Early Stage Of Phase Separation Research Articles

Interdiffusion and Spinodal Decomposition in Electrically Conducting Polymer Blends

The impact of phase morphology in electrically conducting polymer composites has become essential for the efficiency of the various functional applications, in which the continuity of the electroactive paths in multicomponent systems is essential. For instance in bulk heterojunction organic solar cells, where the light-induced electron transfer through photon absorption creating excitons (electron-hole pairs), the control of diffusion of the spatially localized excitons and their dissociation at the interface and the effective collection of holes and electrons, all depend on the surface area, domain sizes, and connectivity in these organic semiconductor blends. We have used a model semiconductor polymer blend with defined miscibility to investigate the phase separation kinetics and the formation of connected pathways. Temperature jump experiments were applied from a miscible region of semiconducting poly(alkylthiophene) (PAT) blends with ethylenevinylacetate-elastomers (EVA) and the kinetics at the early stages of phase separation were evaluated in order to establish bicontinuous phase morphology via spinodal decomposition. The diffusion in the blend was followed by two methods: first during a miscible phase separating into two phases: from the measurement of the spinodal decomposition. Secondly the diffusion was measured by monitoring the interdiffusion of PAT film into the EVA film at elected temperatures and eventually compared the temperature dependent diffusion characteristics. With this first quantitative evaluation of the spinodal decomposition as well as the interdiffusion in conducting polymer blends, we show that a systematic control of the phase separation kinetics in a polymer blend with one of the components being electrically conducting polymer can be used to optimize the morphology.

A computational study of short-range surface-directed phase separation in polymer blends under a linear temperature gradient

The nonlinear Cahn–Hilliard theory and the Flory-Huggins-de-Gennes theory were used to study numerically the surface-directed phase separation phenomena of a model binary polymer blend quenched into the unstable region of its binary symmetric upper critical solution temperature phase diagram. Short-range surface potential within a square geometry, where one side of the binary polymer blend is exposed to a surface with preferential attraction to one component of the blend that is under a linear temperature gradient along the direction perpendicular to the surface, was integrated into the model. The structure factor analysis showed a faster exponential growth at the early stage of phase separation and a slower growth rate at the intermediate stage with a slope of 0.31 within the bulk, which is consistent with the Lifshitz–Slyozov growth law. The investigation of surface enrichment rate at the surface wall demonstrated faster growth rate at the early stage with the slope of 0.5. This growth rate became slower at the intermediate stage with a slope of 0.13 near the surface. The effect of various temperature gradient values on the surface enrichment rate with constant temperature T1* at the surface preferentially attracting one of the polymer components and different temperature T2* at the opposite surface, where T1*>T2*, was studied for the first time. The results showed that the thickness of the wetting layer increased with increasing temperature difference ΔT*, where ΔT*=T1*−T2*. The structure factor analysis of the surface potential h1 effect on the phase separation within the bulk close to the surface showed earlier transition time for higher values of h1. However, there was no difference observed for transition time within the bulk at distances farther away from the surface. As the surface potential increased, spinodal wave became more visible in the bulk and the transition time from complete wetting to partial wetting occurred at a later time on the surface.

Effect of nanoclay on the phase separation behavior of poly(methyl methacrylate)/poly(vinyl acetate) binary polymer blends

The effect of nanoclay on the phase-separation behavior of poly(methyl methacrylate)/poly(vinyl acetate) (PMMA/PVAc) blends has been mainly investigated by small-angle laser light scattering. It is found that the effect of clay on the thermodynamics and kinetics of phase-separation for PMMA/PVAc blends seems inconsistent. The kinetics phase-separation rate decreases, while the thermodynamics parameters, cloud points T c and delay time t D of isothermal phase-separation also decrease, and the variation amplitude depends on the matrix composition. The affinity of clay to PMMA results in the composition difference between the border layer and the polymer matrix and further causes the concentration fluctuation at the early stage of phase separation to reduce T c and t D. On the other hand, the decrease of phase-separation rate is caused by the mechanical barrier effect of clay on the macromolecular diffusion of blend matrix. Hence, such seemingly counterintuitive results on the thermodynamics and kinetics of phase-separation are attributed to different dominant factors.

Surfactant Behavior of Ionic Liquids Involving a Drug: From Molecular Interactions to Self-Assembly

Aggregates formed in an aqueous medium by three ionic liquids CnMImIbu made up of 1-alkyl-3-methyl-imidazolium cation (n = 4, 6, 8) and ibuprofenate anion are investigated. Dynamic light scattering (DLS), cryogenic transmission electron microscopy (cryo-TEM), (1)H nuclear magnetic resonance measurements, and atom-scale molecular dynamics simulations are used to shed light on the main interactions governing the formation of the aggregates and their composition. At high concentration, mixed micelles are formed with a composition that depends on the imidazolium alkyl chain length. For the shortest alkyl chain, micelles are mainly composed of ibuprofenate anions with some imidazolium cations intercalated between the anions. Upon increasing the alkyl chain length, the composition of the aggregates gets enriched in imidazolium cations and aggregates of stoichiometric composition are obtained. Attractive interactions between these aggregates led to the formation of larger aggregates. As suggested by molecular simulations, these larger aggregates might constitute the early stage of phase separation. Transitions from micelles to vesicles or ribbons are observed due to dilution effects and changes in the chemical composition of the aggregates. We also show that aggregation can be probed using simple microscopic quantities such as radial distribution functions and average solvation numbers.

Anomalous phase separation behavior in dynamically asymmetric LCST polymer blends

The interplay of thermodynamic forces and self-generated stresses induced in different compositions, and at different quench depths on the phase behavior of dynamically asymmetric PS/PVME blends are studied. The thermodynamic phase diagram is obtained from dynamic temperature sweep experiments. Phase contrast optical microscopy and rheological measurements including linear viscoelastic behavior and the stress growth behavior are employed to investigate the time evolution of the different phase-separating morphologies. At an intermediate quench depth of 14 °C, in addition to thermodynamically controlled phase separation mechanisms (nucleation and growth, NG, and spinodal decomposition, SD), three different kinds of phase separation behavior are induced by transient gel formation due to self-induced stresses in the early stage of phase separation: (i) conventional viscoelastic phase separation (VPS), (ii) nucleation of aggregate-like PS-rich phase and subsequent formation of a percolated network by coalescence, and (iii) nucleation of an aggregate-like PS-rich phase while the dispersed-matrix morphology is preserved in the later stages of phase separation. While it is generally accepted that VPS occurs at deep quench depths, we observe the VPS mechanism at shallow quench depths which is attributed to dynamic heterogeneity in the phase-separated domains. A dynamic phase diagram which shows the effect of dynamic asymmetry on phase behavior is proposed.

Early Stage Phase Separation in Pharmaceutical Solid Dispersion Thin Films under High Humidity: Improved Spatial Understanding Using Probe-Based Thermal and Spectroscopic Nanocharacterization Methods

Phase separation in pharmaceutical solid dispersion thin films under high humidity is still poorly understood on the submicrometer scale. This study investigated the phase separation of a model solid dispersion thin film, felodipine-PVP K29/32, prepared by spin-coating and analyzed using probe-based methods including atomic force microscopy, nanothermal analysis, and photothermal infrared microspectroscopy. The combined use of these techniques revealed that the phase separation process occurring in the thin films under high humidity is different from that in dry conditions reported previously. The initial stage of phase separation is primarily initiated in the bulk of the films as amorphous drug domains. Drug migration toward the surface of the solid dispersion film was then observed to occur under exposure to increased humidity. PVP cannot prevent phase separation of felodipine under high humidity but can minimize the crystallization of amorphous felodipine domains in the solid dispersion thin films. This study demonstrates the unique abilities of these nanocharacterization methods for studying, in three dimensions, the phase separation of thin films for pharmaceutical applications.

Dynamic structure factor of density fluctuations from direct imaging very near (both above and below) the critical point of SF6

Large density fluctuations were observed by illuminating a cylindrical cell filled with sulfur hexafluoride (SF(6)), very near its liquid-gas critical point (|T-T(c)|< 300 μK) and recorded using a microscope with 3 μm spatial resolution. Using a dynamic structure factor algorithm, we determined from the recorded images the structure factor (SF), which measures the spatial distribution of fluctuations at different moments, and the correlation time of fluctuations. This method authorizes local measurements in contrast to the classical scattering techniques that average fluctuations over the illuminating beam. We found that during the very early stages of phase separation the SF scales with the wave vector q according to the Lorentzian q(-2), which shows that the liquid and vapor domains are just emerging. The critical wave number, which is related to the characteristic length of fluctuations, steadily decreases over time, supporting a sustained increase in the spatial scale of the fluctuating domains. The scaled evolution of the critical wave number obeys the universal evolution for the interconnected domains at high volume fraction with an apparent power law exponent of -0.35 ± 0.02. We also determined the correlation time of the fluctuations and inferred values for thermal diffusivity coefficient very near the critical point, above and below. The values were used to pinpoint the crossing of T(c) within 13 μK.

Determination of Phase Behavior of Poly(ethylene oxide) and Chitosan Solution Blends Using Rheometry

Aqueous solutions of PEO exhibit a lower critical solution temperature (LCST) phase diagram. In this work, phase separation behavior of PEO/water solutions was investigated using small-amplitude oscillatory shear and steady shear rheological measurements. Binodal decomposition temperatures were determined from the sudden changes in the slope of the dynamic temperature sweep of storage modulus and loss tangent. The spinodal decomposition points were also estimated by a mean-field theoretical approach. Comparing the obtained critical points with other conventional methods revealed that rheological measurements are powerful and sensitive to detect even the early stage of phase separation of PEO solutions. This successful method was employed to investigate phase separation and miscibility of chitosan/PEO solutions at different compositions in aqueous acetic acid solutions that have already showed anomalous behavior in a forming process. Lower critical solution temperature (LCST) phase behavior was observed for chitosan/PEO solution blends. Phase separation temperature, miscibility range, and correlation length of the solutions were determined from isochronal dynamic temperature sweep experiments. The effect of chitosan/PEO ratio on the binodal and spinodal decomposition temperatures was studied. Finding phase separation information on polymer solutions through rheological measurement is very promising. Isothermal steady shear rheological measurements were also carried out on chitosan/PEO solutions over a temperature range in which phase separation occurs. Viscosity increase at low shear rates above but in the vicinity of phase separation temperature was observed, which confirms the validity of the theoretical approach employed to determine the critical temperatures through dynamic rheological measurements. Finally, the Flory–Huggins interaction parameters were estimated from critical solution temperature and concentration results.

Microfluidic-Enabled Real-Time Imaging of Lipid Domains in Bilayer Membranes

A microfluidic bilayer lipid membrane system supporting correlated opto-electrical membrane characterization has been developed and applied to the real time imaging of lipid domains by confocal microscopy. Annulus supported bilayers are formed by a novel “kiss and retreat” method and the device supports rapid buffer exchange in both chambers. Additionally, the system has been extended for the creation of solvent-free asymmetric bilayers by a microfluidic adaptation of the monolayer folding method. Because the dynamic folding process itself may be imaged, the very early stages of phase separation and domain growth may be observed. By enabling the simultaneous measurement of transmembrane current and optical observation of lipid microdomains in real-time, the microfluidic platforms open the door to new multi-domain measurements of dynamic lipid-lipid and lipid-ligand interactions that are not feasible using established tools. (left) Confocal image sequences of liquid ordered domains in a bilayer formed by kiss and retreat method. Under ∼5 Pa transmembrane pressure, lo domains are observed to emerge, grow and merge. (right) Confocal image sequence of bilayer formation by monolayer folding, sequentially revealing a single monolayer, the folding process, and the initial emergence of liquid ordered domains.

Hindered Coarsening of a Phase-Separating Microemulsion Due to Dispersed Colloidal Particles

The addition of sterically stabilized colloidal particles to a phase-separating microemulsion leads to dramatic changes in its demixing behavior, especially during the later stages. Our microemulsion is composed of reverse micelles of sodium dodecyl sulfate, pentanol, and water in a dodecane continuous phase which separates into micelle-rich and micelle-poor phases above a lower critical solution temperature. The poly(methyl methacrylate) particles preferentially partition into the less structured, micelle-poor phase. Nucleation of the minority phase or spinodal decomposition close to criticality continue to occur in the presence of particles, albeit with pronounced pretransitional clustering of particles when the micelle-poor phase is in the minority. The coalescence of micelle-poor droplets and the coarsening of micelle-rich domains are both strongly modified due to the presence of colloidal particles. We use our observations of the early stages of phase separation to understand these late stage changes.

The β'-α' Interaction: a Study of early Stages of Phase Separation in a Fe-20%Cr-6%Al-0.5%Ti Alloy

The temporal evolution of the microstructure resulting from phase separation into Fe-rich (α), Cr-rich (α¢), and Fe(Ti,Al) (β¢) phases of a Fe-20Cr-6Al-0.5Ti alloy has been analyzed by thermoelectric power measurements (TEP). The early stages of decomposition and the evolution of the three-dimensional microstructure have been analyzed by atom probe tomography (APT). The roles of Cr, Al, and Ti during the decomposition process have been investigated in terms of solute partitioning between the phases. Analysis of proximity histograms revealed that significant Al and Ti partitioning occurs, which is consistent with theoretical calculations. The results indicate that as the α-α¢ phase separation proceeds, Al and Ti are rejected into the α phase, which causes the β¢ phase to nucleate on the surface of the α¢ phase.

Effect of Polydispersity on the Phase Behavior of Polystyrene (PS)/Poly (Vinyl Methyl Ether) (PVME)

The thermodynamics and kinetics of phase separation in partially miscible blends of poly (vinyl methyl ether) (PVME) and two kinds of polystyrene (PS) with the same weight average molecular weight but different polydispersity were studied. The miscibility of PS/PVME with the monodisperse PS was better than that of PS/PVME with the polydisperse PS. Different morphology was observed for the two kinds of PS/PVME (10/90) blends during the nonisothermal phase separation process. The blend with monodisperse PS presented a co-continuous structure while the blend with polydisperse PS presented a viscoelastic phase separated network structure at a quench depth of 29°C. With increase of the heating rate, the increase of cloud point of PS/PVME (30/70) with polydisperse PS was smaller than that of PS/PVME (30/70) with monodisperse PS. During the isothermal phase separation of the critical composition (20/80) of PS/PVME with a quench depth of 30°C, it was found that the phase morphology of the two kinds of blends was nearly the same at the early stage of phase separation. However, as the dispersed phase, an approximately spherical droplet structure was observed in the blend with monodisperse PS at the late stage of phase separation, which did not appear in the blend with polydisperse PS.

Kinetics of early stages of phase separation in glasses of the PbO-B2O3 system

Kinetics of changes in sizes of phase regions and volumes at the early stages of the phase separation of glasses in the PbO-B2O3 system has been studied using small-angle X-ray scattering (SAXS). The existence of the interval of heating durations in which the changes in the sizes of the phase regions and volumes are small so that it is possible to approximately consider that the structure is in a relatively stable state has been experimentally established. The relative phase volumes and compositions in glasses of the studied system are close to the equilibrium ones already for the sizes (radii) of the regions of the separated phase of about 15–20 A and the Ostwald ripening (coalescence) stage begins. Maxima due to the interparticle interference have been observed in the angular dependences of the SAXS intensities. This indicates ordering in the distribution of the phase regions.

Fluid Inclusion Constraints on the Genesis of Gold in the Darasun District (Eastern Transbaikalia), Russia

Fluid Inclusion Constraints on the Genesis of Gold in the Darasun District (Eastern Transbaikalia), Russia

Spinodal decomposition and phase separation kinetics in nanoclay–biopolymer solutions

AbstractIntensity of light, I(q,t), scattered from homogeneous aqueous solutions, of nanoclay (Laponite) and protein (gelatin‐A), was studied to monitor the temporal and spatial evolution of the solution into a phase‐separated nanoclay–protein‐rich dense phase, when the sample temperature was quenched below spinodal temperature, Ts (=311 ± 3 K). The zeta potential data revealed that the dense phase comprised charge‐neutralized intermolecular complexes of nanoclay and protein chains of low surface charge. The early stage, t &lt; 500 s, of phase separation could be described adequately through Cahn‐Hilliard theory of spinodal decomposition where the intensity grows exponentially, I(q, t) = I0 exp.(2R(q)t). The wave vector, q dependence of the growth parameter, R(q) exhibited a maxima independent of time. Corresponding correlation length, 1/qc = ξc was found to be ≈75 ± 5 nm independent of quench depth. In the intermediate regime, anomalous growth described by I(q, t) ∼ tα with α = 0.1 ± 0.02 independent of q was observed. Rheological studies established that there was a propensity of network structures inside the dense phase. Isochronal temperature sweep studies of the dense phase determined the melting temperature, Tm = 312 ± 4 K, which was comparable with the spinodal temperature. The stress‐diffusion coupling prevailing in the dense phase when analyzed in the Doi‐Onuki model yielded a viscoelastic correlation length, ξv determined from low‐frequency storage modulus, G ′0 ≈ kB T/ξ, which was ξv ≈ 35 ± 3 nm indicating 2ξv ≈ ξc. It is concluded that the early stage of phase separation in this system was sufficiently described by linear Cahn‐Hilliard theory, but the same was not true in the intermediate stage. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 555–565, 2010

Phase separation in polymer blend thin films studied by differential AC chip calorimetry

AC chip calorimetry is used to study the phase separation behavior of 100 nm thin poly(vinyl methyl ether)/poly(styrene) (PVME/PS) blend films. Using the on-chip heaters, very short (10 ms–10 s) temperature jumps into the temperature window of phase separation are applied, simulating laser heating induced patterning. These temperature pulses produce a measurable shift in the glass transition temperature, evidencing phase separation. The effect of pulse length and height on phase separation can be studied. The thus phase separated PVME/PS thin films remix rapidly, in contrast with measurements in bulk. AC chip calorimetry seems to be a more sensitive technique than atomic force microscopy to detect the early stages of phase separation in polymer blend thin films.

Kinetics of phase separation in polymer blends revealed by resonance light scattering spectroscopy

In this work, kinetics of phase separation in the blends of polystyrene (PS) and poly(vinyl methyl ether) (PVME) was investigated by a simple and sensitive method, i.e., resonance light scattering (RLS) spectroscopy. Owing to the aggregation of chromophores (phenyl rings) in the systems when phase separation occurred, RLS intensities were drastically enhanced and hence acted as a characteristic indicator. At the early stage of phase separation, two different RLS behaviors corresponding to spinodal decomposition (SD) and nucleation and growth (NG) were observed. The Cahn-Hilliard (C-H) linearization theory was found not applicable for kinetics analysis of the scattering data at lambda < 346 nm due to RLS effect near the absorption band. Based on a decomposition reaction model, the apparent activation energy of SD phase separation was estimated by the Arrhenius equation. In view of its simplicity and sensitivity of measurement, affordability and availability of instrument, and wide application range of polymer blends, RLS proved to be an effective means for characterization of microstructural variation in polymer blends.

Cure kinetics, phase behaviors, and fracture properties of bismaleimide resin toughened by poly(phthalazinone ether ketone)

AbstractPoly(phthalazinone ether ketone)s (PPEK) were used to toughen bismaleimide (BMI) resin composed of 4,4′‐bismaleimidodiphenyl methane (BMDM) and O,O′‐diallyl bisphenyl A (DABPA). Dynamic differential scanning calorimetry (DSC) of the blends was carried out for kinetic analysis of the curing reaction. The reaction activation energy indicated that the reaction mechanism remained the same even after the incorporation of PPEK. The reaction‐induced phase separation process in BMI/PPEK blends was investigated by optical microscopy (OM). The primary phase structure of the blends was fixed at the early stage of phase separation, and a secondary phase separation was observed as a result of the high viscosity of the blends. Scanning electron microscope (SEM) graphs showed that the morphology of the cured resin changed from a dispersed structure to a phase‐inverted structure with the increase of PPEK content. Compared with the neat resin, the fracture toughness of the modified resin exhibits a moderate increase when PPEK was incorporated. Several toughening mechanisms, such as local plastic deformation, crack deflection, and branches, presumably took part in improving the toughness of BMI/PPEK blends on the basis of the morphology. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers

Preparation and properties of porous glass using fly ash as a raw material

The porous glasses were prepared by a conventional phase separation method using coal fly ash as a raw material, and the properties of these porous glasses were investigated. The composition of coal fly ash is basically composed of SiO 2–Al 2O 3–Fe 2O 3–CaO system and the SiO 2–B 2O 3–Al 2O 3–CaO–Na 2O system of glass was chosen as base glass composition. The pore diameter increases proportional to cube root of heating time ( t 1/3), however, the early stage of phase separation is not clear. It is estimated that the rate determining step may be the diffusion process of structural units involving oxygen ions and the phase separation may take place by the nucleation and growth mechanism, and the relatively larger pores of above 1 μm can be obtained easily. The chemical composition of porous glasses is SiO 2–B 2O 3–Al 2O 3(–CaO–Na 2O). A relatively large amount of fly ash (>40%) can be used successfully for the preparation of porous glass.

The study on poly(ether sulfone) modified cyanate ester resin and epoxy resin cocuring blends

AbstractPoly(ether sulfone) terminated with phenolic hydroxyl groups modified cyanate ester resin and epoxy resin cocuring blends were investigated by differential scanning calorimetry, Fourier transform infrared spectroscopy, scanning electron microscopy, rheometry, and mechanical properties measurement. The results suggested that poly (ether sulfone) (PES) could accelerate the polycyclotrimerization reaction of cyanate ester and cocuring processes between cyanate ester and epoxy of modified blends because of the presence of phenolic hydroxyl groups at the end of the PES molecules. It was found that the evolution of the morphologies and complex viscosities of the modified blends sensitive to molecular weight and content of PES, the tensile strength and elongation at break of the modified blends were correlated with the morphologies of modified blends. Moreover, the evolution of complex viscosities of the modified blends also showed an exponential growth at the early stage of phase separation, which demonstrated experimentally that the coarsening processes of droplets of bisphenol‐A dicyanate and diglycidyl ether of bisphenol A and the final morphologies obtained in the blends modified with PES were affected by viscoelastic behavior. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009