Phase Separation In Polymer Solutions Research Articles

Connecting Structural Characteristics and Material Properties in Phase-Separating Polymer Solutions: Phase-Field Modeling and Physics-Informed Neural Networks.

The formed morphology during phase separation is crucial for determining the properties of the resulting product, e.g., a functional membrane. However, an accurate morphology prediction is challenging due to the inherent complexity of molecular interactions. In this study, the phase separation of a two-dimensional model polymer solution is investigated. The spinodal decomposition during the formation of polymer-rich domains is described by the Cahn-Hilliard equation incorporating the Flory-Huggins free energy description between the polymer and solvent. We circumvent the heavy burden of precise morphology prediction through two aspects. First, we systematically analyze the degree of impact of the parameters (initial polymer volume fraction, polymer mobility, degree of polymerization, surface tension parameter, and Flory-Huggins interaction parameter) in a phase-separating system on morphological evolution characterized by geometrical fingerprints to determine the most influential factor. The sensitivity analysis provides an estimate for the error tolerance of each parameter in determining the transition time, the spinodal decomposition length, and the domain growth rate. Secondly, we devise a set of physics-informed neural networks (PINN) comprising two coupled feedforward neural networks to represent the phase-field equations and inversely discover the value of the embedded parameter for a given morphological evolution. Among the five parameters considered, the polymer-solvent affinity is key in determining the phase transition time and the growth law of the polymer-rich domains. We demonstrate that the unknown parameter can be accurately determined by renormalizing the PINN-predicted parameter by the change of characteristic domain size in time. Our results suggest that certain degrees of error are tolerable and do not significantly affect the morphology properties during the domain growth. Moreover, reliable inverse prediction of the unknown parameter can be pursued by merely two separate snapshots during morphological evolution. The latter largely reduces the computational load in the standard data-driven predictive methods, and the approach may prove beneficial to the inverse design for specific needs.

Mechanical Slowing Down of Network-Forming Phase Separation of Polymer Solutions.

Phase separation is a fundamental phenomenon leading to spatially heterogeneous material distribution, which is critical in nature, biology, material science, and industry. In ordinary phase separation, the minority phase always forms droplets. Contrary to this common belief, even the minority phase can form a network structure in viscoelastic phase separation (VPS). VPS can occur in any mixture with significant mobility differences between their components and is highly relevant to soft matter and biomatter. In contrast to classical phase separation, experiments have shown that VPS in polymer solutions lacks self-similar coarsening, resulting in the absence of a domain-coarsening scaling law. However, the underlying microscopic mechanism of this behavior remains unknown. To this end, we perform fluid particle dynamics simulations of bead-spring polymers, incorporating many-body hydrodynamic interactions between polymers through a solvent. We discover that polymers in the dense-network-forming phase are stretched and store elastic energy when the deformation speed exceeds the polymer dynamics. This self-generated viscoelastic stress mechanically interferes with phase separation and slows its dynamics, disrupting self-similar growth. We also highlight the essential role of many-body hydrodynamic interactions in VPS. The implications of our findings may hold importance in areas such as biological phase separation, porous material formation, and other fields where network structures play a pivotal role.

Novel multiscale simulations on the membrane formation via hybrid induced phase separation process based on dissipative particle dynamics

A novel simulation methodology, that was based on dissipative particle dynamics (DPD) combined with information from molecular dynamics (MD) and calculations of an extended Maxwell-Stefan (MS) equations, was established to obtain multiscale analysis on the dynamics of hybrid induced phase separation (HIPS), taking polyvinylidene fluoride-diphenyl carbonate/water–ethanol (PVDF-DPC/water–ethanol) system as an example. Firstly, MS equations were derived to describe the heat transfer across the interface between the casting solution and coagulation to get the temperature profiles of the casting solution. Secondly, MD simulations for the PVDF-DPC solution were conducted to acquire bond and angle distributions of the PVDF chains. And compared to the DPD particle distributions, a novel DPD force field to describe the real flexibility of PVDF chains and a coarse-graining mapping between the MD and DPD were developed. Thirdly, based on the quenching time calculated by the MS equations and the DPD time scale, DPD simulations were carried out to demonstrate the dynamics of phase separation and membrane formation for different parts of the polymer solution via HIPS and compared to that via thermally induced phase separation (TIPS). The simulation results indicated that the mass transfer had a decisive effect on the phase separation of polymer solution near the interface and resulted in a highly asymmetric structure with a denser polymer top layer and porous sublayer, and this effect was together decided by the quenching rate and polymer solidification. For the inner parts of the polymer solution, a mainly isotropic structure was formed due to heat transfer. Based on experimental verification and simulation results, it can be known that the multiscale simulation was much closer to reality and can provide deep insights into the membrane formation prepared via the HIPS process.

Plant cell-like tip-growing polymer precipitate with structurally embedded multistimuli sensing ability

Soft systems that respond to external stimuli, such as heat, magnetic field, and light, find applications in a range of fields including soft robotics, energy harvesting, and biomedicine. However, most of the existing systems exhibit nondirectional, nastic movement as they can neither grow nor sense the direction of stimuli. In this regard, artificial systems are outperformed by organisms capable of directional growth in response to the sense of stimuli or tropic growth. Inspired by tropic growth schemes of plant cells and fungal hyphae, here we report an artificial multistimuli-responsive tropic tip-growing system based on nonsolvent-induced phase separation of polymer solution, where polymer precipitates as its solvent dissolves into surrounding nonsolvent. We provide a theoretical framework to predict the size and velocity of growing precipitates and demonstrate its capability of sensing the directions of gravity, mechanical contact, and light and adjusting its growing direction in response. Exploiting the embedded physical intelligence of sensing and responding to external stimuli, our soft material system achieves multiple tasks including printing 3D structures in a confined space, bypassing mechanical obstacles, and shielded transport of liquids within water.

Tuning the thermodynamic, optical, and rheological properties of thermoresponsive polymer solutions via silica nanoparticle shape and concentration

HypothesisThe shape and quantity of hydrophilic silica nanoparticles (NPs) can be used to tune the microstructure, rheology, and stability of phase-separating polymer solutions. In thermoresponsive polymer systems, silica nanospheres are well-studied whereas anisotropic NPs have little literature precedent. Here, we hypothesize that NP shape and concentration lower the onset of rheological and turbidimetric transitions of aqueous poly(N-isopropyl acrylamide) (PNIPAM) solutions. ExperimentsDifferential scanning calorimetry (DSC), Fourier-transform infrared spectroscopy (FTIR), turbidimetry, and oscillatory rheology are utilized to examine interactions between NPs, PNIPAM, and water and to track changes in phase separation and rheological properties due to NP concentration and shape. FindingsNP addition reduces phase separation enthalpy due to PNIPAM-NP hydrogen bonding interactions, the degree to which depends on polymer content. While NP addition minorly impacts thermodynamic and optical properties, rheological transitions and associated rheological properties are dramatically altered with increasing temperature, and depend on NP quantity, shape, and polymer molecular weight. Thus NP content and shape can be used to finely tune transition temperatures and mechanical properties for applications in stimuli-responsive materials.

Phase-field simulation for the formation of porous microstructures due to phase separation in polymer solutions on substrates with different wettabilities

The porous microstructure has been widely observed in a variety of polymer solutions that have been broadly applied in many industry fields. Phase separation is one of the common mechanisms for the formation of the porous microstructure in binary polymeric mixtures. Previous studies for the formation of porous microstructures mostly focus on the separation of the bulk phase. However, there is a paucity of investigation for the phase separation of polymer mixtures contacting the solid substrate. When the polymeric liquid mixtures interact with the solid substrate, the wetting boundary condition has to be taken into account. In this work, we present a phase-field model which is coupled with the wetting boundary condition to study the phase separation in binary polymer solutions. Our consideration is based on the polymerization-induced phase separation, and thermally induced phase separation by using the Flory–Huggins model. By taking the wetting effect into account, we find that polymer droplets spontaneously occur in the microstructure, even though the bulk composition is outside the spinodal region. This phenomenon is caused by the surface composition resulting from the wetting effect that was often overlooked in literature. For the phase separation in the binary polymer mixture, we also study the impact of the temperature gradient on the microstructural evolution. The porosity, the number of droplets, and the mean radius of the droplets are rationalized with the temperature gradient.

Formation of Core-Sheath Polymer Fibers by Free Surface Spinning of Aqueous Two-Phase Systems.

Core-sheath fibers have numerous applications ranging from composite materials for advanced manufacturing to materials for drug delivery and regenerative medicine. Here, a simple and tunable approach for the generation of core-sheath fibers from immiscible solutions of dextran and polyethylene oxide is described. This approach exploits the entanglement of polymer molecules within the dextran and polyethylene oxide phases for free surface spinning into dry fibers. The mechanism by which these core-sheath fibers are produced after contact with a solid substrate (such as a microneedle) involves complex flows of the phase-separating polymer solutions, giving rise to a liquid-liquid core-sheath flow that is drawn into a liquid bridge. This liquid bridge then elongates into a core-sheath fiber through extensional flow as the contacting substrate is withdrawn. The core-sheath structure of the fibers produced by this approach is confirmed by attenuated total reflection Fourier-transform infrared spectroscopy and confocal microscopy. Tuning of the core diameter is also demonstrated by varying the weight percentage of dextran added to the reservoir from which the fibers are formed.

Chain conformations and phase separation in polymer solutions with varying solvent quality

AbstractMolecular dynamics simulations are used to investigate the conformations of a single polymer chain, represented by the Kremer‐Grest bead‐spring model, in a solution with a Lennard‐Jones liquid as the solvent when the interaction strength between the polymer and solvent is varied. Results show that when the polymer‐solvent interaction is unfavorable, the chain collapses as one would expect in a poor solvent. For more attractive polymer‐solvent interactions, the solvent quality improves and the chain is increasingly solvated and exhibits ideal and then swollen conformations. However, as the polymer‐solvent interaction strength is increased further to be more than about twice the strength of the polymer‐polymer and solvent‐solvent interactions, the chain exhibits an unexpected collapsing behavior. Correspondingly, for strong polymer‐solvent attractions, phase separation is observed in the solutions of multiple chains. These results indicate that the solvent becomes effectively poor again at very attractive polymer‐solvent interactions. Nonetheless, the mechanism of chain collapsing and phase separation in this limit differs from the case with a poor solvent rendered by unfavorable polymer‐solvent interactions. In the latter, the solvent is excluded from the domain of the collapsed chains while in the former, the solvent is still present in the pervaded volume of a collapsed chain or in the polymer‐rich domain that phase separates from the pure solvent. In the limit of strong polymer‐solvent attractions, the solvent behaves as a glue to stick monomers together, causing a single chain to collapse and multiple chains to aggregate and phase separate.

Partition of Block Copolymers in Phase-Separating Polymer Solutions.

The distribution of the AB diblock copolymer in a phase-separating solution composed of immiscible A and B homopolymers in a common solvent has been investigated theoretically. We have utilized the mixing Gibbs energy density for the interfacial phase based on mean-field lattice theory to this four-component system. Distributions of the AB diblock copolymer in the A and B homopolymer-rich bulk phases and the interfacial region between the separating bulk phases are calculated as a function of the B-block content, degrees of polymerization of the copolymer and A and B homopolymers, as well as interaction parameters among the A and B monomer units and the solvent. The copolymer prefers to distribute more in the interfacial region rather than separating bulk phases at a higher copolymer degree of polymerization and a higher interaction parameter between A and B monomer units. The theory is also compared with Asano et al.'s experimental results [ Langmuir 2015, 31, 7488-7495] for polystyrene-b-poly(ethylene glycol) copolymer added to the phase-separating solution of polystyrene and poly(ethylene glycol) homopolymers dissolved in chloroform.

The effect of the viscosity ratio on band formation in sheared mixtures of phase-separated polymer solutions with ultralow interface tension

The microscopic structure and dynamics of droplet dispersions formed by phase separation of two Newtonian polymer solutions were observed under simple shear. The phases had equal volumes. Under certain conditions, these dispersions show long, wormlike, extended structures after several minutes of shearing. The development of these structures was recorded in microscopic movies. These were interpreted by calculating two-point grey value correlation functions. The divergence of the correlation in the flow direction was taken as a signature of the onset of the extended structures. It was demonstrated that a viscosity ratio far from the unity of the two phases is necessary for the formation of the long shear-stabilised structures. At a viscosity ratio close to unity, the systems maintain isolated droplets in a dynamic equilibrium of coalescence and break-up. The structure formation is the result of shear-induced coalescence. The need for a difference in viscosity between the phases suggests a relation with shear banding seen in shear-thinning systems.

Dynamic Coupling of Optically Evolved Assembling and Swarming of Gold Nanoparticles with Photothermal Local Phase Separation of Polymer Solution

Optical trapping of gold nanoparticles (Au NPs) at a glass/solution interface initially generates a periodically aligned structure of a few NPs along the direction perpendicular to a linearly polar...

Green preparation of hierarchically structured hemostatic epoxy-amine sponge

Materials with highly absorptive property have been used to control bleeding by creating a physical barrier at the wound. Additionally, studies have suggested that macroporosity, cationic chemical groups, and microparticle morphology are advantageous attributes for hemostatic materials. Yet, no study to date has capitalized on these properties synergistically and integrates them into the development of multifunctional materials for enhanced performance in hemorrhage hemostasis. Here we present the preparation of such a new hemostatic sponge material using a facile and green method. The process simply involves epoxy-amine click reaction in water coupled with the phenomenon of temperature induced phase separation of polymer solution. It enables the convergence of multiple key features with hemostasis significance into the material. Specifically, phase separation results in the epoxy-amine (EA) sponge being hierarchically structured, with macroporosity for high water uptake while its microparticle structural units amenable for encapsulation of hydrophobic guests. Additionally, the EA sponge is inherently rich in amine groups, and thus can potentially exhibit a positive charge-based procoagulant effect. The material is pliable, and demonstrates excellent hemocompatibility and cytocompatibility. It also shows improved hemostatic performance over commercial products of gauze and gelatin sponge, supported by findings of in vitro whole blood clotting study, as well as in vivo mouse-tail amputation and rat liver injury models. This new versatile material has shown strong promises to warrant further development of such a multifaceted intervention to significantly enhance hemostatic management of bleeding.

Nonspherical Coacervate Shapes in an Enzyme-Driven Active System.

Coacervates are polymer-rich droplets that form through liquid–liquid phase separation in polymer solutions. Liquid–liquid phase separation and coacervation have recently been shown to play an important role in the organization of biological systems. Such systems are highly dynamic and under continuous influence of enzymatic and chemical processes. However, it is still unclear how enzymatic and chemical reactions affect the coacervation process. Here, we present and characterize a system of enzymatically active coacervates containing spermine, RNA, free nucleotides, and the template independent RNA (de)polymerase PNPase. We find that these RNA coacervates display transient nonspherical shapes, and we systematically study how PNPase concentration, UDP concentration, and temperature affect coacervate morphology. Furthermore, we show that PNPase localizes predominantly into the coacervate phase and that its depolymerization activity in high-phosphate buffer causes coacervate degradation. Our observations of nonspherical coacervate shapes may have broader implications for the relationship between (bio)chemical activity and coacervate biology.

Precision cell delivery in biphasic polymer systems enhances growth of keratinocytes in culture and promotes their attachment on acellular dermal matrices.

Current approaches for precision deposition of cells are not optimized for moist environments or for substrates with complex surface topographic features, for example, the surface of dermal matrices and other biomaterials. To overcome these challenges, an approach is presented that utilizes cell confinement in phase-separating polymer solutions of polyethylene glycol and dextran to precisely deliver keratinocytes in well-defined colonies. Using this approach, keratinocyte colonies are produced with superior viability, proliferative capacity, and barrier formation compared with the same number of cells dispersedly seeded across substrate surfaces. It is further demonstrated that keratinocytes delivered in colonies to the surface of acellular dermal matrices form an intact epidermal basal layer more rapidly and more completely than cells delivered by conventional dispersed seeding. These findings demonstrate that delivery of keratinocytes in phase-separating polymer solutions holds potential for enhancing growth of keratinocytes in culture and production of functional skin equivalents.

Progress Report on Phase Separation in Polymer Solutions.

Polymeric porous media (PPM) are widely used as advanced materials, such as sound dampening foams, lithium-ion batteries, stretchable sensors, and biofilters. The functionality, reliability, and durability of these materials have a strong dependence on the microstructural patterns of PPM. One underlying mechanism for the formation of porosity in PPM is phase separation, which engenders polymer-rich and polymer-poor (pore) phases. Herein, the phase separation in polymer solutions is discussed from two different aspects: diffusion and hydrodynamic effects. For phase separation governed by diffusion, two novel morphological transitions are reviewed: "cluster-to-percolation" and "percolation-to-droplets," which are attributed to an effect that the polymer-rich and the solvent-rich phases reach the equilibrium states asynchronously. In the case dictated by hydrodynamics, a deterministic nature for the microstructural evolution during phase separation is scrutinized. The deterministic nature is caused by an interfacial-tension-gradient (solutal Marangoni force), which can lead to directional movement of droplets as well as hydrodynamic instabilities during phase separation.

Mesoscopic simulations of thermally-induced phase separation in PVDF/DPC solutions

We present a phase-field model of thermally-induced phase separation in polymer solutions, calibrated for the polyvinylidene fluoride (PVDF)/diphenyl carbonate (DPC) system. Large-scale three-dimensional computer simulations were performed for isotropic and anisotropic thermal quenches, and the evolution and structure of the resulting two-phase morphology is analyzed. Isotropic quenches, in which the temperature is uniformly reduced below the binodal temperature, were conducted to understand the initiation and coarsening of the polymer-rich and polymer-poor phases throughout time. Anisotropic quenches, in which the system is cooled from one particular surface, were also conducted to understand how gradients in the characteristic domain size develop for varying conditions. In these anisotropic quenches, we observe the formation of a dense skin layer adjacent to the cooling surface, the thickness of which depends on several parameters including the polymer volume fraction, the assumed bath temperature that is maintained at the cooling surface, and the rate of thermal conduction through the polymer solution. The model here can be adapted to other polymer/solvent systems by modifying the thermodynamic and kinetic parameters specific to the two species.

Phase Separation in Polymer Solutions under Extension

The results obtained recently by the authors during investigation of the phase behavior of polymer solutions under uniaxial tension are reviewed. The main attention is given to the dilute solutions of unentangled semiflexible macromolecules. The effect of the solution extension rate on the coil–stretched coil transition for semiflexible chains is considered, and stability of the system of stretched coils is analyzed in terms of their segregation followed by formation of the concentrated polymer phase. The spinodal and binodal of the system are determined depending on temperature and extension rate. The kinetics of solution segregation to polymer and solvent, where three stages, namely, (i) spinodal decomposition accompanied by the formation of regions with increased and reduced polymer contents, (ii) formation of the microfibrillar network, and (iii) network collapse yielding separation of the solvent from the polymer phase are identified, is described.

Diffusion and phase separation at the morphology formation of cellulose membranes by regeneration from N-methylmorpholine N-oxide solutions

Phase separation of polymer solutions initiated by the addition of a nonsolvent is the main method for the preparation of polymer membranes. Depending on the application, such membranes must have a different pore size, which depends on the numerous parameters of the forming process. The liquid–liquid phase separation has been considered for cellulose solutions in N-methylmorpholine N-oxide (NMMO) interacting with various alcohols (methyl, ethyl, isopropyl, and isobutyl). Kinetics of cellulose regeneration was investigated by laser interferometry technique to understand the mechanism of cellulose film structure formation in the NMMO process. Influence of temperature, coagulant nature, and cellulose content on the process kinetics and morphology of the films was studied and corresponding interdiffusion coefficients were calculated. Based on the solubility parameters, triple phase diagrams of the systems were calculated. Formation of different morphologies was explained primarily by the different position of the composition path, the bimodal curve, and the gelation line in the phase diagrams. The second important parameter was the different rate of mutual diffusion of the NMMO and coagulants, due to the difference in the viscosity of the latter. Using methanol or ethanol as coagulation baths leads to obtaining the nanoporous structure of cellulose films, whereas isopropanol and isobutanol favors macropore formation.

Morphological-evolution pathway during phase separation in polymer solutions with highly asymmetrical miscibility gap.

Microstructural evolution during thermally induced phase separation in a polymer solution with a highly asymmetrical miscibility gap is analyzed using Flory-Huggins thermodynamics and nonlinear Cahn-Hilliard kinetics. Computer simulation results demonstrate that, in contrast to systems with symmetric miscibility gaps, interesting morphological-evolution pathways (MEPs) including cluster-to-percolation and percolation-to-cluster transitions are identified. These MEPs are rationalized according to asynchronous evolution of the two product phases. For a highly asymmetric polymer system, the initial solution composition is also found to play a critical role in determining the MEPs. According to the simulation results a map of MEPs in terms of initial solution composition and aging time of phase separation is established to guide future microstructural design in asymmetrical polymer solutions.

Interpolymer complexes as modifiers of the structure and properties of polymeric materials

Systematic analysis of the effect of various factors on the behavior of polymer solutions used for forming the porous structure by impregnation of nonwoven fibrous substrates in the process of fabrication of synthetic composite materials and coatings is performed. Scientific foundations of a targeted control of the phase separation of polymer solutions on their modification with interpolymer complexes of different compositions and structures in order to obtain products and materials with a porous structure and set of performance characteristics required for various applications.