Hierarchically porous membranes with multiple channels: Fabrications in PVDF/PMMA/PLLA blend and enhanced separation performance

In this work, separation performance of hierarchically porous membranes (HPMs) with multiple channels have been investigated with the combination of finite element simulation and experiment. The simulation results indicate that in addition to m-channels (i.e., channels in microns) in reference (with only pores in microns), nanopores in HPMs not only contribute to n-channels (channels in nanometers), but also connect them with m-channels, producing mixed channels. In experiment, PVDF HPMs have been prepared in poly(vinylidene fluoride)/poly(methyl methacrylate)/poly(l-lactic acid) (PVDF/PMMA/PLLA) blend based on phase separation and crystallization templates. Phase separation produces bi-continuous structures of PLLA and PVDF/PMMA with size of microns. In the latter, PMMA has been expelled out during the crystallization of PVDF, resulting in the bi-continuous structures of them in nanometers. Upon etching with chloroform, both PLLA and PMMA have been removed, yielding pores in microns and nanometers respectively. Relative to reference, HPMs exhibit enhanced separation performance, which can be attributed to the shorter diffusion length, extra channels, enlarged contact angle difference and enhanced driving force for separation. Both simulation and experiment results indicate that multiple channels play more important role in the improvement of flux in the case of smaller m-channel diameter. Our results provide an efficient solution for trade-off effect in separation based on HPMs with multiple channels.