Skin layer ultrathin defect-free

An ideal PV membrane consists of an ultrathin defect-free nonporous skin supported by a porous support. The selectivity of the membrane is derived from this skin layer. However, the porous support also plays an important role in the overall performance of the membrane as discussed later. 

Asymmetric polyimide membranes with an ultrathin defect-free skin layer were fabricated by the dry-wet process [15]. Composition of casting solution used for the preparation of asymmetric membranes was 12 wt.% polyimide, 55 wt.% methylene chloride, 23 wt.% 1,1,2-trichloroethene, and 10 wt.% butanol. In the dry process (solvent evaporation) the evaporation period was changed from 15 to 600 s, while in the wet process (coagulation process) the coagulation media was methanol. It was possible to control the thickness of the skin layer by controlling the evaporation period. From this AFM study, it was observed that the nodule formation was controlled by evaporation time, while the coagulation media controlled the roughness parameter. 

In this regard, there is an excellent review article on MMMs for gas separation, with a detailed discussion on the morphology of the interface between the inorganic particles and the polymer matrix (Chung et al. 2007). Unlike many other articles, this deals with asymmetric membranes for both flat sheets and hollow fibers aimed at the formation of an ultrathin defect-free mixed-matrix skin layer. 

Effect of Evaporation Condition Previous studies on more traditional applications have investigated the effect of increased air velocity, that is, forced-convection conditions for a combination of dry/wet phase inversion techniques to produce defect-free, ultrahigh flux asymmetric membranes with ultrathin skin layers [115-117]. To investigate the effect of evaporation condition on the release rate of drug, tablets were dip coated with CA solution containing 10% CA, 80% acetone, and 10% water and allowed to dry by blowing air across the surface with a blower (forced convection). As a comparison, tablets coated with the same solution were air dried under natural free-convection conditions. 

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