Phase Separation In Polymer Solutions Research Articles

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.

Phase separation of polymer solutions on a solvent surface

Study of the dynamics of liquid droplets of dilute and semidilute polymer solutions on the surfaces of liquid subphases representing solvents for corresponding polymers has shown that a spot of a rather stable layer is formed on an air–liquid interface. The spot spreads over a liquid subphase surface to yield a monomolecular polymer layer. At the same time, the solvent passes into the solution, so that the polymer or its concentrated solution remains on the subphase surface. The polymer does not dissolve in the bulk subphase for several hours. The stability of the polymer spot has been explained under the assumption that the interfacial surface possesses elastic properties and hinders the penetration of macromolecules into the bulk subphase. Desolvation of macromolecules followed by phase separation occurs on the surface. The initial rate of the phase separation of the solution is rather high, while the time dependence of the diameter of the spreading spot is described by a scaling law with an exponent almost equal to 2/3.

A multi-fluid model for microstructure formation in polymer membranes.

We develop a multi-fluid model for a ternary polymer solution using the Rayleighian formalism of Doi and Onuki, and give an efficient pseudo-spectral method for solving both the diffusion and momentum equations that result. Subsequently, we find that the numerical simulation is capable of describing systems at the micron length-scale and easily reaches millisecond time-scales. In addition, we characterize the model thermodynamics and kinetics including the (i) phase behavior, (ii) structure of the interfaces, (iii) mutual diffusion coefficients, (iv) bulk spinodal decomposition kinetics with and without hydrodynamics and (v) spinodal decomposition in the presence of an interface with a non-solvent bath. We obtain good qualitative agreement with the expected thermodynamic and kinetic behavior. We also show that a linear stability analysis of the diffusion equation quantitatively predicts the fastest growing mode obtained from simulation and gives insight into the phase separation process relevant for the evolution of microstructure in phase-separating ternary polymer solutions.

Removal of Heavy Metal Ions by Poly(vinyl alcohol) and Carboxymethyl Cellulose Composite Hydrogels Prepared by a Freeze–Thaw Method

Environmentally friendly composites made of poly(vinyl alcohol) and carboxymethyl cellulose (PVA/CMC) hydrogels were proposed. The PVA/CMC hydrogels were prepared by freeze–thaw processes and applied to adsorb heavy metal ions including Ag+, Ni2+, Cu2+, and Zn2+. The phase separation of polymer solutions during freezing stages led to the formation of crystallites and resulted in insoluble hydrogels. The PVA/CMC hydrogels were effectively acquired, confirmed by at least 71% of insoluble gels found in the gel fraction. In addition, the gel fraction, physical properties, and swelling ratios were clearly modulated by the compositions of PVA and CMC among the samples of pure PVA, P2C1 (containing two-thirds of PVA and one-third of CMC), P1C1, and P1C2 hydrogels. For pure PVA hydrogels, the swelling ratio was 416%, while the P1C2 hydrogels exhibited a much higher swelling ratio (1437%). For the metal adsorption, the experimental results indicated that the P2C1 hydrogels presented not only the highest adsorption...

Three-dimensional phase-field simulations of membrane porous structure formation by thermally induced phase separation in polymer solutions

We present a three-dimensional phase-field simulation of membrane porous structure formation via spinodal decomposition by the thermally induced phase separation (TIPS) method. The free energy of the polymer solution system was described based on the Flory–Huggins theory, and the mobility was calculated from the solvent self-diffusion coefficients estimated by the free-volume theory of Vrentas and Duta. We explored the effects of initial polymer concentration, quench temperature, and gradient energy parameter on the membrane morphology evolution. The initial domain size increased with the decreasing driving force for the spinodal decomposition and the increasing contribution of the gradient energy to the total energy. The morphology evolution during the intermediate and late stage of spinodal decomposition also depended on the membrane morphology formed at the initial stage, such as bicontinuous or droplet structures. We also simulated the membrane formation under the influence of a polymer concentration gradient in the TIPS process. The polymer concentration gradient led to the formation of membranes with anisotropic pore structures.

On the nature of phase separation of polymer solutions at high extension rates

ABSTRACTExperiments with stretching moderately concentrated polymer solutions have been carried out. Model experiments were carried out for poly(acrylonitrile) solutions in dimethyl siloxane. Just the choice of concentrated solutions allowed for a clear demonstration of a demixing effect with the formation of two separate phases—an oriented polymer fiber and solvent drops sitting on its surface. An original experimental device for following all subsequent stages in the demixing process was built. It combined two light beams, one transverse to the fiber and a second directed along (inside) the fiber, the latter played the role of an optical line. This gives a unique opportunity to observe processes occurring inside a fiber. The process of demixing starts from the volume phase separation across the whole cross section of a fiber at some critical deformation and the propagation of the front of demixing along the fiber. Then a solvent cylindrical skin appears which transforms into a system of separate droplets. New experimental data are discussed based on a comparison of the current different points of view on the phenomenon of deformation‐induced phase separation: thermodynamic shift of the equilibrium phase transition temperature, growth of stress‐induced concentration fluctuations in two‐component fluids, and mechanically pressing a solvent out from a polymer network. The general belief is that a rather specific (so‐called “beads‐on‐a‐string”) structure of a filament is realized in stretching dilute solutions: beads of a polymer solution connected by oriented polymer bridges forming a single object. The situation in stretching moderately concentrated solutions appears quite different: real phase separation was observed. So, the alternative phenomenon to the formation of the “beads‐on‐a‐string” structure has been experimentally proven. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 559–565

Liquid filament instability due to stretch-induced phase separation in polymer solutions

The instability in a jet of a viscoelastic semidilute entangled polymer solution under high stretching is discussed. Initially the jet is stable due to solution elasticity stopping the fluctuation growth. The modeling shows that a jet stretching results in a radial gradient in the polymer distribution: the polymer is concentrated in the jet centre, whereas the solvent is remaining near the surface. This viscous liquid shell demonstrates Raleigh-type instability resulting in the formation of individual droplets on the oriented filament. All process stages were observed experimentally.

Porous Polymer Nanostructures Fabricated by the Surface-Induced Phase Separation of Polymer Solutions in Anodic Aluminum Oxide Templates

We study the formation of porous polymer nanostructures fabricated by the surface-induced phase separation of polymer solutions in anodic aluminum oxide (AAO) templates. Poly(methyl methacrylate) (PMMA) and tetrahydrofuran (THF) are used to investigate the evolution process of the surface-induced phase separation. With the longer immersion time of the AAO template in the polymer solution, the size of the solvent-rich droplet is increased by the coarsening process, resulting in the formation of porous polymer nanostructures. The coarsening mechanism is further evaluated by changing the experimental parameters including the immersion time, the polymer concentration, the polymer molecular weight, and the solvent quality. Under conditions in which polymer solutions have higher viscosities, the coarsening process is slowed down and the formation of the porous nanostructures is prohibited. The prevention of the porous nanostructures can also be realized by adding water to the PMMA/THF solution before the immersion process.

WO3 nanonodule-decorated hybrid carbon nanofibers for NO2 gas sensor application

WO3 nanonodule-decorated carbon nanofibers (CNFs) of various diameters were fabricated by single-nozzle co-electrospinning using two phase-separated polymer solutions. Using more polyvinylpyrrolidone (PVP) solution decreased the CNF diameter from 130 to 40 nm and increased the Brunauer–Emmett–Teller (BET) surface area from 147 to 276 m2 g−1. A spin-coating method was used to deposit the hybrid CNFs on the sensor substrate to minimize the contact resistance between them. In addition, ultraviolet (UV) irradiation lowered the desorption energy level of the NO2 gas between the transducer materials during the recovery time. As a result, the recovery time decreased to ca. 7 min using UV light with an intensity of 75 mW cm−2. The sensitivity of the hybrid CNF gas sensors increased with decreasing diameter of the CNFs; the minimum detectable level (MDL) was 1 ppm at room temperature for 40 nm hybrid CNFs dispersed uniformly on the electrode. Furthermore, increasing the amount of decorated WO3 nanonodules on the CNF surface enhanced the sensitivity to NO2 gas. The NO2 sensor made with the hybrid CNFs had sensing performance comparable to those made with conventional metal oxide-based nanomaterials and pristine carbon nanotubes (CNTs).

Viscoelastic interface motion in semidilute polymer solutions

We study interface motion in phase separated semidilute polymer solutions near the critical point. We derive the non-Markovian equation for motion of an interface taking into account the viscoelastic effects due to the transient polymer network and the hydrodynamic interaction between interfaces. This equation indicates that the velocity of an interface is affected by the history of the past velocity when the characteristic length scale of domains is shorter than , where d0 is the capillary length and ξve is the viscoelastic length. We also study growth of a droplet in metastable polymer solutions. The growth is much decelerated when the radius of a droplet is smaller than .

Spinodal decomposition of polymer solutions: molecular dynamics simulations of the two-dimensional case

As a generic model system for phase separation in polymer solutions, a coarse-grained model for hexadecane/carbon dioxide mixtures has been studied in two-dimensional geometry. Both the phase diagram in equilibrium (obtained from a finite size scaling analysis of Monte Carlo data) and the kinetics of state changes caused by pressure jumps (studied by large scale molecular dynamics simulations) are presented. The results are compared to previous work where the same model was studied in three-dimensional geometry and under confinement in slit geometry. For deep quenches the characteristic length scale ℓ(t) of the formed domains grows with time t according to a power law close to . Since in this problem both the polymer density ρp and the solvent density ρs matter, the time evolution of the density distribution PL(ρp,ρs,t) in L × L subboxes of the system is also analyzed. It is found that in the first stage of phase separation the system separates locally into low density carbon dioxide regions that contain no polymers and regions of high density polymer melt that are supersaturated with this solvent. The further coarsening proceeds via the growth of domains of rather irregular shapes. A brief comparison of our findings with results of other models is given.

Phase-Separating Aqueous Polymer Solutions as Simple Experimental Models for Cytoplasm

The interior of biological cells is crowded with macromolecules, heterogeneous in composition, and dynamic. We are developing simple experimental models for intracellular environments based on aqueous polymer solutions. Macromolecular crowding is modeled by inclusion of polymers such as polyethylene glycol or dextran; when both polymers are present aqueous phase separation can occur. Phase separation provides microcompartments corresponding to the dextran-rich and PEG-rich phase domains and enables control over the local concentration of not only the polymers themselves but also any molecules that accumulate in one of the phases by partitioning. Local enrichment of tenfold or greater can be maintained between the phases for solutes such as nucleic acids or proteins, and can be used to drive reactions that are concentration-dependent. Encapsulation of the aqueous two-phase systems within lipid vesicles having a semipermeable membrane provides a primitive model of biological cells capable of microcompartmentation, polarity, and asymmetric division. The synthesis and characterization of artificial cells and cell-like environments may provide new insight into how fundamental chemical and physical phenomena common to all cells may have shaped the development of early cells and underlie many of the seemingly complex behaviors of modern cells.

Collective Cohesion Forces in Polymers

Collective cohesion forces are a set of normal cohesion forces the origins of which exhibit a regularity in their spatial distribution. Atoms, molecules or segments that undergo their action display collective coupled motions where the neighbours do not change or identity. Such particular kind of motion can be called “rigidity”. The repetitive character of the polymeric chain renders the segments particularly susceptible to generate collective cohesion forces but this tendency is not the same for each of them. Rigid segments have a lower chemical potential than fluid segments. This is the case for the crystalline parts of the semi-crystalline polymers. The tendency of some sequences to generate collective cohesion forces is so great that in most cases rigid domains are spontaneously formed where the spatial regularity does not extend to the three dimensions, what makes them less detectable by diffraction methods. But these rigid domains being connected with other zones deprive the last ones from macroscopic mobility and no crystallization can take place. Moreover, as evidenced by dipolar measurements, even in true solutions given sequences of segments are regularly involved in ephemeric rigid domains, and have therefore a lower chemical potential. Phase separation occurs when, as a consequence of the change of solvent or temperature, some rigid domains become permanent. As a consequence of this inherent inequality between the segments, the criterion of phase separation in polymer solutions is the existence of two states α and β in which the lowest chemical potential or the segments is the same. This fundamentally differs from the liquid-liquid treatment of Flory.

Intrinsic Contact Angle of Aqueous Phases at Membranes and Vesicles

Biomimetic membranes exposed to several aqueous phases are studied theoretically. Specific examples are provided by lipid vesicles and phase separating polymer solutions. The theory reveals the existence of an intrinsic contact angle, theta(in), between the membranes and the aqueous phases. An explicit relation is derived by which this intrinsic angle can be determined from the effective contact angles as measured by optical microscopy. A detailed comparison between theory and experiment provides strong evidence that the intrinsic contact angle theta(in) represents a material parameter, in contrast to the effective contact angles.

Cloud point phenomena for POE-type nonionic surfactants in imidazolium-based ionic liquids: Effect of anion species of ionic liquids on the cloud point

Cloud point temperatures, T c, of polyoxyethylene (POE)-type nonionic surfactants in a room temperature ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF 6), were measured and compared with those previously obtained for the surfactant solution in bmimBF 4. The T cs for bmimPF 6 solution are higher than those for bmimBF 4 solution by approx. 40 °C. This means that the surfactant molecules are more solvophilic in bmimPF 6 compared to bmimBF 4. The analysis of 1H NMR chemical shift measurements proved that the higher solvophilicity of the surfactants in bmimPF 6 is attributed to weaker hydrogen-bond interaction between bmim cation and PF 6 - anion than that between bmim cation and BF 4 - anion. This interpretation is consistent with the interaction energy parameters derived from the thermodynamic analysis of cloud point curve applying the Flory–Huggins model for phase separation in polymer solution. The present work demonstrates that the property of imidazolium-based ionic liquids as a solvent is determined by a balance of interactions among imidazolium cation, counter anion, and solute molecule.

Cloud point phenomena for POE-type nonionic surfactants in a model room temperature ionic liquid

The cloud point phenomenon has been investigated for the solutions of polyoxyethylene (POE)-type nonionic surfactants (C 12E 5, C 12E 6, C 12E 7, C 10E 6, and C 14E 6) in 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF 4), a typical room temperature ionic liquid (RTIL). The cloud point, T c , increases with the elongation of the POE chain, while decreases with the increase in the hydrocarbon chain length. This demonstrates that the solvophilicity/solvophobicity of the surfactants in RTIL comes from POE chain/hydrocarbon chain. When compared with an aqueous system, the chain length dependence of T c is larger for the RTIL system regarding both POE and hydrocarbon chains; in particular, hydrocarbon chain length affects T c much more strongly in the RTIL system than in equivalent aqueous systems. In a similar fashion to the much-studied aqueous systems, the micellar growth is also observed in this RTIL solvent as the temperature approaches T c . The cloud point curves have been analyzed using a Flory–Huggins-type model based on phase separation in polymer solutions.

Band formation on shearing in phase-separated polymer solutions

In a phase-separated mixture of two Newtonian polymer solutions sheared in a cone-plate geometry adapted for microscopic observation at a zero-velocity plane, shear-induced coalescence of droplets of the broken-up phase, followed by band formation, was observed. Initially wormlike structures developed into doughnut-shaped bands, disconnected from the walls of the geometry. The shear rate and composition inside the bands differed from that in the outside solution. The shear-shear rate instability preceding the band formation could be qualitatively described by a van der Waals-loop-shaped shear rate dependence of droplet deformation.

Microcapsules from Self-Assembled Colloidal Particles Using Aqueous Phase-Separated Polymer Solutions

We report a new way of producing microcapsules consisting of a shell of aggregated colloids (also referred to as colloidosomes) using aqueous phase-separated polymer solutions (water-in-water emulsions) as a template. The extremely low interfacial tension of the template allows the production of reversible colloidosomes that spontaneously disintegrate depending on environmental conditions, making them ideal for controlled release purposes. Also, colloidosomes can have an elongated shape such that they may be used as microfluidic membranes or artificial arteries. Because the method described here does not use any organic solvents, it enables the use of sensitive materials such as cells and proteins.

Phase behavior of polymer mixtures with nonadditive hard-sphere potential

The phase behavior of a binary mixture of homopolymers in which macromolecules are composed of tangent hard spheres was studied. The interaction of unlike units is characterized by the contact distance (1/2)(σA + σB)(1 + Δ), where σ i is the diameter of the ith sphere (unit) and Δ is the nonadditivity parameter. The effect of nonadditivity was taken into account by means of the perturbation theory relative to the additive system (Δ = 0) considered earlier (Polymer Science, 47, 2146 (2005)) in terms of the Percus-Yevick approximation. The theoretical consideration presented is completely analytical. It was found that a polymer mixture experiences phase separation with an increase in pressure; the two-phase region extends with an increase in both the size ratio between the units α = σA/σB and the length of the chain per se. Closed phase diagrams were first predicted for athermal mixtures; such diagrams appear at Δ < 0 and certain values of α. It was shown that the thermodynamics of an incompressible mixture of hard-chain molecules at α = 1 follows the Flory-Huggins theory with the temperature-independent interaction parameter. Phase separation in polymer solutions with the nonadditive hard-sphere potential was also analyzed.

Design and preparation of polymeric scaffolds for tissue engineering

Polymeric scaffolds for tissue engineering can be prepared with a multitude of different techniques. Many diverse approaches have recently been under development. The adaptation of conventional preparation methods, such as electrospinning, induced phase separation of polymer solutions or porogen leaching, which were developed originally for other research areas, are described. In addition, the utilization of novel fabrication techniques, such as rapid prototyping or solid free-form procedures, with their many different methods to generate or to embody scaffold structures or the usage of self-assembly systems that mimic the properties of the extracellular matrix are also described. These methods are reviewed and evaluated with specific regard to their utility in the area of tissue engineering.