Nonsolvent Mixture Research Articles

Stretching of porous poly (l-lactide-co-ε-caprolactone) membranes regulates the differentiation of mesenchymal stem cells.

Background: Among a variety of biomaterials supporting cell growth for therapeutic applications, poly (l-lactide-co-ε-caprolactone) (PLCL) has been considered as one of the most attractive scaffolds for tissue engineering owing to its superior mechanical strength, biocompatibility, and processibility. Although extensive studies have been conducted on the relationship between the microstructure of polymeric materials and their mechanical properties, the use of the fine-tuned morphology and mechanical strength of PLCL membranes in stem cell differentiation has not yet been studied. Methods: PLCL membranes were crystallized in a combination of diverse solvent-nonsolvent mixtures, including methanol (MeOH), isopropanol (IPA), chloroform (CF), and distilled water (DW), with different solvent polarities. A PLCL membrane with high mechanical strength induced by limited pore formation was placed in a custom bioreactor mimicking the reproducible physiological microenvironment of the vascular system to promote the differentiation of mesenchymal stem cells (MSCs) into smooth muscle cells (SMCs). Results: We developed a simple, cost-effective method for fabricating porosity-controlled PLCL membranes based on the crystallization of copolymer chains in a combination of solvents and non-solvents. We confirmed that an increase in the ratio of the non-solvent increased the chain aggregation of PLCL by slow evaporation, leading to improved mechanical properties of the PLCL membrane. Furthermore, we demonstrated that the cyclic stretching of PLCL membranes induced MSC differentiation into SMCs within 10days of culture. Conclusion: The combination of solvent and non-solvent casting for PLCL solidification can be used to fabricate mechanically durable polymer membranes for use as mechanosensitive scaffolds for stem cell differentiation.

Solvent sintered sPVDF-co-HFP electro-spun nanofiber membrane for aqueous zinc flow battery

Sintering of the nanofiber membranes results in fine-tuning of the membrane properties without disturbing the core structure of the fibers. Here we have demonstrated the robust method with solvent and nonsolvent mixture to tune the pores of an electrospun poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-co-HFP) membrane and its utility in an aqueous zinc‑iodine flow battery (ZIFB). The method effectively altered the membrane's pores while improving the inter-fiber adhesion between nanofibers, which also gave the membrane improved mechanical properties. The sintered membrane exhibited its competence in ZIFB by delivering high Coulombic efficiency (CE) and Energy efficiency (EE) of 96 % and 82 % at 40 mA cm−2 over 200 stable charge/discharge cycles without obvious decay. The nanofiber membrane's pores were precisely adjusted to prevent electrolyte cross-over while promoting the battery's ability to heal itself after developing zinc dendrites. In comparison to the best-reported membranes in the literature, we have obtained the best power density of 201 mW cm−2 at 50 mA cm−2 and a capacity of 71.5 mAh cm−2. Other polymer nanofibers compatible with the investigated solvent/nonsolvent system can be used with the technique, indicating its broad applicability and utility.

Experimental phase diagram for the PVDF – DMAc– water ternary system with new topology: Method of construction, thermodynamics, and structure formation of membranes

Ternary mixtures of a semicrystalline (SC) polymer with a solvent and a nonsolvent are widely used primarily for preparation of membranes by nonsolvent induced phase separation (NIPS). A detailed analysis of the published studies devoted to phase equilibrium in such systems showed that the phase diagrams were either incomplete or presented in a schematic form. In the present paper, we propose a new method of plotting the ternary phase diagrams for the SC polymer – solvent – nonsolvent mixtures. This method is based on finding the location of the isotherm intersection with the curves on the temperature–composition pseudobinary phase diagrams for the SC polymer mixtures with solvent and binary mixtures of solvent and nonsolvent of different composition. The coordinates of the intersection points were used to plot for the first time the complete ternary phase diagram for the widely used mixture of polyvinylidene fluoride (PVDF) with dimethyl acetamide (DMAc) and water. The curves on the phase diagram separate the regions of homogeneous mixtures, coexistence of two liquids, macroscopically uniform gels (swollen SC polymer) and coexistence of such gels with liquid. Using the plotted phase diagram, a new understanding of the NIPS mechanism was proposed and confirmed by direct microscopic observation of the NIPS process and SEM analysis of the membranes obtained following this way. An unexpected result of the study is discovery of the crystalline complexes in the mixtures of PVDF and DMAc, which was confirmed using both DSC and XRD techniques.

Self-Organized Wrinkling in Thin Polymer Films under Solvent–Nonsolvent Solutions: Patterning Strategy for Microfluidic Applications

Self-organized wrinkling instabilities in thin polymer films have instigated a field of versatile surface patterning and have spurred several research efforts in developing micro- and nanopatterned templates for a wide range of applications. Here, we report for the first time a distinct class of wrinkles in a thin polymer (polystyrene, PS) film coated on a substrate under a mixture of organic solvent and aqueous nonsolvent. The solvent (dimethyl formamide, DMF) softens and swells the polymer and paves the way for wetting of the hydrophilic substrate (≥46 mJ/m2) by the solvent–nonsolvent (S-NS) mixture, leading to wrinkle formation. It is investigated that selective delamination-induced wrinkling is a generic phenomenon and takes place in various polymers as well as different combinations of solvent–nonsolvent mixtures. The surface energy of the substrate and the composition of the solvent–nonsolvent mixture play a critical role as wrinkling is not observed on substrates with lower surface energy (<46 mJ/m2). An isotropically distributed yet disordered self-organized wrinkle network of hollow buried channels is formed, and it is illustrated that these can be exploited to generate a mesh of microwires and harnessed to form highly directional patterns using electron beam lithography, which can turn the new leaf for nano- and microfluidic device fabrication platforms.

Precision Polymer Particles by Flash Nanoprecipitation and Microfluidic Droplet Extraction

We comparatively review two versatile approaches employed in the precise formation of polymer particles, with length scales from 10s of nm to to 100s μm, from ternary polymer(s), solvent and nonsolvent mixtures. Flash nanoprecipitation (FNP) utilizes an opposing jet arrangement to mix a dilute polymer solution and a nonsolvent in confinement, inducing a rapid (∼millisecond) chain collapse and eventual precipitation of nanoparticles (NPs) of 10–1000 nm diameters. FNP of polymer mixtures and block copolymers can yield a range of multiphase morphologies with various functionalities. While droplet solvent extraction (DSE) also involves the exposure of a polymer solution to a nonsolvent, in this case the polymer solution is templated into a droplet prior to solvent extraction, often using microfluidics, resulting in polymer particles of 1–1000 μm diameter. Droplet shrinkage and solvent exchange are generally accompanied by a series of processes including demixing, coarsening, phase inversion, skin formation, and kinetic arrest, which lead to a plethora of possible internal and external particle morphologies. In the absence of external flow fields, DSE corresponds effectively to nonsolvent induced phase separation (NIPS) in a spherical geometry. In this review, we discuss the requirements to implement both approaches, detailing consequences of ternary solution phase behavior and the interplay of the various processes underpinning particle formation and highlighting the similarities, differences, and complementarity of FNP and DSE. In addition to reviewing previous work in the field, we report comparative experimental results on the formation of polystyrene particles by both approaches, emphasizing the importance of solution phase behavior in process design.

Surface modification of poly (L-lactic acid) films to improve electrical conductivity by surface entrapment/in situ polymerization methods

Electrical properties play an important role in the interaction of cells and biomaterials. The aim of this study was to increase surface electrical conductivity of poly (L-lactic acid) (PLLA) films by polyaniline (PANI) using surface entrapment (SE) and surface entrapment in situ polymerization (SEIP) methods. The entrapment modification technique was performed by the reversible gelation of the PLLA surface region following exposure to a solvent and non-solvent mixture, while PANI chains were diffused into the swollen PLLA surface. In SEIP method, PANI was entrapped through in situ chemical oxidative polymerization of aniline on PLLA films. The effects of PANI concentration, the ratios of solvents and immersion time on the electrical conductivity were studied by SE method. The effects of aniline concentration, doping effect, temperature and time of polymerization were evaluated by SEIP method too. The results obtained by attenuated total reflection Fourier transformer infrared spectroscopy, scanning electron microscopy and energy dispersive x-ray mapping analysis confirmed the presence of PANI on the surface of PLLA films. UV-visible analysis was employed to investigate the doping effect on the reduction of energy and conductivity of the films. Experimental results also indicated that the electrical conductivity, uniformity and density of the SEIP modified films were significant in comparison to those of the modified films by SE method.

Porous carbon prepared from polyacrylonitrile for lithium-sulfur battery cathodes using phase inversion technique

In this paper, a time and resource efficient way of preparing porous carbon cathode for lithium-sulfur batteries with superior cycle stability has been demonstrated. In this simple work, we used commercially available polymer as carbon source and non-solvent mixture as porogen or pore former. The cathode has been fabricated by using polyacrylonitrile as base polymer via phase inversion route. By this technique, a highly porous substrate material is generated by dipping a semi-gelled film of polyacrylonitrile in non-solvent mixture. After oxidative crosslinking followed by pyrolysis under inert atmosphere results a highly porous nitrogen doped carbon material, which was further hybridized with sulfur via melt diffusion of elemental sulfur. This cathode material shows although relatively low specific capacity (Cycle 1: ca. 1050 mA h/gsulfur, cycle 500: ca. 400 mA h/gsulfur), but excellent cycle stability over 500 charging-discharging cycles is displayed.

Phase Inversion Strategy to Fabricate Porous Carbon for Li‐S Batteries via Block Copolymer Self‐Assembly

AbstractIn this paper, an easy, facile way of preparing porous carbon cathode for lithium–sulfur batteries with superior cycle stability is demonstrated. The highly porous cathode is fabricated by using self‐assembled nanostructured block copolymer as template. The block copolymer template is formed via phase inversion route followed by filling the empty spaces by oligomeric phenolic resin, thermopolymerization of the resin, and finally by carbonization. Semi‐gelled film of self‐assembled block copolymer is immersed in solution of nonsolvent mixture. After washing away of low molecular additive, a highly porous mat is generated. After successful filling with carbon precursor inside the template, crosslinking and carbonization under inert atmosphere result in a highly porous carbon material, which is further hybridized with sulfur via melt diffusion of elemental sulfur. This new fabrication route of template formation by phase inversion requires no solvent/thermal annealing step to generate nanostructure aided by block copolymer. The cathode material shows excellent cycle stability over 240 charging–discharging cycles, rate handling over 100 cycles.

Mechanism of formation of hollow fiber membranes for membrane distillation: 1. Inner coagulation power effect on morphological characteristics

Hollow fiber membrane morphology and inner surface structure were analyzed based on the thermodynamic and kinetics of the phase inversion process. Different nonsolvent mixtures formed by N,N-dimethyl acetamide (DMAC) and distilled water were considered. By means of Hansen solubility parameters, lower interaction between the nonsolvent and the mixed solvent (DMAC/trimethyl phosphate, TMP) was observed when greater amount of DMAC was added in the nonsolvent. The spinning solution became thermodynamically more stable and kinetically showed slower coagulation rates, being both related to the resultant membrane formation. In this first study poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) hollow fiber membranes were prepared by the dry/wet spinning technique employing the above cited nonsolvent mixtures as an internal coagulant. Their effect on the internal structure of the membrane evolved towards a more open-porous inner surface when increasing the solvent content in the bore liquid. Various characterization techniques were used in order to analyze the adequacy of these membranes for desalination by direct contact membrane distillation (DCMD). It was found that all the properties together with the permeability of the hollow fibers improved when the coagulation power of the nonsolvent was reduced.

Mechanism of formation of hollow fiber membranes for membrane distillation: 2. Outer coagulation power effect on morphological characteristics

Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) hollow fiber membranes with porous and skinless outer surfaces were prepared for desalination by direct contact membrane distillation (DCMD). N,N-dimethyl acetamide (DMAC)/distilled water mixtures extruded from the outermost channel of the spinneret were employed as external coagulants along the gap of the spinning process. During membrane formation, various types of interactions studied via Hansen solubility parameters were identified at the outer layer of the nascent fiber. Several characterization techniques and DCMD experiments were carried out in order to analyze the effects of the wet gap on the membrane morphology, properties and desalination performance. The skin-layer was avoided and permeate flux was enhanced when using the DMAC/water mixtures as the external coagulants. Increasing the concentration of DMAC in the nonsolvent mixture weakened the external coagulation power and the outer layer morphology of the fiber became more porous with larger surface pore sizes. Open-porous inner surface, skinless outer layer and spongy cross-section were observed for the hollow fiber membranes prepared with the highest amounts of DMAC in water, 50 and 60wt%, used simultaneously as internal and external coagulants. The hollow fiber membrane prepared with the coagulant mixture 50wt% DMAC in water exhibited good mechanical properties and excellent MD membrane characteristics, leading to the highest DCMD permeate flux with a good salt rejection factor.

Transferable thin films with sponge-like porous structure via improved phase separation

Abstract This study is the first to report highly efficient production of homogeneous large-scale thin polymer films with a sponge-like porous structure via an improved phase separation method. A mixture of tetrahydrofuran and methanol, which is used as a highly volatile solvent/nonsolvent pair, is crucial to induce phase separation and thus determines the resulting structure. The obtained porous structure is composed of intermingled sticks in a size range of 50–100 nanometers and bumps with a diameter of about 200 nanometers, revealing a morphology that resembles a bird's nest. The pore size and porosity of the porous film are finely controlled by varying the mixture composition. Furthermore, by employing a water-soluble interlayer before the phase separation process, the obtained porous film, even with extremely large area, can be easily detached from solid substrates without damaging the film. Notably, it possesses a skin-free layer at the bottom side where it contacts the interlayer surface. The formation mechanism of the sponge-like porous structure was studied and it was found that the progressive enrichment of methanol during the evaporation process is favorable for absorbing water vapor from the air into the solution, which is responsible for phase separation. In addition, a water-miscible mixture of solvent and nonsolvent, which can delay the liquid-liquid demixing process, is essential for the formation of the sponge-like porous structure.

Natural macromolecule based carboxymethyl cellulose as a gel polymer electrolyte with adjustable porosity for lithium ion batteries

A porous membrane of carboxymethyl cellulose (CMC) from natural macromolecule as a host of a gel polymer electrolyte for lithium ion batteries is reported. It is prepared, for the first time, by a simple non-solvent evaporation method and its porous structure is fine-adjusted by varying the composition ratio of the solvent and non-solvent mixture. The electrolyte uptake of the porous membrane based on CMC is 75.9%. The ionic conductivity of the as-prepared gel membrane saturated with 1 mol L−1 LiPF6 electrolyte at room temperature can be up to 0.48 mS cm−1. Moreover, the lithium ion transference in the gel membrane at room temperature is as high as 0.46, much higher than 0.27 for the commercial separator Celgard 2730. When evaluated by using LiFePO4 cathode, the prepared gel membrane exhibits very good electrochemical performance including higher reversible capacity, better rate capability and good cycling behaviour. The obtained results suggest that this porous polymer membrane shows great attraction to the lithium ion batteries requiring high safety, low cost and environmental friendliness.

Influence of the coagulation medium on the performance of poly(ether sulfone) flat‐sheet membranes

ABSTRACTPoly(ether sulfone) flat‐sheet membranes were fabricated via phase inversion with different nonsolvent mixtures. The effect of the nonsolvent water with the addition of various amounts of ethanol, acetone, or isopropyl alcohol on the membrane morphology (as measured with scanning electron microscopy and atomic force microscopy) and the filtration performance were investigated. For the statistical evaluation of the fabrication process, on average, six membranes were produced. The pure water flux (PWF) and macromolecule retention were determined via filtration experiments. The presence of coagulation additives resulted in modified precipitation kinetics and thermodynamics, yielded different membrane structures, and therefore, influenced the performance. The results show that the addition of ethanol, acetone, and isopropyl alcohol in low concentrations (up to 10%) to water led to an increasing PWF. Higher concentrations led to a decrease in PWF. For high concentrations (&gt;30%), a change in the membrane morphology from fingerlike to spongelike structures was expected, and this was experimentally proven for the case of ethanol. One main finding was the similarity of the influence of the used additives on the membrane performance. This was to be expected from Flory–Huggins theory for additives with high water miscibility; hence, under these circumstances, entropic and not energetic reasoning dominated the phase‐inversion process. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41645.

Coalescence of Repelling Colloidal Droplets: A Route to Monodisperse Populations

Populations of droplets or particles dispersed in a liquid may evolve through Brownian collisions, aggregation, and coalescence. We have found a set of conditions under which these populations evolve spontaneously toward a narrow size distribution. The experimental system consists of poly(methyl methacrylate) (PMMA) nanodroplets dispersed in a solvent (acetone) + nonsolvent (water) mixture. These droplets carry electrical charges, located on the ionic end groups of the macromolecules. We used time-resolved small angle X-ray scattering to determine their size distribution. We find that the droplets grow through coalescence events: the average radius (R) increases logarithmically with elapsed time while the relative width σR/(R) of the distribution decreases as the inverse square root of (R). We interpret this evolution as resulting from coalescence events that are hindered by ionic repulsions between droplets. We generalize this evolution through a simulation of the Smoluchowski kinetic equation, with a kernel that takes into account the interactions between droplets. In the case of vanishing or attractive interactions, all droplet encounters lead to coalescence. The corresponding kernel leads to the well-known "self-preserving" particle distribution of the coalescence process, where σR/(R) increases to a plateau value. However, for droplets that interact through long-range ionic repulsions, "large + small" droplet encounters are more successful at coalescence than "large + large" encounters. We show that the corresponding kernel leads to a particular scaling of the droplet-size distribution-known as the "second-scaling law" in the theory of critical phenomena, where σR/(R) decreases as 1/√(R) and becomes independent of the initial distribution. We argue that this scaling explains the narrow size distributions of colloidal dispersions that have been synthesized through aggregation processes.

Enhanced emission of organic and polymeric luminogens containing 2,4,6-triphenylpyridine moieties: crystallization- and aggregation-enhanced emission

Organic and polymeric deep-blue luminogens based on the 2,4,6-triphenylpyridine (TPP) structure were prepared via the facile Chichibabin reaction and their fluorescence behavior in relation to the aggregation-enhanced emission (AEE) properties were evaluated in this study. For the low-mass luminogens, the emission behavior of TPP and cyano-functionalized TPP (TPP-CN) in solvent–nonsolvent pairs was related to the crystallization-enhanced emission (CEE) behavior, which can also be verified by the enhanced emission in the crystalline structures developed by the solvent-fuming process. For a polymeric system, a low Mw PTPP oligomer consisting wholly of TPP repeating units and a PTPPF copolymer containing alternative TPP and 9,9-dioctylfluorene units were prepared through Suzuki coupling reactions and in contrast to TPP and TPP-CN, both of them were amorphous materials with strong solution fluorescence that remained relatively intact in different solvent–non-solvent mixtures. The constant emission behavior was therefore explained by the restricted molecular rotation of the rigid chain segments in PTPP and PTPPF. The solid PTPPF was a highly emissive material with a high quantum yield (ΦF) of 80%.

The influence of membrane formation parameters on structural morphology and performance of PES/PDMS composite membrane for gas separation

AbstractIn this study, influence of membrane preparation parameters on structural morphology and performance of polyethersulfone/polydimethylsiloxane (PES/PDMS) composite membrane was investigated for gas separation. Asymmetric PES flat sheet membranes were composed by phase inversion method and used as supports. PES composite membranes were fabricated by coating silicone rubber as selective layer on the top surface of support. Effects of different concentrations of PES and PDMS, solvent type, and support thickness on membrane performance were investigated for separation of oxygen from nitrogen. The optimized superior membrane was further modified using polyvinylidenfluoride, methanol and ethanol as additives in PES solutions and/or in water coagulation bath to promote the membrane capability. The results showed that addition of ethanol and methanol in cast solution and coagulation bath can greatly affect the morphology and hence the performance of the prepared membranes. The permeance changes have the contrary trend with solubility parameter difference between solvent and nonsolvent mixture, for instance when this parameter difference was lowest, higher permeance was obtained. Support and coating polymer concentration can control the permeance. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Surface Engineering of Polycaprolactone by Biomacromolecules and their Blood Compatibility

Improving blood compatibility of biodegradable polymers is an area of intensive research in blood contacting devices. In this study, curdlan sulphate and heparin-modified poly (caprolactone) (PCL) hybrids were developed by physically entrapping these molecules on the PCL surface. This modification technique was performed by reversible gelation of the PCL surface region following exposure to a solvent and nonsolvent mixture. The presence of these biomacromolecules on the PCL surface was verified by atomic force microscopy (AFM) and scanning electron microscopy and energy dispersive X-ray analysis (SEM-EDAX) analysis, while wettability of the films was investigated by dynamic contact angle measurements. The blood compatibilities of the surface-modified films were examined using in vitro platelet and leukocyte adhesion and thrombus formation. Mouse RAW 264.7 macrophage cells were used to assess the cell adhesion and inflammatory response to the modified surface by quantifying mRNA expression levels of proinflammatory cytokines namely TNF-α and IL-1β using real-time polymerase chain reaction (RT-PCR). A lower platelet and leukocyte adhesion and activation was observed on the modified films incubated with whole human blood for 2 h. The thrombus formation on the PCL was significantly decreased upon immobilization of both curdlan sulphate (39%, *p<0.05) and heparin (28%, *p<0.01) when compared to bare PCL (80%). All of these results revealed that improved blood compatibility was obtained by surface entrapment of both curdlan sulphate (CURS) and heparin (HEP) onto PCL films. Both PCL-CURS and PCL-HEP films reduced RAW 264.7 macrophage cell adhesion (*p<0.05) with respect to the base unmodified PCL. The cellular inflammatory response was suppressed on the modified substrates. The mRNA expression levels of proinflammmatory cytokines (TNF-α and IL-1β) were upregulated on bare PCL, while it was significantly lower on PCL-CURS and PCL-HEP substrates (**p<0.001). Thus, this biomacromolecule entrapment process can be applied on PCL in order to achieve improved blood compatibility and reduced inflammatory host response for its future blood contacting applications.

Influence of Casting Solution Characteristics on Cellulose Acetate Membranes: Rheology and Atomic Force Microscopy

Cellulose acetates with different substitution degrees were investigated as to their rheological properties and morphology. Atomic force microscopy (AFM) studies on membranes showed ordered domains, in which pores and nodules with different sizes and intensities are distributed. The substitution degree, as well as the history of the membranes formed from the solutions in acetone and water nonsolvent mixtures, influenced the aspect of surface images.

Surface Properties of Cellulose Acetate

Cellulose acetates with different substitution degrees were investigated as to their surface tension properties and morphology. Atomic force microscopy (AFM) studies on membranes, obtained from solution in acetone/water nonsolvent mixtures, evidenced pores and nodules of different size and intensity, depending on the substitution degrees of cellulose acetate and on the water contents in nonsolvent mixtures. Modification of hydrophilicity, put into evidence by the apolar components and by the electron acceptor and electron donor parameters of the polar components, was correlated with the pores volumes — influenced both by their diameter and depths, and by surface roughness as illustrated by AFM images. Surface wettability trends were also studied by means of the free energy of hydration between compounds and water.

Chirality in Poly(phenylene-alt-bithiophene)s: A Comprehensive Study of Their Behavior in Film and Nonsolvents

New chiral, alternating copolymers of 2,5-dialkoxybenzenes and 3,3‘-dialkoxy-2,2‘-bithiophenes were prepared and their (chir)optical properties were studied. The polymers were prepared by a Stille coupling reaction and show high conjugation lengths in both solution and films. Fluorescence and UV−vis spectroscopy indicate that the polymers are present as rigid, highly conjugated polymer strands in solution. In a nonsolvent mixture, the polymers aggregate and, if chiral substituents are employed, supramolecular chirality is observed. In annealed films, very large Cotton effects are observed.