Abstract
A facile method combining micro-molding with thermally-induced phase separation (TIPS) to prepare superhydrophobic isotacticpolypropylene (iPP) microporous membranes with micron-submicron hierarchical structures is proposed in this paper. In this study, the hydrophobicity of the membrane was controlled by changing the size of micro-structures on the micro-structured mold and the temperature of the cooling bath. The best superhydrophobicity was achieved with a high water contact angle (WCA) of 161° and roll-off angle of 2°, which was similar to the lotus effect. The permeability of the membrane was greatly improved and the mechanical properties were maintained. The membrane prepared by the new method and subjected to 60h vacuum membrane distillation (VMD) was compared with a conventional iPP membrane prepared via the TIPS process. The flux of the former membrane was 31.2 kg/m2·h, and salt rejection was always higher than 99.95%, which was obviously higher than that of the latter membrane. The deposition of surface fouling on the former membrane was less and loose, and that of the latter membrane was greater and steady, which was attributed to the micron-submicron hierarchical structure of the former and the single submicron-structure of the latter. Additionally, the new method is expected to become a feasible and economical method for producing an ideal membrane for membrane distillation (MD) on a large scale.
Highlights
Membrane distillation (MD) is a thermally driven process that uses hydrophobic porous membranes as the barrier
Superhydrophobic iPP microporous membranes were fabricated by a novel approach of micro-molding & thermally-induced phase separation (TIPS)
This newly developed process ensured that the membrane surface replicated the micro-structures from the micro-structured mold, and the submicron-structures generated from the TIPS process “grew on” the micro-structures
Summary
Membrane distillation (MD) is a thermally driven process that uses hydrophobic porous membranes as the barrier. The driving force lies in the vapor pressure difference between porous hydrophobic membrane surfaces, which allows the volatile components in the solution to permeate to another side [1,2,3]. Several conventional and commercial hydrophobic materials, such as polyvinylidenefluoride (PVDF), polytetrafluoroethylene (PTFE), and isotacticpolypropylene (iPP), are used to prepare MD-related membranes [6,7]. Pore wetting and membrane fouling occur when the hydrophobicity of the membrane is insufficient, which causes failure of the membrane distillation process, and degrades the salt rejection [8,9]. It is a continuing challenge to fabricate MD membranes with anti-wetting or superhydrophobic characteristics [10]
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