Question

please provide me the tips to produce defect-free membrane (with references)

Thanks in advance

Answer 1

There are lots of methods to produce defect-free membranes.

1. A modified dual-bath method of spinning was employed to produce or fabricate defect-free membrane utilizing an immiscible liquid protective layer.

2. Thermally Induced Phase Separation technology is also a manufacturing method for these types of membranes. Temperature processing allows the preparation of a defect-free membrane with excellent mechanical performance.

3. These types of membranes are also formed by casting an appropriate drope followed by forced convective evaporation of solvent to obtain a dry phase separated asymmetrical structure. The structure is then washed in a precipitation liquid and dried. Advantage of the this production method is the excellent reproducibility of the thin selective layer. Moreover, these membranes can be used without post-treatments of any kind, thereby simplifying their production.

To obtain highly productive defect-free membranes according to this method, the proper dope formulation and nascent membrane formation procedures must be followed prior to immersion in a nonsolvent quench bath. Various post-formation procedures can be used in some cases to facilitate drying, to protect the thin selective layer, or to repair damage caused during or after the formation process.

The dope should contain (a) polymer/copolymer or polymer blend, (b) a solvent system, and (c) a nonsolvent system. The polymer or copolymer blend can be chosen from either commercially available glassy polymers or tailored materials with desirable intrinsic gas separation properties. The solvent system must be chosen to both fully dissolve the polymer and to tolerate the presence of the nonsolvent system so that the point of incipient phase separation can be reached while maintaining a dope rheology that is appropriate for casting or spinning the nascent membrane structure.

The solvent system must contain at least one component with a higher vapor pressure than any of the components of the nonsolvent system at the temperature of the nascent membrane formation.

On the other hand, the nonsolvent system must contain at least one component with a lower vapor pressure than the most volatile component in the solvent system at the temperature of the nascent membrane formation. Additional components in the dope with even lower vapor pressures than the most volatile component may be used to control the transport properties of the nascent membrane. This additional capability allows control of efflux rates of originally present dope components as well as influx rates of precipitating components.

Dry phase separation in the outer region of the nascent membrane is induced by the removal of volatile components such that the remaining solution in this thin region becomes thermodynamically unstable. The removal of volatile components to induce the dry phase separation can occur as a result of either free or preferably forced convection from the surface of the nascent membrane.

The use of forced convection to induce the dry phase separation yields superior membrane reproducibility and defect free selective layers.

The gas or vapor used as the convective agent can include any pure or mixed stream with the ability to remove volatile components from the surface of the nascent membrane. Free convection may require subsequent treatments to produce a defect-free selective layer.

Following the dry phase separation process, the nascent membrane must be immersed in a nonsolvent precipitation liquid to cause phase inversion, thereby producing an integrally-skinned asymmetric membrane with an ultra-thin separating layer.

The precipitation liquid can be either aqueous, or organic, inorganic or any mixture of aqueous, organic and inorganic components that are miscible with a sufficient fraction of the remaining dope components in the nascent membrane at the time of immersion to allow their removal into the bath to produce a self-sustaining membrane.

The selection of the precipitation liquid is important to obtaining acceptable properties in the final asymmetric structure. The preferred embodiment of a suitable precipitation liquid should show an absorption level in the neat polymer of less than 50% and preferably less than 30% and even more preferably less than 10%.

After the membranes have been removed from the precipitation bath, washed and dried, gas permeation test data indicate that the specific method used to induce dry phase separation has a strong influence on whether or not post treatments of the dried membranes are needed to achieve defect-free selective layers.