeate side conductivity only increases to about 20μS/cm in the 50-60h membrane distillation test,which indicates TA-APTES modification significantly improved the anti-pollution performance of the membrane against the dyes.（5）Because of the time-consuming and complicated process of fabricating first and then constructing the micro-/nano-hierarchical structure on the surface of the membrane by two-steps,this thesis focuses on the direct preparation of UHMWPE membrane with the micro-/nano-hierarchical structure for membrane distillation utilizing UHMWPE/good solvent/poor solvent system.The UHMWPE microgel dispersions were obtained by adding poor solvent into dilute UHMWPE good solvent solution and fast cooling in the L-L phase separation zone of TIPS,which were vacuum-filtering onto a nonwoven fabric to obtain an intrinsically superhydrophobic UHMWPE composite membrane.The best preparation conditions for the composite membrane are:the volume ratio of good solvent xylene to poor solvent cyclohexanone is 1:5,the cooling temperature is 30℃（M_η=1.6×10⁶ UHMWPE）and 0℃（M_η≥4.0×10⁶ UHMWPE）,and the loading amount of UHMWPE is 2.52 mg/cm²,which can eliminate the defects on the membrane surface and have the best hydrophobic performance.The composite membrane prepared by M_η=4.0×10⁶ UHMWPE（4.0m X/C-c）has a water contact angle of 154.6°,sliding angle of 2.5°,distillation flux of 21.2 kg/（m² h）and a smaller particles structure than the membrane prepared by M_η=1.6×10⁶ UHMWPE（1.6m X/C-c）.Finally,For the Na Cl and Ca CO₃ anti-scaling test,the membrane4.0m X/C-c with small particle structure has a higher water recovery of over 93.6%,and the conductivity just increases from 2.3 to 5.7μS/cm in 83.0 h.In addition,the membrane has higher rejection ratio of over（99.9%）for anionic dyeing wastewater（100 ppm methyl orange+2 wt%Na Cl solution）.（6）Finally,utilizing UHMWPE shish-kebab crystals to construct micro-/nano-hierarchical structure UHMWPE composite membrane for membrane distillation was explored in this thesis.Large-size（19×10 cm²）UHMWPE shish-kebab composite membranes were prepared by wrapping PET fabric mesh on the surface of homemade iron frame and then placed in dilute UHMWPE solution with stirring and shearing,and the effects of preparation conditions such as crystallization temperature and stirring rate on the structural properties of shish-kebab membranes were investigated.The optimal crystallization temperature is 85℃and the stirring rate is 900 rpm.The composite membrane（M85-900）has the smallest“kebab”size,moderate membrane thickness,highest water contact angle（155.1°）,porosity（77%）,and the distillation flux（35.2 kg/（m² h））.In addition,the surface morphology and membrane distillation properties did not change significantly when immersed in acid（p H=1）or alkali（p H=13）solutions for 7 days.The anti-surfactant wetting property was improved by immersing M85-900 into a low-concentration fluorine resin（0.6 wt%）solution to further reduce the surface energy,and although the distillation flux decreased to 26.7kg/（m² h）,M85-900/F0.6%maintained a stable distillation flux and salt rejection rate in the surfactant-resistant membrane distillation test,and the significant decrease in salt rejection did not occur until 0.2 m M of sodium dodecyl sulfate（SDS）was added to the feed side.Finally,when the water flow direction parallel to the“shish”orientation direction,just as fish gills reduce sediment deposition,the resulting micro-turbulence can impede the deposition of salt crystals on membranes surface,and the membrane has better resistance to Na Cl,Ca CO₃ scaling,and in this condition,membrane（M85-900/F0.6%）has a high-water recovery of 92.5%and a low conductivity increase（from 2.8 to 8.3μS/cm in 70 h）.