A new mechanism for the formation of macrovolds m phase-mverslon membranes IS proposed It 1s based on the observed difference m type of demlxmg of a thm film of a polymer solution when immersed m a nonsolvent bath delayed or instantaneous demlxmg The explanation for macrovold formation assumes local condltlons of delayed demlxmg m front of a certain layer of nuclei already formed, due to a change m mterfaclal composltlons at the advancing coagulation front, as compared to the ongnal composltlon at the interface film-bath Effects of vanatlons m membrane formation condltlons which can be calculated using the model of diffusive mass transport m thm films of polymeric solutions m combmatlon with phase separation m phase-mverslon membranes, completely support the mechanism as proposed

Using equations and boundary conditions derived in Part I*, calculations have been performed on the ternary diffusion processes that occur in acellulose acetate (CA) -acetone casting solution immersed into a water bath. The necessary concentration-dependent thermodynamic and hydrodynamic parameters have been derived from experimental data on the three limiting binary mixtures. Calculations show that immersion of the polymer solution into the coagulation bath results in an instantaneous increase of the polymer concentration at the surface of the solution. For a CA-acetone casting solution the thickness of this concentrated surface layer will increase until the onset of liquid-liquid demixing by means of nucleation and growth of the diluted phase that fixes the asymmetric polymer distribution in the film. The moment of onset of the demixing process depends on the thickness of the film. However, addition of a certain minimum amount of water to the casting solution results in an instantaneous onset of liquid-liquid demixing upon immersion, yielding a membrane with a very thin skin layer. The model calculations have been confirmed by light transmission measurements performed on immersed casting solutions. *J. Membrane Sci., 34 (1987) 45. 0376-7388/87/$X3.50 0 1987 Elsevier Science Publishers B.V.

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SynopsisThe demixing behavior on cooling of ternary systems of cellulose acetate/solvent/water has been examined for CA concentrations up to 40 wt% CA in several solvents. Cloud points have been measured as a function of cooling rate. The rapid process of liquid-liquid demixing can be discriminated from the slow process of aggregate formation by examining the dependence of the cloud point on the cooling rate and by structure analysis of quenched solutions with scanning electron microscopy. The appearance of aggregate formation depends strongly on the type of solvent. Slow cooling of ternary solutions in which acetone is the solvent leads to aggregate formation long before liquid-liquid demixing occurs.In addition, isothermal sol-gel transitions have been measured for quenched solutions at varying gelation times. It is concluded that gelation is not always preceded by aggregate formation.

A new method for the preparation of gas separation membranes in a one-step procedure is presented, where common, non-volatile solvents can be used in the polymer solution. It concerns contacting of a polymer solution with two successive nonsolvent baths, whereby the first bath initiates the formation of a dense top layer and the second bath gives the actual polymer precipitation. Membranes made by this method will have high gas selectivity and do not need any additional coating. The new technique was used to make polyethersulfone (PES) hollow fibres from solutions consisting of 35% (w/w) polymer and 10% glycerol in N-methylpyrrolidone (NMP). High selectivities were obtained when using glycerol or 1-pentanol as the first nonsolvent and water as the second one. For a feed gas of 25 vol.% of CO, in methane the intrinsic selectivity of PES [a (CO&H,) x 501 was easily obtained, without the necessity of an additional coating step. By a step-wise, liquid exchange removal of residual fluids in the fibres, an improvement in flux could be obtained. This was accompanied by a somewhat lower selectivity compared to that of directly air-dried fibres.