An Investigation of Pore Collapse in Asymmetric Polysulfone Membranes

Abstract

Porous polysulfone membranes prepared by phase inversion can be tailored to suit filtration requirements by the choice of solvent and coagulant. In the current research polysulfone membranes were prepared by inverting a solution in N-methyl pyrrolidinone (NMP) in isopropanol to form uniform sized pores. Phase inversion resulted in the formation of an asymmetric membrane. The membranes have a characteristic "skin" which is supported by a highly porous substructure. Water-wet membranes experience capillary force during water evaporation. Since the modulus of the membranes is lower than the capillary force, the membrane walls shrink and thicken giving rise to a condensed structure.

The "skin" regulates permeation through the membranes which is essential for filtration. A change in the pore structure of the skin alters the permeability. The current research investigates the influence of amine plasma treatments on the surface pore structure of polysulfone membranes. The permeation of a rhodamine dye through the plasma treated membranes and through non-plasma treated membranes is used to examine the influence of the plasma treatment. Furthermore, the influence of plasma treatment on the loss of water from the membranes leading to pore collapse is also explored.

The study revealed that a plasma ablates the skin, increasing the permeation. An ammonia plasma treatment produced more etching, and hence increased permeation compared to permeation for an aniline plasma-treated membrane. A one-minute aniline plasma treatment only caused a moderate increase in permeation. Plasma treatments introduced significant surface modification by the introduction of new functionalities. However, permeation was not influenced by the surface modification.

Water trapped in the pores is essential to maintain the pore structure of the membrane. The surface treatment dictates the pore size and therefore, the convection allowing water evaporation, leading to pore collapse. Heat treating also increases the rate of water removal. Using thermogravimetric analysis (TGA) and scanning electron microscopy (SEM) the role of heat and surface treatments on the extent of pore collapse was investigated. The ammonia plasma treated samples showed maximum water loss and the control samples showed a minimum loss of water when stored at room temperature. All the samples stored at 90 °C exhibited equivalent water loss. Water loss was not affected by the plasma treatments.