Immiscibility Loop Research Articles

Cooperative progress of crystallization and spinodal decomposition in the blends of dissimilar polycarbonates

We investigated the structure development of the blends of bisphenol A polycarbonate (BPA-PC) and cyclohexane dimethylene polycarbonate (CHD-PC) during isothermal annealing from the miscible state. The crystallization of BPA-PC was found to be induced in the blends and small distorted crystallites developed in the interconnected structure by annealing within the immiscibility loop type phase boundary. The analysis for the time variation of the light scattering intensity revealed that 1) liquid-liquid phase separation occurred via spinodal decomposition without induction period, 2) crystallization is induced at early stage of the spinodal decomposition, 3) crystallization was largely accelerated in the blend composition in which the apparent diffusion coefficient for spinodal decomposition was larger, 4) crystallization progresses cooperatively with liquid-liquid phase separation at late stage of the spinodal decomposition. Owing to the cooperative progress, the glass transition of the blend estimated by DMA shifted to that of the neat component polymer during the crystallization.

On the metastable liquid miscibility gap in Pd-Ni-P

Based on the results of experiments performed at temperatures T ≳ 613 K, a thermodynamic model was recently proposed to explain: (i) the origin of a metastable liquid miscibility gap (MLMG) in Pd-Ni-P, an alloy system with a negative heat of mixing. It originates from the superposition of two distinct liquid Gibbs free energy blades; and (ii) a liquid-liquid phase transition (LLT) just outside the MLMG. Since Gibbs free energy is a function of T, the MLMG was revisited in this work for its T dependence, but this time with T ≲ 613 K. It was found that the MLMG has the shape of a truncated right angle prism, instead of a right angle prism as suggested previously. It is similar to, but not the same as an immiscibility loop in a binary liquid mixture. The revised MLMG is in good agreement with the predictions of the thermodynamic model.

Effects of molecular complexation on phase equilibria in mixtures of urea/polyethylene glycol derivatives and electrochemical performance of urethane based polymer electrolyte membranes for solid-state lithium ion battery

Phase diagram approach has been extended to the determination of phase diagram of a binary pair of urea with polyethylene glycol (PEG) in order to providence guidance to the formulation window of isotropic polymer electrolyte membranes (PEMs) with emphasis on the electrochemical performance. Effect of molecular weight of PEG on solid-liquid phase diagrams of its mixtures with urea has been demonstrated experimentally and theoretically. The observed change of eutectic, immiscibility loop, azeotrope type phase diagrams with increasing PEG molecular weight (or decreasing chain-ends) has been analyzed in the context of the combined Flory-Huggins (FH) theory of liquid-liquid demixing and phase field (PF) theory of crystal solidification. This rich phase behavior has been captured via the variation of crystalline-crystalline interaction strength of the combined FH-PF free energies. Subsequently, the phase diagram of poly (ethylene glycol) diacrylate (PEGDA)/urea was constructed in the framework of the aforementioned FH-PF theory. The incorporation of urea not only dissociates lithium cation from its salt, but also enhances the ionic conductivity of urea-modified polymer electrolytes dramatically. However, small molecule hydrophilic urea was found to be electrochemically unstable in the charge/discharge cycling test. To alleviate such shortcoming, urea (or urethane) linkage was imparted into the network chains by synthesizing PEG-bis(carbamate dimethacrylate) (PEGBCDMA) and subsequently photopolymerizing in the presence of SCN plasticizer and LiTFSI salt to afford the ion conductive PEM network. The as-synthesized PEGBCDMA based PEM consisting of PEGBCDMA/SCN/LiTFSI (20/40/40 ratio by weight) turned out to be completely amorphous and transparent before and after photo-crosslinking. The incorporation of urethane linkages in the PEM network exhibited enhanced ionic conductivity, improved thermal stability, and superior tensile properties with improved capacity retention during charge/discharge cycling, suggesting potentials for application in solid-state lithium ion battery.

The interplay between wetting and demixing in non-additive symmetric mixtures at selective walls

The Monte Carlo simulation method in the grand canonical ensemble has been used to study the wetting behavior of highly non-additive symmetric mixtures in contact with selective walls. We have focused on the behavior of the mixtures, which exhibit closed immiscibility loops in the bulk, and have investigated the interplay between the surface selectivity and the wetting properties of the adsorbed mixtures. Different wetting behaviors has been found, depending on the values of the adsorption energies of the mixture components. We have considered the systems in which the wetting at non-selective walls occurs at the temperatures below the onset of the demixing transition in the bulk. When the average value of adsorption energies is fixed, the wetting temperature has been found to be unaffected by the weak selectivity of the wall. In this case, the reentrant mixing has been observed in thick adsorbed films. However, when the wall selectivity increases beyond a certain threshold value, the reentrant mixing in the film has been suppressed. This leads to the increase of the wetting temperature. In the case, when the adsorption energy of one component is fixed, while the adsorption energy of the second component changes, the wetting temperature has been found to exhibit non-monotonous changes with the difference between the adsorption energies of the mixture components.

Non-additive symmetric mixtures at selective walls

The results of Monte Carlo simulation of adsorption and wetting behaviour of a highly non-additive symmetric mixture at selective walls is discussed. We have concentrated on the interplay between the surface induced demixing in the adsorbed films and the properties of the bulk mixture, which exhibits a closed immiscibility loop. It has been shown that the wetting behaviour depends on the absolute values of the parameters determining the strengths of interaction between the mixture components and the surface, as well as on their difference. In general, an increase of the difference between the adsorption energies of the components leads to a decrease of the wetting temperature. In the cases when the wetting of non-selective walls occurs at the temperatures above the onset of demixing transition in the bulk, an increasing wall selectivity leads to a gradual decrease of the wetting temperature towards the triple point, in which the vapour coexists with the mixed and demixed liquid phases. When the wetting temperature at the non-selective wall is located below the onset of the demixing transition in the bulk mixture, an increase of the adsorption energy of the selected component causes the developing adsorbed films to demix and to show the reentrant mixing upon approaching the bulk coexistence. At the temperatures above the onset of the demixing transition in the bulk, the adsorbed films remain demixed up to the bulk coexistence and undergo the first-order wetting transition. A rather unexpected finding has been the observation of a gradual increase of the wetting temperature at highly selective walls.

Miscible and Core−Sheath PS/PVME Fibers by Electrospinning

We demonstrate that miscible and phase-separated fibers of the blend between polystyrene (PS) and poly(vinyl methyl ether) (PVME) can be electrospun by appropriate selection of the solvent. Solutions in benzene allow preparing miscible fibers with composition ranging from pure PS to PS/PVME 70/30. Results indicate that the rapid solvent evaporation during electrospinning has no significant influence on the phase behavior of polymer blends when the system is miscible throughout the ternary phase diagram. On the other hand, fibers electrospun from chloroform solutions show a clear phase separation because of a closed immiscibility loop. They are nevertheless highly malleable, in contrast with the brittle films obtained by solution casting. The confinement and solvent evaporation kinetic lead to a core−sheath structure, with the interior composed of the PS-rich phase and the surface highly enriched in PVME, as opposed to microdomains in cast films. The shell can be extracted rapidly and efficiently by a 10 s treatment in water, converting the mat from fully wettable to highly hydrophobic.

Driving force for phase segregation in polymer mixtures

Using the general bulk thermodynamics and a molecular equation-of-state model, we provide a heuristic argument on driving forces for phase segregation in polymer mixtures. Particular attention is given to the immiscibility loop emerged under pressure for a binary mixture of polymers of relatively low sizes, which differ in their compressibility. As the disparity in the equation-of-state properties governs the entire phase diagram for a given system, such compressibility effect alters its character as being entropically or enthalpically contributing to phase segregation according to the lower or upper critical solution (order-disorder) transition.

Polymer Chain Collapse in Supercritical Fluids. 1. Molecular Simulation Results

AbstractA few years ago we reported the first observation, by computer simulations, of polymer chain collapse near the lower critical solution temperature (LCST).1 In the present work, we extended the above study to understand the underlying physics of a single polymer chain collapse near LCST and its relationship to phase boundaries in the T‐x plane. Effects of solvent and monomer sizes, and solvent and monomer energetic interactions are studied. Using Monte Carlo simulations, the mean end‐to‐end distance (R) and gyration radius (Rg) are calculated for a single chain in a supercritical fluid solvent over a broad range of densities, pressures and temperatures. In general, the chain collapses as temperature increases at constant pressure. Upon a further temperature increase, the chain expands again to approach the athermal limit provided that the temperature is sufficiently high. The collapse is related to an LCST phase boundary while the expansion represents the signature of an upper‐critical solution temperature (UCST) suggesting the existence of a closed‐immiscibility loop. By manipulating the strength of the energetic interactions as well as the solvent‐to‐monomer size ratio, the size of the size of the immiscibility loop can be fine‐tuned. The relationship among size and the segment‐solvent energetic interaction are correlated by a conformational parameter (Ψ) for the first time. By monitoring the Ψ behavior, it is possible to predict solution's phase behavior, transition zone from LCST‐UCST in a closed‐loop miscibility behavior. The above relationship between chain conformation to phase boundaries may be useful in understanding phase stability in compressible polymer‐solvent mixtures.

Tuning the Phase Behavior of Polystyrene-block-poly(n-alkyl methacrylate) Copolymers by Introducing Random Copolymer for Methacrylate Block

Block copolymers have gained increasing interest because of their self-assembly into periodic morphologies such as lamellae, hexagonally packed cylinders, spheres (body-centered cubic), and gyroids with nanometer length scale. The phase behavior of a block copolymerdepends on the volume fraction ( f ) of oneof the blocks, the degree of polymerization (N), and the Flory-Huggins segmental interaction parameter (χ). Among many block copolymers, weakly interacting polystyrene-block-poly(n-alkyl methacrylate) copolymers (PS-b-PnAMA) show various phase behaviors depending on the length of alkyl chain in PnAMA: order-to-disorder transition (ODT), lower disorder-to-order transition (LDOT), and a closed-loop (immiscibility loop) with both LDOT and upper order-to-disorder transition (UODT) upon heating. The phase behavior of PS-bPnAMA was successfully explained by a compressible random phase approximation developed by us. Very recently, we found closed-loop phase behavior for polystyrene-block-poly(n-butyl-ran-n-hexyl)methacrylate copolymers (PS-b-Pn(B-r-H)MA) similar to polystyrene-block-poly(n-pentyl methacrylate) copolymers (PS-b-PnPMA), whereas PS-bPnHMA showed only ODT-type phase behavior, but PS-bPnBMA exhibited LDOT-type phase behavior. Also, the pressure coefficient of the transition temperatures for PS-b-Pn(B-r-H)MA (dTLDOT/dP=þ595 C/kbar and dTUODT/dP = -700 C/kbar) was much greater than that of PS-b-PnBMA (dTLDOT/dP=þ147 C /kbar) and PS-b-PnHMA (dTUODT/ dP=-60 C/kbar). Since the carbonnumbers in the alkyl group of PnBMA and PnHMA are 4 and 6, respectively, the random copolymer, Pn(B-r-H)MA, consisting of the same molar ratio of PnBMA and PnHMA, would exhibit a similar phase behavior to PnPMA with n=5. Namely, an average force field of Pn(B-r-H) MA having the same weight fraction of PnBMA and PnHMA would be similar to that of PnPMA. Ruzette et al. reported that polystyrene-block-poly(methacrylate-ran-lauryl methacrylate) copolymers (PS-b-P(MMA-rLMA)) exhibited the LDOT, even though both PS-b-PMMA and PS-b-PLMA exhibited the ODT. They explained this interesting phase behavior by the fact that the specific volume and the solubility parameter of P(MMA-r-LMA) are very similar to those of neat PnBMA, which exhibited the LDOT. They also found that the dT/dP of the LDOT of PS-b-P(MMA-r-LMA) was 150 C/kbar, which is similar to that (147 C/kbar) of PS-bPnBMA. Thus, it is interesting to investigate whether the closed-loop phase behavior is observed for polystyrene-block-poly(n-alkylran-n0-alkyl methacrylate) copolymers with very different values of n and n0. To prove this postulation, we chose polystyreneblock-poly(n-octyl-ran-methyl) methacrylate copolymers (PS-bPn(O-r-M)MA) (namely, n=8 and n0=1). We found that when the total molecular weight and the composition of the random copolymer block were judiciously controlled, PS-b-Pn(O-r-M)MA exhibited closed-loop phase behavior having both LDOT and UODT within an experimentally accessible temperature range. This is an interesting result because both PS-b-PMMA and PS-b-PnOMA showed only ODT during heating. This is distinctly in contrast to PS-b-Pn(B-r-H)MAwhere PS-b-PnBMA showed LDOT, whereas PS-b-PnOMA exhibited UODT. We consider that the closed-loop phase behavior observed for PS-bPn(O-r-M)MA is attributed to the fact that the average force field of Pn(O-r-M)MAwith∼70/30 (w/w) PnOMA/PMMAcomposition would be similar to that of PnPMA. Furthermore, PS-bPMMA exhibited small barotropicity with dTODT/dP=þ23 C/ kbar (that is, themiscibility between PS andPMMAdecreases with increasing P), while PS-b-PnOMA showed very weak baroplasticity with dTODT/dP = -5 C/kbar (that is, the miscibility between two block components is slightly enhanced upon pressurization). However, PS-b-Pn(O-r-M)MA showed excellent baroplasticity similar to PS-b-PnPMA and PS-b-Pn(B-r-H)MA. We explained this phase behavior by using a compressible random phase approximation.

The global phase behavior of the two-component systems with intracomponent association: Flory approach

Within the Flory approach we study the phase diagrams of two-component fluids, the molecules of each component A(f(A)), B(f(B)) bearing f(A) (f(B)) functional groups capable of forming thermoreversible A-A and B-B bonds. We develop a general procedure to classify these diagrams depending on the values of four governing parameters -- entropies and normalized energies of A-A and B-B bonds, and give full topological classification of phase diagrams with f(A,B)> or =3. We show that these phase diagrams can have immiscibility loops and up to four critical points.

Effect of solvents on closed-loop phase behavior of block copolymers

We studied the effect of solvent selectivity on the closed-loop phase behavior of a polystyrene- block-poly( n-pentyl methacrylate) copolymer. It was found that the lower disorder-to-order transition temperature (LDOT) and upper order-to-disorder transition temperature (UODT) consisting of the closed-loop were very sensitive to the selectivity of the solvent. With the addition of very small amounts of non-selective solvents such as di- n-octyl phthalate and dimethyl phthalate, the LDOT increased rapidly, whereas the UODT decreased dramatically; thus, the immiscibility loop was shrunk greatly. On the other hand, both the LDOT and UODT decreased with increasing amount of dodecanol, a highly selective solvent to poly( n-pentyl methacrylate) block. However, the decrease in the LDOT was greater than that of the UODT, leading to an increased immiscibility loop.

Miscibility Enhancement on the Immiscible Binary Blend of Poly(vinyl phenol) and Poly(acetoxystyrene) with Poly(ethylene oxide)

AbstractSummary: The miscibility and hydrogen‐bonding behaviors of ternary polymer blends of poly(ethylene oxide) (PEO)/poly(vinyl phenol) (PVPh)/poly(acetoxystyrene) (PAS) were investigated by using DSC and Fourier transform infrared spectroscopy (FTIR). The PEO is miscible with both PVPh and PAS based on the observed single Tg over the entire composition range. FTIR was used to study the hydrogen‐bonding interactions between PEO with PAS and PVPh, respectively. Quantitative analyses show that the strength of hydrogen‐bonding strength is of the order of the hydroxyl‐ether inter‐association of PVPh/PEO blend > the hydroxyl‐hydroxyl self‐association of pure PVPh > the hydroxyl‐carbonyl inter‐association PVPh/PAS blend at room temperature. Furthermore, the addition of PEO is able to enhance the miscibility of immiscible PVPh/PAS binary blends at lower (20 wt.‐%) or higher (60 and 80 wt.‐%) PEO content. However, there exists a closed immiscibility loop in the phase diagram at 40 wt.‐% PEO content due to the “Δχ” and “ΔK” effects in this hydrogen‐bonded ternary polymer system. Therefore, an interesting and unusual sandwich phase diagram has been observed in this ternary polymer blend.Ternary phase diagram of the PEO/PAS/PVPh system.magnified imageTernary phase diagram of the PEO/PAS/PVPh system.

Local Monomer Activation Model for Phase Behavior and Calorimetric Properties of LCST Gel-Forming Polymers

We investigate the phase diagrams and calorimetric signatures of gel-forming polymers with a lower critical solution temperature (LCST). We construct a model that captures many of the observed properties of this type of polymer. The model assumes the existence of a sharp internal conversion, at a characteristic temperature T*, in which monomers are activated from effectively hydrophilic to associative-hydrophobic states. The location of the gel transition is determined by Flory's model of gelation, while phase separation of polymer and solvent is determined by assuming that the effective immiscibility χ parameter is generated solely from gel-forming contacts. The resulting phase diagrams exhibit an immiscibility loop and a gel−sol boundary. At low temperatures both the phase and the sol−gel boundaries are almost independent of concentration. The gel transition does not produce a signature in the calorimetric properties (heat capacities at constant concentration), but the internal transition produces a clear signal near the gel transition point. The phase separation produces a finite positive jump in the excess heat capacity but is immediately followed by an exothermal feature. We discuss examples of experimentally studied polymeric systems with properties consistent with those of the model.

Effects of Fluctuations and Compressibility on the Phase Behavior of Loop-Forming Diblock Copolymers

The recently developed Hartree analysis for compressible diblock copolymers has been performed to study the thermodynamic origin of an immiscibility loop and its unprecedented pressure dependence observed in a diblock copolymer from polystyrene and poly(n-pentyl methacrylate). Certain specific interactions (SI) were incorporated in the theory by adding an entropic component into cross-contact interactions. An effective Flory-type interaction parameter χcRPA, which carries not only the change in contact interactions but also compressibility difference between blocks, was shown to play a central role here. The χcRPA plotted against temperature exhibited a loop character with a maximum because of the input SI. The effects of both fluctuations and compressibility difference were found through χcRPA to yield a loop phase diagram for the copolymer in a feasible temperature range along with the peculiar dependence of the loop on copolymer chain sizes and pressure. The resultant theoretical phase behavior was shown to be harmonious with the experimental observations for the copolymer.

Investigating the Effect of Miscibility on the Ionic Conductivity of LiClO4/PEO/PCL Ternary Blends

We demonstrate that miscibility affects the ionic conductivity of ternary polymer blends of lithium perchlorate (LiClO4), poly(ethylene oxide) (PEO), and poly(ε-caprolactone) (PCL). Although individually these three binary blends are fully miscible, a closed immiscibility loop exists in the ternary blend phase diagram as a result of the complicated interactions among LiClO4, PEO, and PCL. The addition of PCL suppresses the crystallization of PEO and results in higher ionic conductivity. FTIR spectroscopy studies indicate that an excess PCL content causes immiscibility, which results in PCL being excluded from the ternary blends. Consequently, the maximum ionic conductivity (6.3 × 10-7 S cm-1) at ambient temperature of ternary blends having a fixed LiClO4 content (25 wt %) is at a composition of 25/60/15 (LiClO4:PEO:PCL).

Criteria dilemma of phase behavior in ternary blends comprising polystyrene, poly(alpha-methyl styrene), and poly(4-methyl styrene)

A ternary blend system of polystyrene (aPS), poly(α-methyl styrene) (PαMS), and poly(4-methyl styrene) (P4MS) (blends of three styrenic homopolymers of similar molecular structures) was investigated by using differential scanning calorimetry (DSC), polarized-light optical microscopy (POM), scanning electron microscopy (SEM), and solid-state 13C cross-polarization/magic angle spinning nuclear magnetic resonance (CP/MAS NMR). This ternary system of aPS/PαMS/P4MS exhibits a small miscibility loop only at relatively high percentages of PαMS (≥80%), while most of the aPS/PαMS/P4MS compositions exhibit two coexisting aPS/PαMS and aPS/P4MS phases in the immiscibility loop. In the immiscible loop, SEM measurements revealed evidence in contrasting with DSC for criteria of miscibility. DSC characterization revealed a single T g for most ternary blend compositions; however, SEM graphs apparently indicated sub-micron phase domains, except for several PαMS-rich compositions (>80%). The T 1 H and T 1 ρ H techniques using NMR were found to resolve the dilemma between the conventional thermal analysis and microscopy results, and indeed supported that the ternary blends at above 80% PαMS are completely homogeneous on the molecular level. Attempts have been made to resolve the seemingly contradictory interpretations on the complex ternary phase structures and domains from the DSC, and microscopy results.

Effects of slit-like pore confinement on the closed loop immiscibility in symmetric binary model mixtures: fundamental measure density functional approach

We investigate the adsorption of model symmetric binary mixtures, exhibiting closed loop immiscibility in the bulk phase, in slit-like pores by using a density functional approach. Our focus is on changes in the closed loop phase diagram owing to Confinement. We have found that, in general, confinement narrows immiscibility loops in the temperature-selectivity plane.

Phase diagrams classification of the systems with thermoreversible alternating association (the Flory approach)

We consider incompressible blends of f1- and f2-functional monomers Af1 and Bf2, capable of forming thermoreversible bonds AB only between unlike monomers with an equilibrium Arrhenius association constant k=k0 exp(−ε/T). Effects of the parameters f1, f2, k0, and ε on the global phase behavior of these systems are analyzed within the conventional Flory approach. An explicit topological classification of all types of phase diagrams is given via building (i) the phase portraits, i.e., separation of the plane (ln k0,ε) into the regions corresponding to topologically similar phase diagrams, and (ii) typical phase diagrams on the (volume fraction; T) plane for all regions of the phase portraits. The phase behavior of the systems with alternating association is found to be rather sensitive to the values of the parameters (ln k0,ε) and diverse. In particular, seven possible types of phase diagrams with topologically different binodals are found. Nontrivial phase behavior in the systems with alternating association includes closed immiscibility loops, triple points, metastable critical points, and even completely metastable phases. The main trend governing all the variety of these phase diagrams is an association-induced increase in the thermodynamic stability of blends of stoichiometric composition. In particular, low-temperature blends of stoichiometric composition turn out to be thermodynamically stable if (−ε/2Θ)>(f1+f2)−1, where Θ is the value of the Θ temperature measured in energetic units.

Mean‐field and fluctuation correction analyses for a diblock copolymer melt exhibiting an immiscibility loop

AbstractA study on a diblock copolymer melt that can form certain specific interactions between dissimilar monomers is performed first with a mean‐field approach and then with a fluctuation correction approach. Flory's interaction parameter χ possesses both enthalpic and entropic contributions because of the specific interactions. It is found that not only a lower critical ordering transition but also an immiscibility loop with an upper critical ordering transition can be developed in the copolymer by the presence of the specific interactions and the entropic component in χ. The mean‐field loop phase diagram is shown to feature a typical sequence of microphase transitions upon both heating and cooling with two continuous transition points at a symmetric composition. It is revealed that the fluctuation effects remove both continuous transition points to significantly shrink the loop. The pressure effects on the phase behavior of the copolymer are also discussed. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1889–1896, 2003

Ternary miscibility in blends of three polymers with balanced binary interactions

AbstractThis study demonstrates and discusses ternary miscibility in a three‐polymer blend system based on balanced binary interactions. A truly miscible ternary blend comprising poly(ε‐caprolactone) (PCL), poly(benzyl methacrylate) (PBzMA), and poly(vinyl methyl ether) (PVME), was discovered and reported. Miscibility with phase homogeneity (excluding the PCL crystalline domain) in a wide composition range has been demonstrated using criteria of thermal transition behavior, cloud point, and microscopy characterization. At ambient temperature, the three‐polymer ternary system is completely miscible within the entire composition range (i.e., no immiscibility loop). However, at slightly elevated temperatures above the ambient. phase separation readily occurred in this originally miscible ternary blend. A quite low “lower critical solution temperature” (LCST) near 75°C was found for the ternary blend, which is much lower than any of those for the binary pairs. Balanced interactions with no offsetting Δχ among the three binary pairs were a key factor leading to a ternary miscible system.