No abstract

A simple way to generate isoporous membranes with tailored pore sizes is shown. Block copolymers of different compositions are blended in solution, and membranes are obtained by solution casting followed by nonsolvent-induced phase separation. This enables the preparation of integral asymmetric membranes with a defined pore size for given sets of block copolymers just by choosing the right blend composition.

Nanoporous membranes with tailored size pores and multifunctionality derived from self-assembled block copolymers attract growing interest in ultrafiltration. The influence of the structure of block copolymer in the membrane casting solution on the formation of integral asymmetric isoporous block copolymer membranes using the nonsolvent induced phase separation process (NIPS) has been one of the long-standing questions in this research area. In this work we studied the principal role of the solvent on the micellization and self-assembly of asymmetric polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) diblock copolymers by using a combination of dynamic light scattering (DLS) and small-angle X-ray scattering (SAXS). Our results indicate a significant effect of the solvent selectivity on the optimal casting concentration and solution structure. In addition, morphological characterization of the resulting membranes demonstrates considerable influence of the solvent system on the ordering and uniformity of the pores and pore characteristics in the separation layer as well as porous substructure of the final membranes.

Isoporous asymmetric polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) hollow fiber membranes were successfully made by a dry-jet wet spinning process. Well-defined nanometer-scale pores around 20-40 nm in diameter were tailored on the top surface of the fiber above a non-ordered macroporous layer by combining block copolymer self-assembly and non-solvent induced phase separation (SNIPS). Uniformity of the surface-assembled pores and fiber cross-section morphology was improved by adjusting the solution concentration, solvent composition as well as some important spinning parameters such as bore fluid flow rate, polymer solution flow rate and air gap distance between the spinneret and the precipitation bath. The formation of the well-organized self-assembled pores is a result of the interplay of fast relaxation of the shear-induced oriented block copolymer chains, the rapid evaporation of the solvent mixture on the outer surface and solvent extraction into the bore liquid on the lumen side, and gravity force during spinning. Structural features of the block copolymer solutions were investigated by small angle X-ray scattering (SAXS) and rheological properties of the solutions were examined as well. The scattering patterns of the optimal solutions for membrane formation indicate a disordered phase which is very close to the disorder-order transition. The nanostructured surface and cross-section morphology of the membranes were characterized by 2 scanning electronic microscopy (SEM). The water flux of the membranes was measured and gas permeation was examined to test the pressure stability of the hollow fibers.

The self-assembly of block copolymers proposes interesting strategies for design and fabrication of ordered nano/microdomain structures. Recently, large attention has been given to the preparation of integral asymmetric flat sheet membranes with cylindrical domains forming an isoporous top layer. In this work, this strategy is extended to the formation of nanoporous hollow fiber membranes from polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) solutions via phase inversion process. In this way, the self-assembly of block copolymers into an ordered morphology via solvent evaporation is combined with microdomain alignment by shear flow in the die. The influence of the experimental parameters on the morphology of the hollow fiber is discussed, such as solution concentration and viscosity, extrusion pressure within the spinneret and air gap distance between the spinneret and the precipitation bath (evaporation time). The evaluation of the surface morphology of the membranes by scanning electron microscopy (SEM) confirms the strong effect of shear flow and solution viscosity on the formation of nanoporous structures in hollow fiber spinning.