Polymer-poor Phase Research Articles

Segregative phase separation of strong polyelectrolyte complexes at high salt and high polymer concentrations.

The phase behavior of polyelectrolyte complexes and coacervates (PECs) at low salt concentrations has been well characterized, but their behavior at concentrations well above the binodal is not well understood. Here, we investigate the phase behavior of stoichiometric poly(styrene sulfonate)/poly(diallyldimethylammonium) mixtures at high salt and high polymer concentrations. Samples were prepared by direct mixing of PSS/PDADMA PECs, water, and salt (KBr). Phase separation was observed at salt concentrations approximately 1 M above the binodal. Characterization by thermogravimetric analysis, FTIR, and NMR revealed that both phases contained significant amounts of polymer, and that the polymer-rich phase was enriched in PSS, while the polymer-poor phase was enriched in PDADMA. These results suggest that high salt concentrations drive salting out of the more hydrophobic polyelectrolyte (PSS), consistent with behavior observed in weak polyelectrolyte systems. Interestingly, at the highest salt and polymer concentrations studied, the polymer-rich phase contained both PSS and PDADMA, suggesting that high salt concentrations can drive salting out of partially-neutralized complexes as well. Characterization of the behavior of PECs in the high concentration limit appears to be a fruitful avenue for deepening fundamental understanding of the molecular-scale factors driving phase separation in these systems.

Spray-dried paracetamol/polyvinylpyrrolidone amorphous solid dispersions: Part II – Solubility and in vitro drug permeation behavior

Amorphous solid dispersions (ASDs) comprising an active pharmaceutical ingredient (API) and polymer are a frequently described approach in the formulation of new drug candidates. This study aimed to evaluate the saturation solubility and dissolution behavior of ASDs consisting of paracetamol (PCM) and polyvinylpyrrolidone/vinyl acetate (PVP/VA) in water and their influence on the transepithelial in vitro permeation of PCM. With increasing amounts of PVP/VA, the water solubility of ASDs containing PCM increased up to six times compared to that of a saturated PCM solution. In the case of preparations with 30 % PCM, two-phase separation was observed in water at room temperature, consisting of a polymer-rich phase with high API loading and an aqueous, polymer-poor phase. This result was attributed to the thermoresponsive behavior of PVP/VA with lower critical solution temperature (LCST). As the PCM content in the ASD increased, the LCST decreased. This behavior was analyzed by measuring the demixing temperature (Tdem) values with differential scanning calorimetry (DSC). Furthermore, the permeation behavior of PCM from these phase-separated preparations through Caco–2 cells was analyzed. Additionally, the effect of these preparations on cell viability was evaluated using the MTT assay. Preparations with relatively high PCM concentrations showed a reduction in cell viability.

Thermodynamic analysis of polymer solutions for the production of polymeric membranes

Phase separation is a commonly used mechanism for the preparation of porous filtration membranes. In order to control the membrane structures for obtaining required membrane characteristics and performances, the examination of the thermodynamics of the membrane formation mechanisms is essential. This is why several studies have already been conducted to determine the phase diagrams of polymeric casting solution systems. However, most of the commonly used methods have certain limitations. This is the reason why a new method for the investigation of ternary polymeric systems was developed and evaluated in this study. The new method provides reproducible data which does not only provide information on the position of the binodal curve but also on the compositions of the phases which are formed after the phase separation. The tie-lines of the ternary system polyethersulfone/N-methyl-2-pyrrolidon/water were determined at different temperatures and compared to cloud point titrations conducted under the same experimental conditions. It could be shown that the location of the miscibility gap of the examined system is not visibly dependent on the phase separation temperature within the examined temperature range, especially in the region of the polymer-poor phases. This finding was comparable in all experiments, irrespectively of the method which was used. However, in the region of the polymer-rich phases within the phase diagram, the results of both methods differ from each other as the binodal determined by the tie-line method showed a temperature-dependent shift which cannot be found for the binodal curves extrapolated from cloud point measurements. Apart from determining the binodal curves, the molecular weight distributions of the polymer in the polymer-poor and the polymer-rich phases were determined in the frame of the tie-line determination. Hereby it was found that the distribution in the polymer-poor phase shows a temperature-dependence, as the average molecular weight raised from around 6 kDa at 10 °C to above 10 kDa at 40 °C.

Vertical Phase Segregation Induced by Dipolar Interactions in Planar Polymer Brushes

We present a generalized theory for studying structural properties of a planar dipolar polymer brush immersed in a polar solvent. We show that an explicit treatment of the dipolar interactions yields a macroscopic concentration dependent effective “chi” (the Flory–Huggins-like interaction) parameter. Furthermore, it is shown that the concentration dependent chi parameter promotes phase segregation in polymer solutions and brushes so that the polymer-poor phase consists of a finite/nonzero polymer concentration. Such a destabilization of the homogeneous phase by the dipolar interactions appears as vertical phase segregation in a planar polymer brush. In a vertically phase segregated polymer brush, the polymer-rich phase near the grafting surface coexists with the polymer-poor phase at the other end. Predictions of the theory are directly compared with prior reported experimental results for dipolar polymers in polar solvents. Excellent agreements with the experimental results are found, hinting that the di...

Synthesis and character investigation of new collagen Hydrolysate/polyvinyl alcohol/hydroxyapatite Polymer-Nano-Porous Membranes: I. Experimental design optimization in thermal and structural properties.

Development of bioorganic-inorganic composites has drawn eyes to extensive attention in biomedical fields and tissue engineering. So many attempts to prepare hydroxyapatite (HA), in conjunction with various binders including polyvinyl alcohol (PVA), and collagen has performed for late 20years. We applied a method based on the phase separation for making of polymer porous membranes. This procedure is induced through the addition of a small quantity of water (polymer-rich phase) to a solution with HA precursors (polymer-poor phase). Thermal and structural composite properties of collagen Hydrolysate (CH)-PVA/HA Polymer-Nano-Porous Membranes were analyzed by Design of experiment that was undertaken using D-optimal approach, to select the optimal combination of nano composites precursor. The resulted composite characters were investigated by Fourier transform infrared, scanning electron microscopy (SEM) and thermal gravimetric analysis. Based on the SEM images, this new method could be clearly concluded to porous CH-PVA/HA hybrid materials. Finally the hemocompatibility of nanocomposite membranes were evaluated by the hemolysis study.

Preparation of epoxy monoliths via chemically induced phase separation

Epoxy porous monoliths were prepared from a commercial epoxy resin, D.E.R. 331, that cured with a tertiary amine, 2,4,6-tris-(dimethylaminomethyl) phenol, in the presence of a solvent, diisobutyl ketone (DIBK). During the curing process, polymers were formed and a decrease in its solubility in DIBK; the solution thus phase-separated, usually referred to as chemically induced phase separation. The phase separation formed interconnected polymer-poor phase that then became interconnected pores after the removal of DIBK. By varying the content of DIBK from 32 to 40 vol.%, epoxy monoliths with interconnected pores were prepared, with surface pore size ranging from 0.20 to 2.33 μm, overall porosity from 0.41 to 0.60, and ethanol permeability from 10 to 4,717 L/(m2 h−1 bar−1). The glass transition temperatures of the epoxy monoliths, measured with differential scanning calorimetry, were all higher than 100 °C, and temperatures of 5 % weight loss, analyzed by thermal gravimetry, were higher than 350 °C, evidencing the monoliths’ high thermal stability. Also, the monolith morphology was found to be strongly related to the reaction mechanism of polymerization. The results indicate that the mechanism of chain initiation and propagation associated with the tertiary amine can effectively form monoliths with interconnected pores, which cannot be easily prepared with a stepwise polymerization mechanism associated with using primary amine as the curing agent.

Direct Observation by Laser Scanning Confocal Microscopy of Microstructure and Phase Migration of PVC Gels in an Applied Electric Field

The fluorescent probe lucigenin was incorporated in poly(vinyl chloride) (PVC) gels, and laser scanning confocal microscopy (LSCM) was used to clarify the internal structures of the gels. From the two-dimensional and three-dimensional information by LSCM, we first observed the internal structure of the PVC gel at a wet status, where the PVC gels comprised a polymer-rich phase and a polymer-poor phase uniformly with a three-dimensional network structure. After an electric field was applied, an effect of the electric field resulted in the change of internal structure in the gels. The polymer-poor phase moved from the cathode to the anode and the polymer-rich phase formed linelike arrangement between electrodes due to the attraction force. On the other hand, the freeze-dried PVC gels with/without in-situ dc voltage casting were particularly fabricated to confirm above results by the field emission scanning electron microscopy (FE-SEM). It was found that many craters remained on the surface of the gel near the anode due to sublimation in freeze-drying. This phenomenon did not appear on the surface near the cathode. The results of in-situ dc voltage casting also suggested that a substantial amount of polymer-poor phase was moved and fixed at the anode. Thus, results of both LSCM and in-situ dc voltage casting corresponded to the effect of electric field on PVC gels and provided a convincing evidence for the interpretation of the deformation mechanism of PVC gel actuators by an applied electric field.

Morphology and Formation Mechanism of Poly(Vinylidene Fluoride) Membranes Prepared with Immerse Precipitation: Effect of Dissolving Temperature

Poly(vinylidene fluoride) (PVDF) membranes were prepared by the immersion precipitation method. The effects of polymer dissolving temperature for the dopes on the morphology, crystallization and performance of prepared membranes were examined. Polymer dissolving temperature was varied from 50 to 120°C. N,N-dimethylacetamide (DMAc) and de-ionized water were used as solvent and non-solvent, respectively. Based on the membrane morphology and the schematic phase diagram of the ternary system, the membrane formation mechanism was analyzed theoretically. The binodal liquid-liquid demixing took place first for the nucleation and growth of droplets in the polymer poor phase; then consequently the spinodal liquid-liquid demixing occurred in the polymer rich phase. The demixings together resulted in the prepared membranes having a cross-section composed of interconnected globule-like particulates with bi-continuous structured surfaces. The dissolving temperature of the dopes had a remarkable effect on the morphology of the cross-section, even when the solution underwent a long time cooling before the demixing. The increase of the diameter of the particulates with the dissolving temperature was theoretically analyzed according to the conditions of the polymeric solution.

Porometry Studies of the Polysulfone Membranes on Addition of Poly(ethylene Glycol) in Gelation Bath During Preparation

Polysulfone membranes are prepared through phase separa- tion technique, introduced by Loeb-Sourirajan. The viscous polymer solution (in dimethyl formamide) is first spread into the thin film, and then immersed in gelation medium (water). The influence of poly- meric additive, (poly(ethylene glycol) (PEG)) in the formation of the pores during phase separation in gelation bath (i.e. from the polymer poor phase, which appears at the phase separation) is explored. The effects of different molecular weight (Mw) of PEG in the gelation bath regarding the differential filter flows of nitrogen as well as their concentration are reflected from the porometry studies. The pore dis- tribution is shifted as the molecular weight of PEG used in the gela- tion bath. The bubble point and mean flow pore diameters vary with the concentration as well as their molecular weight of PEG.

A Study of the Microstructural and Diffusion Properties of Poly(vinyl alcohol) Cryogels Containing Surfactant Supramolecular Aggregates

Surfactant-containing poly(vinyl alcohol) (PVA) cryogels have been prepared by drying and reswelling hydrogel patches, previously obtained by the freeze/thaw procedure, in decyltrimethylammonium bromide (C10TAB) aqueous solutions. The microstructural and diffusive properties of the resulting material have been characterized by a combined experimental strategy. Gravimetric measurements show that the cryogel maximum swelling is not affected by the surfactant. The surfactant concentration within the cryogel, measured by ion chromatography, is the same as that in the rehydrating surfactant solution. Electron paramagnetic resonance (EPR) spin-probe and small-angle neutron scattering (SANS) measurements show that surfactant self-aggregation in the gel is similar to that in water, occurring at the same critical concentration and resulting in the formation of micellar aggregates whose structure is not affected by the cryogel polymeric scaffold. However, both the micelle intradiffusion coefficients, measured by PGSE-NMR, and the spin-probe correlation times, measured by EPR, indicate that dynamic processes in the hydrogel are much slower than in bulk water. A quantitative analysis of these results suggests that the cryogel polymer-poor domains, in which surfactant molecules are solubilized, have an average dimension of approximately 0.1 microm. Interestingly the experimental data also show that the polymer-poor phase contains more polymer than expected, suggesting that the spinodal decomposition, which occurs during the freezing step of cryogel preparation, is not complete or prevented by ice formation.

Effects of mixed solvent on gelation of poly(vinyl alcohol) solutions

The relationship between the polymer–solvent interaction and gelation behavior of poly(vinyl alcohol) (PVA) solutions prepared from ethylene glycol/water (EG/water) mixed solvents was investigated using a viscometer, light scattering, FTIR, X-ray, and pulsed NMR analyses. The viscometric result showed that the affinity to PVA for water is higher than that for EG. The light scattering result showed that the spinodal decomposition rate of the PVA solution decreases rapidly as the water content in the EG/water mixed solvent is increased. On the other hand, the FTIR and X-ray results both indicated that the crystallinity of the PVA gel decreases with water content. These results imply that the water molecules must improve the affinity of the solvent to PVA to inhibit the aggregation or crystallization of PVA chains. The pulsed NMR measurement results showed that the spin–spin relaxation times related to the polymer-rich and polymer-poor phases of the PVA gel increase, and the fractional amount of the polymer-poor phase increases while that of the polymer-rich phase decreases with increasing water content. These facts indicated that the increase in the mobility of PVA chains must give rise to the difficulty in chain aggregation of PVA solutions with increasing water content. Two transition temperatures were found in the phase transition of the polymer-rich phase. The lower transition temperature was attributed to the destruction of the denser chain entanglements in the polymer-rich phase and the higher transition temperature was mainly concerned with the melting of the crystallites. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 1113–1120, 2001

Microencapsulation of pyrrolnitrin from Pseudomonas cepacia using gluten and casein

For the formulation and sustained-release delivery of pyrrolnitrin, a biologically anti-fungal antibiotic, a microencapsulation technique using gluten and casein as the coating materials and employing simple coacervation was developed. The core materials were the dried fermentation broth of Pseudomonas cepacia containing pyrrolnitrin (Powder A) and pyrrolnitrin-adsorbed Microken powder (Powder B). Irregular microcapsules with particle sizes ranging from 5 to 80 μm and possessing a smooth surface after hardening with glutaraldehyde were obtained. For the encapsulation of Powder B, about 51–69% of pyrrolnitrin was retained in the microcapsules; compared with only 27–48% for the encapsulation of Powder A. These low yields were due to the decomposition of pyrrolnitrin, as well as to its release into the polymer-poor phase during the coacervation process. Microcapsules prepared by each of the dispersion systems conferred significant delayed-release of pyrrolnitrin, compared to its rapid release from the core materials without encapsulation. The sustained release properties depended on both the core-to-wall ratio and the dosage of hardening agent.

Chain collapse and phase separation in poor-solvent polymer solutions: A unified molecular description

We present a molecular theory of the phenomena of single-chain collapse and phase separation into a polymer-rich and a polymer-poor phase, which occur in polymer solutions below the Θ temperature. The theory extends the Fourier self-consistent approach of Allegra and Ganazzoli from the study of single-chain properties to that of an ensemble of interacting chains. We derive an expression for the free energy of a ‘‘Gaussian cluster’’ made up of ν chains of length N (ν=1,2,3,...; N≫1). In the limit ν→∞ this yields a mean-field expression for the solution free energy per chain as a function of the reduced temperature τ=(T−Θ)/T, the polymer volume fraction φ and the mean-square radius of gyration of the chains. From this we calculate the chain dimensions in solution and several thermodynamic properties, such as the osmotic pressure and the polymer–solvent phase diagram. We find that the contraction ratio of the chain radius of gyration is a single-valued function of (τB+K1Fφ)√N, where B and K1 specify the strength of the two- and three-body interactions and F is a polymer-dependent positive constant. We provide numerical evidence for a possible universality of the binodal line for different polymer-solvent systems; the spinodals do not share this characteristic of universality.

Changes in morphology and transport characteristics of polysulfone membranes prepared by different demixing conditions

A polymer solution film, which is cast from a homogeneous polysulfone (PS) solution (15 wt%) in dimethylformamide (DMF), demixes by liquid-liquid phase separation due to nucleation of polymer-poor phase with sorption of water vapor from atmosphere. The separated two liquid phases continue to grow until the polymer-rich phase solidifies. However, the polymer-rich phase in the liquid-liquid phase-separated solution can demix again before precipitation of the phase. The demixing of the polymer-rich phase on the top layer of the film is induced by kinetically fast demixing conditions, such as rapid mass transfer between nonsolvent (water) and solvent (DMF) in a nonsolvent bath. Morphology of solidified membranes shows that when a membrane structure is established by water vapor sorption alone, the liquid-liquid phase separated solution film forms closed cell-like structures in a whole cross section including surface region and has round pores on the top surface. Even though pore sizes on the surface of the membrane are more than 1 μm, those pores do not work as active pores for membrane performance. A skin structure of a membrane precipitated by fast or instantaneous demixing in a water bath includes nodules or polymer aggregates which can be distinguished from the cell-like structures formed by the liquid-liquid phase separation due to nucleation of the polymer-poor phase. The membrane including nodules in the skin region has higher surface area and broader pore size distribution than the membrane which consists of the cell-like structures. The crack-like pores on the surface of the former contribute to the permeation characteristics of the membrane.

Characterization of the surface layer of integrally skinned polyimide membranes: Relationship with their mechanism of formation

AbstractA series of dense films and membranes was prepared using different processes; their surface morphology was investigated by transmission electron microscopy using the replication technique. On the basis of TEM results, SEM observations of cross sections of membranes and gas‐separation properties of membranes, two kinds of membrane morphology are evidenced in relation to their preparation conditions and final properties. The first one gives an ultrafiltration‐type membrane. Large nodules are observed on the surface of the membranes. It is suggested that these are formed by liquid–liquid separation that occurs by nucleation and growth of the polymer‐rich phase. The second one yields gas‐separation‐type membranes. No nodules are evident at the surface of these membranes. The skin is formed by a step that concentrates the initial solution. It is followed by the transformation of the solution in the glassy state. Liquid—liquid separation takes place below the first skin layers by nucleation and growth of the polymer‐poor phase. © 1993 John Wiley & Sons, Inc.

Self-diffusion study of poly (ethylene oxide)-poly (propylene oxide)-poly (ethylene oxide) block copolymers in aqueous solution

The self-diffusion of poly (ethylene oxide)-poly (propylene oxide)-poly (ethylene oxide) block copolymers dissolved in deuterated water was investigated by means of pulsed field gradient NMR (PFG-NMR). The polymer forms micelles in the solution and, with increasing temperature, clouding and phase demixing occurs. The self-diffusion coefficient indicates the association of the polymer molecules in the vicinity of the cloud point because of its maximum with increasing temperature. Above the cloud point, two kinds of diffusing species are observed due to phase separation. The faster diffusing species is attributed to the polymer-poor phase. The self-diffusion coefficient of the polymer-rich phase species decreases with increasing temperature above the cloud point due to further association and dehydration. The correlation length of the diffusing associates, calculated from the self-diffusion coefficient and the viscosity by means of the Stokes-Einstein equation is nearly independent of temperature and concentration up to 30 wt-% polymer concentration. The correlation length is about 1.4 nm. It shows a slight maximum at the cloud point.

Discussions on the formation mechanism of surface pores in reverse osmosis, ultrafiltration, and microfiltration membranes prepared by phase inversion process

Discussions are made on the formation mechanism of pores at the surface of reverse osmosis, ultrafiltration, and microfiltration membranes. Pores of three different origins are proposed: the network pore originated from the space between polymer segments in the network of a polymer aggregate, the aggregate pore originated from the interstitial space between polymer aggregates, and the phase separation pore originated from the large polymer-poor phase which appears at the liquid-liquid phase separation and is surrounded by agglomerates of the polymer aggregates. A proper adjustment in the proportion of the above three pores of different origins and the control of their sizes is required to design a membrane for a specific separation purpose.

Solution Crystallization of Poly-3,3-Bis(Chloromethyl)-Oxacyclobutane

Poly-3,3-bis(chloromethyl)-oxacyclobutane crystallizes isothermally from dilute solution in the form of lamellar single crystals, whose shape and thickness depend on the crystallization temperature, along with a globular material of distinctly different morphology. The variation in crystal shape with crystallization temperature is suggested to arise from the presence of two different types of folds in a given fold domain, the relative number depending on the temperature. The globular material is believed to arise from a phase separation occurring during rapid quenching of the hot polymer solution to the crystallization temperature, the polymer-rich phase crystallizing as globules and the polymer-poor phase as single crystals.