The Microstructure Construction,regulation And Mechanism Of Bimodal Microporous Hydrophilic Polypropylene Hollow Fiber Membrane

Polypropylene hollow fiber membrane(PPHFM)was widely used in water treatment,membrane distillation,gas separation and biomedical applications due to the impact resistance,corrosion resistance,large unit membrane area and high separation efficiency.However,PPHFM prepared by melt spinning-stretching(MS-S)had low membrane water flux and poor anti-pollution performance in the field of water treatment or aqueous solution separation because of low pore porosity and poor hydrophilicity,which was due to single pores structure,the cluster of lamellae and the hydrophobicity of PP.In order to solve these two problems effectively,a novel bimodal microporous membrane structure with two different grades and independent pore size distribution based on the theory of lamellae separation and phase separation was designed in this paper.The bimodal microporous structure was used to improve the porosity and hydrophilic of PPHFM,and also improve the pure water flux and anti-pollution property of membrane.In addition,the bimodal microporous structure was regulated by improving the compatibility of the blend system.The construction and regulation mechanism models of the bimodal microporous structure were established.The effect of the membrane forming process on the bimodal microporous structure of the membrane was explored.The main contents of this paper are as follows:1.PP/MAH/EVOH blends with different EVOH contents were obtained by blending PP with different contents of EVOH and quantitative compatibilizer(PP-g-MAH).The hydrophilic PPHFM with bimodal microporous structure was prepared by the MS-S.The relationship between the phase morphology,crystallization behavior,rheological behavior,crystal structure of hollow fibers,structure and properties of membrane was studied.The construction mechanism model,rejection model and anti-pollution models of the bimodal microporous structure were established.The hydrophilic PPHFM with bimodal microporous structure showed the high porosity of 83.7%.The surface oxygen content of membrane was up to 5.93%,which showed the improvement of hydrophilicity.The pure water flux of membrane increased to 322.0±8.8 L/m~2·h,which was 117.56%higher than that of PPHFM.The anti-pollution performance of membrane was effectively improved,and the flux recovery rate was increased by up to 25%.The rejection of the 0.1%carbon solution was up to 99.88%,showing good rejection property.The small micropore in the bimodal microporous structure was formed by the separation of the oriented PP lamellae.The large micropore was formed by the separation of compatible interface between PP phase and EVOH phase.The microfiber in large micropore was formed by the separation of interacting phase interface.With the increase of EVOH content,the size of EVOH islands in the PP/MAH/EVOH blends increased,and the interaction of interface between the two phases was weakened.Therefore,the pore size of the large micropore formed by the separation of the two-phase interface increased.EVOH as an effective nucleating agent for PP matrix decreased the size of PP spherulites,and increased the nucleation density and total crystallization rate of PP spherulites in PP/MAH/EVOH blends.The thickness of the lamellae in the PP/MAH/EVOH hollow fibers was lower than that of the PP hollow fiber.However,the movement of the PP molecular chain and the PP crystal growth process would be hindered after excessive addition of EVOH.Therefore,the pore size of the small micropores in the PP/MAH/EVOH hollow fiber membranes(PMEVOH-HFMs)were smaller than that of PPHFM.When the mass ratio of PP/EVOH was 90/10,the membrane had the best bimodal microporous structure and also showed the best performance.2.Based on the principle of two-phase interfaces separation,the regulation of bimodal microporous structure was achieved by adding different contents of PP-g-MAH to improve the compatibility of the PP/EVOH/MAH blends.The regulation mechanism model of bimodal microporous structure was established.The results showed that the interaction of the two-phase interfaces in the PP/EVOH/MAH blends enhanced with the increase of PP-g-MAH content.The interfaces were difficult to separate due to the stronger interaction of the interfaces.Therefore,the size of large micropore was gradually increased.As the compatibility of PP/EVOH/MAH blends gradually improved,the nucleation temperature and growth temperature of PP phase in the PP/EVOH/MAH blends both shifted to the low temperature direction,indicating that the crystallization process of the PP phase was hindered.The thickness of the lamellae in the crystal structure of PP/EVOH/MAH hollow fibers were gradually reduced,while the orientation of PP lamellae increased first and then decreased.Although the thicker of lamellae correspond to the larger size of smaller micropore,when the PP-g-MAH content was less than 5 wt%,the pore sizes were all around 183 nm due to the large decrease in the orientation of lamellae in the hollow fibers.The large micropore formed by the separation of the two-phase interface was mainly controlled by changing the PP-g-MAH contents.When 5 wt%PP-g-MAH was added,the perfect bimodal microporous structure could be obtained,and the membrane showed the best performance.3.The maximum effecting factors in the MS-S method were studied,including the take-up speed,annealing temperature and stretching ratio.The results showed that there was an extreme value in the take-up speed.When the take-up speed was 395m/min,the PMEVOH-10 hollow fiber(PP/PP-g-MAH/EVOH mass ratio was 90/5/10)and PMEVOH-10-HFM showed the best structure and performance.There was also an extreme value in the annealing temperature.When the annealing temperature was150?,the PMEVOH-10 hollow fiber and PMEVOH-10-HFM showed the best structure and performance.The large and small micropores in bimodal microporous structure were simultaneously formed and enlarged during the stretching process,and the pore size of bimodal microporous increased first and then decreased as the stretching ratio increasing.When the stretching ratio was 200%,the size of bimodal micropores reached the maximum value,and the corresponding membrane showed the maximum porosity and pure water flux.The pore size of bimodal microporous structure and porosity of membrane could be controlled by stretching different ratios.There was no significant effect on the rejection property of PMEVOH-10-HFMs.However,as the stretching ratio was increased,the anti-pollution property of membrane would slightly reduced due to the pore size of membrane increased.