(43d) Controlling the Self Assembly of Pores in Water Purification Membranes
Current ultrafiltration membranes are made using a phase inversion process. Micrographs of these membranes reveal structures which do not have a well defined pore size; instead the structure resembles overcooked spaghetti with the voids forming a polydisperse distribution of pore sizes. Among this distribution, the larger pores can compromise any desired separation of solutes.
One potential way to improve this separation is to reduce this polydispersity with more monodisperse pores. Such pores can be made using the structures formed by the self assembly of block copolymers. The structure of interest is that of cylinders of polylactide (PLA) surrounded by a continuous polystyrene matrix. The PLA can be selectively etched to make monodisperse pores. However, alignment of the PLA domains perpendicular to the plane of the membrane is a major hurdle to making these materials into viable membranes. Thermodynamics often favors cylinders which are parallel to the plane of the membrane, requiring the application of external forces (e.g. electric fields or surface treatments) to turn the cylinders to the desired perpendicular orientation.
This work kinetically traps the perpendicular cylinders by taking advantage of the concentration gradients which develop while solvent evaporates from a cast film. It thus circumvents the need for external fields. Experimental conditions found to produce 100 cm2 of membrane with cylinders perpendicular to the plane of the membrane are identified. Moreover, a theory predicting when these cylinders should form is developed and verified. Thin films made using this technique show rejection of dissolved polyethylene glycol solutes consistent with monodisperse 23 nm pores.
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