Regulation of dual-scale structure of polypropylene/poly(ethylene-co-vinyl alcohol) hollow fiber membranes with bimodal pore base on lamellae optimization during annealing

Solvent-free manufacturing process of high performance polymer hollow fiber membranes has always been a desirable direction. Therefore, maleic anhydride grafted polypropylene (PP-g-MAH) as compatibilize, the polypropylene (PP)/poly(ethylene-co-vinyl alcohol) (EVOH) hollow fiber membranes (PMEHFMs) with bimodal pore were manufactured via melt-spinning and stretching. Bimodal pore structure was constructed for improving the porosity and retaining the rejection performance. Based on the effect of EVOH on nucleation behavior and crystal growth of PP phase, the effect of annealing temperature on microscopic lamellar structure of hollow fibers with adding EVOH was investigated in detail, and the results were expected to guide the regulation of dual-scale structure of bimodal pore. With rising annealing temperature, long period of crystal (L) gradually increased, but thickness of lamellae (Lc) and rigid amorphous (Ltr) firstly increased and then decreased when annealing temperature increased to 160 °C, and optimum values were obtained when annealing temperature was 150 °C. The optimum crystallinity and lamellar orientation of annealed hollow fiber annealed at 150 °C were 48.48% and 0.863, respectively. As the annealing temperature increased, the most probable pore size of small scale pores in PMEHFMs increased firstly and then stabilized at 183 nm, which was correspond to the change of lamellar thickness in annealed hollow fibers. Whereas, the most probable pore size of large scale pores did not show a significant change trend. The porosity increased firstly and then decreased with the increase of annealing temperature, which was also correspond to the change of pure water fluxes of PMEHFMs. The porosity and pure water flux of PMEHFM reached the maximum when the annealing temperature was 150 °C. [Display omitted] •The hydrophilic PMEHFMs with bimodal pore structure are prepared by green melt-spinning and stretching.•The role of annealing temperature in controlling the dual-scale structure of bimodal pore of PMEHFMs is elucidated.•A model for the refinement of lamellae in hollow fiber during annealing is established.