Structure Regulation of Polypropylene/Poly(ethylene-co-vinyl alcohol) Hollow Fiber Membranes with a Bimodal Microporous Structure Prepared by Melt-Spinning and Stretching: The Role of Melt-Draw Ratio

High-performance polymeric hollow fiber membranes fabricated by a green and sustainable route have become the direction of the future. The bimodal microporous structure was green formed via melt spinning–stretching in the polypropylene (PP)/poly(ethylene-co-vinyl alcohol) (EVOH) hollow fiber membranes (PP/EVOH-HFMs) for improving the porosity and retaining the rejection performance. The relationship between the PP lamellar microstructure, the two-phase interface area, and the EVOH island morphology in PP/EVOH hollow fibers and the bimodal microporous structure in PP/EVOH-HFMs under different melt-draw ratios was established. The pore size of the bimodal microporous structure was controlled by regulating the melt-draw ratio. Small micropore sizes increased from 95 to 183 nm, and large micropore sizes decreased gradually from 3916 to 1054 nm with the melt-draw ratios increased. The porosity and pure water flux of PP/EVOH-HFM reached the maximum when the melt-draw ratio was 6000%. Moreover, the models of phase morphology and microstructure of PP/EVOH blends, PP/EVOH hollow fibers, and PP/EVOH-HFMs were built to explain the regulation mechanism of melt-draw ratio on the bimodal microporous structure of membranes.