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

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

In situ analysis of solvent/nonsolvent exchange and phase separation processes during the membrane formation of polylactides

Membrane formation of polylactides has been studied using in situ analysis techniques. An experimental method based on the use of dark ground optics and reflected light illumination is used to monitor the mass transfer and phase separation dynamics during for mation. Additionally, the phase separation and structure formation has been studied using optical microscopy. The results of the dark ground optics technique for the polymer/solvent/nonsolvent systems poly-L-lactide/chloroform/methanol and poly-DL-lactide/chloroform/methanol showed that the diffusion kinetics were similar for the semicrystalline poly-L-lactide (PLLA) and the amorphous poly-DL-lactide. The influence of the molecular weight of the polymers on the diffusion kinetics was found to be negligible. Increasing the polymer concentration of the casting solution decreased the rate of diffusion. The phase separation of poly-DL-lactide was studied with optical microscopy and found to proceed via liquid-liquid demixing. For poly-L-lactide solutions of relatively low concentration (5-6% w/w), phase separation proceeded via liquid-liquid demixing followed by crystallization. For more concentrated PLLA solutions, phase separation proceeded directly via solid-liquid demixing processes. Additionally, for 6% w/w solutions of poly-L-lactide in dioxane immersed in methanol, precipitation also occurred solely via solid-liquid demixing

Phase transitions during membrane formation of polylactides. I. A morphological study of membranes obtained from the system polylactide-chloroform-methanol

The influence of solid-liquid demixing, liquid-liquid demixing and vitrification on the morphology of polylactide membranes has been investigated. To study the effects of crystallization of polylactides on the membrane and morphology, polylactides of varying stereoregularity were used. The polymers applied were poly--lactide (PLLA) and copolymers with different molar ratios of -lactide and -lactide [poly-L95/D5-lactide (PLA95), poly-L80/D20-lactide (PLA80) and poly-L50/D50-lactide (PDLLA)]. Solutions of polylactides in chloroform cast on a glass plate were immersed in methanol. From solutions containing the slowly crystallizing PLA80 or uncrystallizable PDLLA porous membranes were obtained if the phase separated system was removed from the nonsolvent bath within a few hours after immersion. After longer equilibration times in methanol the structure collapsed. The swelling in the nonsolvent methanol was too high to allow stabilization of the liquid-liquid demixed structure by vitrification. Stable membranes were easily obtained with more rapidly crystallizing polymers like PLLA. Casting solutions with low PLLA concentrations gave membranes with a cellular morphology due to liquid-liquid demixing by nucleation and growth of a polymer poor phase. Crystallization only played a role in the fixation of the liquid-liquid demixed structure. At increasing PLLA concentrations the demixing sequence gradually reversed to crystallization followed by liquid-liquid demixing. In these cases membranes with porous spherulites or spherulites surrounded with a cellular layer were obtained

A morphological study of membranes obtained from the systems polylactide-dioxane-methanol, polylactide-dioxane-water and polylactide-N-methyl pyrrolidone-water

The influence of liquid-liquid demixing, solid-liquid demixing, and vitrification on the membrane morphologies obtained from several polylactide-solvent-nonsolvent systems has been investigated. The polymers investigated were the semicrystalline poly-L-lactide (PLLA) and the amorphous poly-DL-lactide (PDLLA). The solvent-nonsolvent systems used were dioxane-water, N-methyl pyrrolidone-water and dioxane-methanol. For each of these systems it was attempted to relate the membrane morphology to the ternary phase diagram at 25°C. It was demonstrated that for the amorphous poly-DL-lactide the intersection of a glass transition and a liquid-liquid miscibility gap in the phase diagram was a prerequisite for the formation of stable membrane structures. For the semicrystalline PLLA a wide variety of morphologies could be obtained ranging from cellular to spherulitical structures. For membrane-forming combinations that show delayed demixing, trends expected on the basis of phase diagrams were in reasonable agreement with the observed membrane morphologies. Only for the rapidly precipitating system PLLA-N-methyl pyrrolidone-water were structures due to liquid-liquid demixing obtained when structures due to solid-liquid demixing were expected. Probably, rapid precipitation conditions promote solid-liquid demixing over liquid-liquid demixing, because the activation energy necessary for liquid-liquid demixing is lower than that for crystallization

Block copolymers of poly(L-lactide) and poly(ε-caprolactone) or poly(ethylene glycol) prepared by reactive extrusion

Blends of poly(L-lactide) (PLLA) and poly(-caprolactone) (PCL) were prepared in a co-rotating twin screw miniextruder (40 rpm, 200°). It was attempted to prepare multiblock copolymers by allowing a controlled number of transesterification reactions. Various cat-alysts (n-Bu3SnOMe, Sn(Oct)2, Ti(OBu)4, Y(Oct)3, para-toluene sulphonic acid) were introduced to promote these transesterification reactions. However, PLLA degradation by ring-closing depolymerization was the dominant reaction in every case. Alternatively, after showing that L-lactide can be conveniently polymerized in the extruder, L-lactide and hy-droxyl functionalized prepolymers of PCL or poly(ethylene glycol) (PEG) were fed to the extruder in the presence of stannous octoate. Monomer conversions of over 90% and effective transformation of all hydroxyl end groups present were generally reached. Di-and triblock copolymers could be prepared in this way with characteristics very similar to polymers prepared in a batch-type process, but with considerably reduced reaction times in a fashion, which is, in principle, scaleable to a continuous process for the production of such block copolymers

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

Modification of the twist angle in chiral nematic polymer films by photoisomerization of the chiral dopant

A method for the production of polarization sensitive recordings in liquid crystalline polymers is presented. The system is based on local modification of the twist angle of chiral nematic polymer films. The twist angle of the polymer film is varied by modifying the chemical structure of the chiral dopant. Here a photoisomerizable menthone derivative is used that has a fivefold difference in helical twisting power between the E and the Z isomer. The twist angle of the film can be varied between –90° and 0° by introducing a nonphotosensitive chiral dopant with opposite twisting sense. Complex pictures with gray scales can be recorded in the films with black and white contrasts higher than 20