Phase separation processes in polymer solutions in relation to membrane formation

This review covers new experimental and theoretical physical research related to the formation of polymeric membranes by phase separation of a polymer solution, and to the morphology of these membranes. Two main phase separation processes for polymeric membrane formation are discussed: thermally induced phase separation and immersion precipitation. Special attention is paid to phase transitions like liquid-liquid demixing, crystallization, gelation, and vitrification, and their relation to membrane morphology. In addition, the mass transfer processes involved in immersion precipitation, and their influence on membrane morphology are discussed

Phase separation phenomena in solutions of polysulfone in mixtures of a solvent and a nonsolvent: relationship with membrane formation

The phase separation phenomena in ternary solutions of polysulfone (PSf) in mixtures of a solvent and a nonsolvent (N,N-dimethylacetamide (DMAc) and water, in most cases) are investigated. The liquid-liquid demixing gap is determined and it is shown that its location in the ternary phase diagram is mainly determined by the PSf-nonsolvent interaction parameter. The critical point in the PSf/DMAc/water system lies at a high polymer concentration of about 8% by weight. Calorimetric measurements with very concentrated PSf/DMAc/water solutions (prepared through liquid-liquid demixing, polymer concentration of the polymer-rich phase up to 60%) showed no heat effects in the temperature range of −20°C to 50°C. It is suggested that gelation in PSf systems is completely amorphous. The results are incorporated into a discussion of the formation of polysulfone membranes

Microstructures in phase-inversion membranes. Part I. Formation of macrovoids

A new mechanism for the formation of macrovoids in phase-inversion membranes is proposed. It is based on the observed difference in type of demixing of a thin film of a polymer solution when immersed in a nonsolvent bath: delayed or instantaneous demixing. The explanation for macrovoid formation assumes local conditions of delayed demixing in front of a certain layer of nuclei already formed, due to a change in the interfacial compositions at the advancing coagulation front, as compared to the original composition at the interface film-bath. Effects of variations in membrane formation conditions which can be calculated using the model of diffusive mass transport in thin films of polymeric solutions in combination with phase separation in phase-inversion membranes, completely support the mechanism as proposed

The formation of nodular structures in the top layer of ultrafiltration membranes

The formation of nodular structures in the top layer of ultrafiltration membranes is considered. A critical review of mechanisms described in the literature is given. Flat-sheet poly(ether sulfone) membranes and hollow-fiber poly(ether sulfone)/polyvinylpyrrolidone membranes were made by coagulation of a polymer solution in a nonsolvent medium under different circumstances. From these experiments, a number of empirical rules are found to describe the resulting morphology of the top layer. A new mechanism for the formation of a nodular structure is proposed. It is based on the small diffusion coefficient of the polymer molecules compared to the diffusion coefficient of solvent and nonsolvent combined with a high degree of entanglement of the polymer network. For unstable compositions, phase separation will proceed by growth in amplitude of concentration fluctuations. The rapid diffusional exchange of solvent for nonsolvent in the top layer leads to vitrification of the maxima of the concentration fluctuations that form the nodules. Complete disentanglement of the polymer chains between the nodules is not reached, which explains the small pores and the low porosity of ultrafiltration membranes

Membranes of semicrystalline aliphatic polyamide nylon 4,6: Formation by diffusion-induced phase separation

The 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 semi-crystalline polymer; phase separation from a solution can occur by solid-liquid (s-l) de-mixing as well as by liquid-liquid (l-l) 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 l-l demixing. When phase separation is induced by water vapor a transition in structure occurs from the cellular type to a morphology typical for s-l phase separated films. At higher polymer concentrations membranes exhibit structures consisting of spheres or smaller crystal-like units resulting from an s-l 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-l demixing as well. In a DIPS process s-l demixing is kinetically competitive with l-l 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-l 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

Metastable liquid-liquid and solid-liquid phase boundaries in polymer-solvent-nonsolvent systems

In general liquid-liquid demixing processes are responsible for the porous morphology of membranes obtained by immersion precipitation. For rapidly crystallizing polymers, solid-liquid demixing processes also generate porous morphologies. In this study, the interference of both phase transitions has been analyzed theoretically using the Flory-Huggins theory for ternary polymer solutions. It is demonstrated that four main thermodynamic and kinetic parameters are important for the structure formation in solution: the thermodynamic driving force for crystallization, the ratio of the molar volumes of the solvent and the nonsolvent, the polymer-solvent interaction parameter, and the rate of crystallization of the polymer compared to the rate of solvent-nonsolvent exchange. An analysis of the relevance of each of these parameters for the membrane morphology is presented

Integrally skinned polysulfone hollow fiber membranes for pervaporation

From polysulfone as polymer, integrally skinned hollow fiber membranes with a defect-free top layer have been spun. The spinning process described here differs from the traditional dry-wet spinning process where the fiber enters the coagulation bath after passing a certain air gap. In the present process, a specially designed tripple orifice spinneret has been used that allows spinning without contact with the air. This spinneret makes it possible to use two different nonsolvents subsequently. During the contact time with the first nonsolvent, the polymer concentration in the top layer is enhanced, after which the second coagulation bath causes further phase separation and solidification of the ultimate hollow fiber membrane. Top layers of ± 1 m have been obtained, supported by a porous sublayer. The effect of spinning parameters that might influence the membrane structure and, therefore, the membrane properties, are studied by scanning electron microscopy and pervaporation experiments, using a mixture of 80 wt % acetic acid and 20 wt % water at a temperature of 70°C. Higher fluxes as a result of a lower resistance in the substructure could be obtained by adding glycerol to the spinning dope, by decreasing the polymer concentration, and by adding a certain amount of solvent to the bore liquid. Other parameters studied are the type of the solvent in the spinning dope and the type of the first nonsolvent

Formation of porous membranes for drug delivery systems

Highly crystalline porous hollow poly (-lactide) (PLLA) fibres suitable for the delivery of various drugs were obtained using a dry-wet spinning process. The pore structure of the fibres could be regulated by changing the spinning systems and spinning conditions. Using the spinning system PLLA-dioxane-water, fibres with a dense toplayer and a spongy sublayer were obtained. The spinning system PLLA-chloroform/toluene-methanol yielded fibres with a very open porous structure. The membrane formation of the former system probably occurs by liquid-liquid demixing followed by crystallization of the polymer rich phase. In the membrane formation process of the spinning system, PLLA-chloroform/toluene-methanol crystallization probably plays a dominant role. The membrane formation process will be related to basic principles of phase separation. The fibres are suitable for the long term zero order delivery of the contraceptive 3-ketodesogestrel and the short term zero order delivery of the cytostatic agent, cisplatin. The drugs are released by dissolution of the drug crystals in the fibre core followed by diffusion through the membrane structure. Short term release of adriamycin could be obtained through an adsorption-desorption mechanism. The pore structures of the fibres have a large influence on the release rates of the drugs investigated. When fibres with dense toplayers were used, low release rates of drugs were observed whereas fibres with well interconnected pore structures over the fibre wall showed very high release rates

On the mechanism of separation of ethanol/water mixtures by pervaporation I. Calculations of concentration profiles

A solution—diffusion model for the permeation of liquid mixtures through polymeric membranes taking into account coupling of fluxes has been developed. The model is applied to the separation by pervaporation of ethanol—water mixtures through cellulose acetate. In order to determine the activities of the permeating components in the polymeric membrane, values of polymer—liquid and liquid—liquid interaction parameters are needed; polymer—liquid interaction parameters have been determined from swelling experiments and liquid—liquid interaction parameters have been calculated from excess free energy of mixing data taken from the literature.\ud \ud Concentration profiles of water and ethanol in cellulose acetate membranes have been calculated using (a) apparent concentration independent diffusion coefficients, and (b) diffusion coefficients with exponential concentration dependence and two adjustable parameters. It is discussed that the transport of ethanol—water mixtures by pervaporation cannot be explained by using concentration independent diffusion coefficient