Dual-template approach to hierarchically porous polymer membranes

Abstract

The non-solvent induced phase separation (NIPS) method is often used to fabricate porous polymer membranes that have a wide variety of applications in the fields of purification and energy materials. Such applications typically require membranes with a well-controlled pore structure across multiple levels of structural hierarchy. However, with the NIPS method, the membrane porosity is typically restricted to a single structural level due to the use of just one type of templating agent, commonly a polymer amphiphile. To overcome this limitation, here we report a new dual-template approach to embed multilevel porosity into poly(ethersulfone) membranes. In this approach, the polymer amphiphile Pluronic® F127 (F127) is used to template the higher level structure (10 μm scale), while a secondary molecular template, 4-(phenyl)azobenzoic acid (PABA), is simultaneously introduced to enable structural control at the 1 μm level. Analysis of the membrane structure by scanning electron microscopy revealed that F127 creates larger finger-like pores, while PABA introduced a more fibrous, interconnected porous sub-structure. Contact angle measurements showed that the PABA additive increases the hydrophilicity of the resultant membranes, while dead-end permeability measurements demonstrated that the hierarchical pore structure results in a 95% increase in permeability compared to a single level pore structure. Solvent extraction studies revealed that quantitative removal of the PABA template could be achieved, without any modification to the secondary level pore structure. This process offered the benefit of removing flakes of crystallized PABA from the larger, finger-like pores, leading to a further 300% increase in permeability. Given the simplicity and versatility of the NIPS method, this dual-template approach presents an efficient route to hierarchically structured, porous polymer membranes whose properties may be tailored for a targeted application.