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

1.5 min. Nishi et al. obtained a similar result.* As can be seen in Figure 7, a large change in concentration at very short times does not lead to a large change in the ratio of excimer-to-monomer fluorescence. As a result of this reduced sensitivity, the value of the dominant growth rate R(&) may be determined only to within a factor of 2. From the slope of the line drawn through the short-time data in Figure 9, R((3,) = 0.9 f 0.3 min-'. Nishi et al. obtained a growth rate for a 50:50 blend with higher molecular weight components annealed at 403 K which was an order of magnitude slower. The diffusion coefficient describing the decomposition process can be calculated provided the wavelength of the dominant concentration fluctuation is known. From optical microscopy, this wavelength has been determined to be of the order of 1 pm.2 From eq 9, the diffusion coefficient is then found to be of the order of -lo-" cm2/s.Finally, the fluorescence results show that phase growth is still taking place at long times where Cahn's model does not hold. From Table I, it appears that the growth rate at long times is several orders of magnitude slower than the rate during the early stages of decomposition. ConclusionsThe technique of excimer fluorescence can be used to study quantitatively the kinetics of phase separation in polymer blends. For a 10% PS/PVME blend annealed at 423 K, Cahn's model for spinodal decomposition adequately describes the process at short times, although the fluorescence results indicate that equilibrium is not reached during this period. The growth rate appears to be several orders of magnitude larger at short times than during the later stages of decomposition. ABSTRACT: A numerical method for the calculation of the binodal of liquid-liquid phase separation in a ternary system is described. The Flory-Huggins theory for three-component systems is used. Binodals are calculated for polymer/solvent/nonsolvent systems which are used in the preparation of asymmetric ultrafiltration or reverse osmosis membranes: cellulose acetate/solvent/water and polysulfone/solvent/water. The values for the binary interaction parameters are taken from literature sources. The effect of a concentration-dependent solvent/nonsolvent interaction parameter is discussed. Although knowledge of the interaction parameters for all compositions in the ternary system is rather poor, fairly good agreement has been found between calculated and experimentally found miscibility gaps when the solvent/nonsolvent parameter is taken to be concentration dependent and the other parameters, the polymer/solvent and the polymer/ nonsolvent interaction parameter, are kept constant. Acknowledgment

Membranes were prepared from a castmg solution of a water-soluble polymer, poly (vmyl pyrrohdone) (PVP), and a membrane forming polymer, poly (ether sulfone), m l-methyl-2-pyrrohdone (NMP) as solvent by immersing them m mixtures of water and NMP It was found that the addltlon of PVP to the ternary system suppresses the formatlon of macrovolds m the sub-layer, while the ultraflltratlon-type top-layer consists of a closely packed layer of nodules Using a model for mass transfer m this quaternary system, it 1s possible to explam the effects of the additive on macrovold formation Strong mdlcatlons are found that the appearance of a nodular structure m the top-layer follows a mechanism of spmodal decomposltlon dunng the very early stages of the lmmerslon step Keywords membrane preparation and structure, theory, thermodynamics, macrovolds, phase-mverslon membranes [ 1,2] In recent, years, Reuvers et al [ 1 ] developed a model for the descrlptlon of mass transfer during the lmmerslon step Two types of de

Structural characteristics in membranes formed by diffusion induced phase separation processes are discussed. Established theories on membrane formation from ternary systems can be extended to describe the effects of high or low molecular weight additives. A mechanism for the formation of nodular structures in the top layer of ultrafiltration membranes is presented. In the last part structures arising from polymer crystallization during immersion precipitation are discussed.

SummaryMembrane distillation can only be applied on liquid mixtures which do not wet, a microporous hydrophobic membrane. Solutions of inorganic material in water have such high values of surface tension ( y,_ 2 72 X lo-" N/m) that the non-wetting condition is fulfilled for a number of hydrophobic membranes. As soon as orgamc solutes are present m the solution, the surface tension ;>I> will be lowered, and if the concentration of organic material becomes too high, wetting of the membrane will occur. By means of theoretical considerations a critical solute concentration or surface tension at which a homogeneous smooth material will be wetted (0 < 90' ) can be calculated. For (micro)porous membranes no such theoretical relation can be derived. Therefore, a simple experimental method is described to measure the maximum allowable concentration for a (micro)porous membrane. On the basis of these measurements, the maximum allowable concentration under process conditions can be determined