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Membrane formation via the combination of self-assembly and the non-solvent induced phase separation (NIPS) process of diblock copolymers is investigated. Several polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) diblock copolymers with different molecular weights and weight percentages of both blocks are tested under different parameters, leading to membrane surfaces with uniform pores of approximately 20–70 nm diameter. The average pore diameter is proved to be adjustable by changing the total molar mass of the block copolymer. The solution composition is an additional parameter controlling the structure formation. The purpose was to explore the upper and lower limits of the membrane structure formation by varying the molecular weight and the composition of the block copolymer. Scanning electron microscopy (SEM) is used to image the surface morphology and the homogeneity of the pore sizes. Primary results of water flux and retention are presented.

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 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 porous membranes with cylinder-forming on the top layer of integral asymmetric membranes. In this work, another example of this effort is shown by manufacturing of nanoporous hollow fibre 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 fibre is discussed, such as type of block copolymer, 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.