Hierarchically porous membranes with isolated-round-pores connected by narrow-nanopores: A novel solution for trade-off effect in separation

Abstract

In this work, a concept of hierarchically porous membranes (HPMs) with isolated-round-pores connected by narrow-nanopores whose separation performance could be enhanced significantly is reported. On one hand, the simulation results provide the direct evidence that the existence of isolated-round-pores can shorten diffusion length and minimize diffusion barrier, which is the reason for the higher flux relative to the reference with only narrow-nanopores; on the other hand, HPMs have been fabricated successfully in ternary blend of PVDF/PMMA/PLLA. Due to the special miscibility in the blend, phase separation produces PLLA islands and PVDF/PMMA matrix. In the matrix, PMMA has been expelled out during the crystallization of PVDF, resulting in bi-continuous structures of them in nanometers. After the extraction with chloroform, the narrow-nanopores (from nano-continuous PMMA) connect the isolated-round-pores (from islands of PLLA) with each other, yielding interpenetrated micro-channels. During separation with HPMs, the selectivity is determined by porous structures in matrix. It has been further improved by the width of the anisotropic narrow pores, contributing to the high rejection ratio. The permeability has been enhanced remarkably via shorter diffusion length and lower diffusion barrier resulted from the isolated-round-pores. Our result provides a novel solution for trade-off effect between permeability and selectivity in separation.