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In this work, poly (vinylidene fluoride) (PVDF) hierarchically porous membranes (HPMs) with isolated large pores and continuous narrow nano-pores have been fabricated from its blend with poly (methyl methacrylate) (PMMA) based on the combination of crystallization template with chemical or supercritical CO2 foaming. On the one hand, the decomposition of azodicarbonamide (ADC, chemical foaming agent) or the release of CO2 can produce isolated large pores. On the other hand, PMMA is expelled during the isothermal crystallization of PVDF in their miscible blend, yielding narrow nano-pores upon etching with a selective solvent. In the case of supercritical CO2, the attained PVDF HPMs fail to improve separation performance because of the compact wall of isolated-large-pore and consequent poor connectivity of hierarchical pores. In the case of ADC, the optimal HPM exhibits much higher flux (up to 20 times) without any loss of selectivity compared with the reference only with nano-pores. The enhanced permeability can be attributed to the shorter diffusion length and lower diffusion barrier from isolated large pores, while the comparable selectivity is determined by narrow nano-pores in THE matrix. [ABSTRACT FROM AUTHOR]

In this work, separation performance of hierarchically porous membranes (HPMs) has been investigated with the combination of experiment and finite element simulation. From the blend of poly (vinylidene fluoride), poly (methyl methacrylate) and sodium chloride (i.e., PVDF/PMMA/NaCl), PVDF HPMs with cubic-large-pores connected through narrow nanopores have been fabricated successfully by etching NaCl and PMMA respectively. In the case of extremely high fill of NaCl, the inorganic fillers distribute all-over the specimen, yielding simultaneously internal and surface cubic-large-pores. Relative to the reference with only nanopores, the attained HPMs exhibit much higher permeability (up to ∼200 times) without any loss of selectivity. Such a significant improvement of separation performance can be attributed to the synergism effect between internal and surface cubic-large-pores. On one hand, the high content of isolated large pores in HPMs corresponds to shorter diffusion length and lower diffusion barrier for fluid; On the other hand, the existence of cubic-large-pores on membrane surface produces not only bigger pores and higher porosity on membrane surface, corresponding to convenient diffusion pathway, but also the enhanced hierarchical roughness including narrow nanopores and cubic-large-pores, providing additional driving force for water/oil separation. [Display omitted] • Simulation results clarify the synergism effect between internal and surface cubic-large-pores. • PVDF HPMs with both internal/surface cubic-large-pores and narrow nanopores have been fabricated successfully. • The attained PVDF HPMs exhibit higher flux without any loss of rejection ratio during separation. [ABSTRACT FROM AUTHOR]