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.

SYNOPSISThe preparation of membranes of nylon 4,6 by diffusion-induced phase separation (DIPS ) using formic acid as a solvent and water as a nonsolvent was studied. Nylon 4,6 is a semicrystalline polymer; phase separation from a solution can occur by solid-liquid (s-1) demixing as well as by liquid-liquid (1-1) demixing. Upon quenching films of solutions with low polymer concentration ( < 17 wt % ) in a nonsolvent bath containing water, the morphology of the membranes show a foam-like structure typical for 1-1 demixing. When phase separation is induced by water vapor a transition in structure occurs from the cellular type to a morphology typical for s-1 phase separated films. At higher polymer concentrations membranes exhibit structures consisting of spheres or smaller crystal-like units resulting from an s-1 phase separation process. The addition of 2 wt % or more of water to polymer solutions with low concentration (up to 15 wt % ) resulted in s-1 demixing as well. In a DIPS process s-1 demixing is kinetically competitive with 1-1 demixing if nuclei are already present in the starting solutions (heterogeneous nucleation), or if a relatively long time is available for crystal nuclei to be formed. The morphology resulting from s-1 demixing is a result of spherulitic crystallization. A certain concentration of nuclei or of precursor particles already present results in a small nucleation density during precipitation and thus large spherulites can be grown; at higher polymer and/or water concentrations the nucleation density increases resulting in an axialitic morphology of the membranes. I NTRO DUCT IONThe use of aliphatic polyamides, such as nylon 6, nylon 6,6, and nylon 11, as a membrane material is of growing importance: most applications are found in microfiltration' for which preparation of nylon membranes has been described in patents. It is expected that aliphatic polyamides will be used in a much broader field because of their favorable mechanical properties and chemical stability.Membranes can be prepared by various techniques one of which is immersion precipitation, a widely used technique.2 A polymer solution is cast * To whom all correspondence should be addressed. as a thin film and subsequently immersed in a nonsolvent bath, thereby inducing diffusion-controlled phase separation (DIPS). The liquid-liquid (1-1) demixing process that usually takes place for amorphous and low-crystalline polymers has been studied extensively in our l a b~r a t o r y .~-~ Solid-liquid ( s-1 ) demixing in membrane formation has also been des~ribed.67~ Lloyd et al. studied the thermally induced phase separation process (TIPS) of isotactic polypropylene and polyethylene solutions. In recent work an extensive study on the different possibilities of demixing processes of a binary system was presented.'-'' Our objective is to investigate the process of s-1 demixing behavior of films cast from a nylon 4,6 solution in a diffusion-induced demixing process in a ternary system.Crystallization phenomena in (quasi) ternary 13

The binary Flory-Huggins interaction parameters for the ternary membrane-forming system nylon, formic acid and water have been obtained from literature data, swelling values and melting point depression. Nylon 4,6 nylon 6 and a copolymer of nylon 4,6 and 6 were examined. The isothermal crystallization boundaries were determined experimentally and the binodal miscibility gap was calculated for these ternary systems. It was found that the crystalline state is the thermodynamically favourable state for each system. Experimental data and calculated phase diagrams are discussed in relation to membrane formation. Although the thermodynamic properties of the system dictate the phase separation that can take place, the kinetic parameters are of equal significance with respect to membrane formation.

Mass transfer during membrane formation by means of phase inversion for a polymeric system with both a solid-liquid and a liquid-liquid equilibrium was studied on the basis of the theory developed by Reuvers and Smolders. During the first moments of immersion in the coagulation bath, the concentrations at the interface between bath and film are governed by the virtual liquid-liquid equilibrium. This equilibrium no longer exists at a larger time scale. The interfacial concentrations as a result of the local liquid-liquid equilibrium during mass transfer are located deeply in the crystallization region or solid-liquid demixing area and after an induction time the solid-liquid phase separation (crystallization) takes place when membranes are formed with an initial polymer concentration of 20% or larger. The calculated initial concentration profiles show a shallow pattern in polymer content for the films with initial concentration of 20 and 25%. From the calculated initial concentration profiles an isotropic morphology in the final membrane can be expected. A steep increase of the polymer concentration at the interface was observed for the more concentrated films correlated with a skinned morphology in the final membrane.

Intermediate stages during membrane formation by means of immersion precipitation were studied by cryo-substitution for the system nylon 4,6, formic acid and water. The presence, nature and size of solid particles was determined as a function of time and of the distance from the interface. The spherulitic nature of these particles was confirmed by staining the samples. It was shown that at a relatively low nucleation density the concentration profile in the film was hardly influenced by a starting phase separation process, while in a situation with a relatively high number of nuclei per volume concentration patterns must be considerable altered.