Preparation of hemispherical polymer particles via phase separation induced by microsuspension polymerization

Micrometer-sized, hemispherical polystyrene (PS) particles were successfully prepared by microsuspension polymerization of homogeneous styrene/hexadecane (HD) droplets dispersed in polyoxyethylene nonylphenyl ether (Emulgen 931) aqueous solution, followed by rapid removal of HD from formed PS/HD particles with a “Janus” structure. It was important for the formation of the morphology of Janus particles in thermodynamically stable state to carry out the polymerization slowly. The formation of by-product small PS particles by emulsion polymerization was suppressed by the additions of CuCl₂ as a water-soluble inhibitor and NaCl to decrease the solubility of styrene in the aqueous phase.     

Formation of microporous Teflon PFA membranes via thermally induced phase separation

Poly(tetrafluoroethylene-co-perfluoro-(propyl vinyl ether)) (Teflon^® PFA) membranes of a variety of structures have been produced through thermally induced phase separation of Teflon^® PFA-chlorotrifluoroethylene melt-blends of different compositions. A phase diagram of the two component system was constructed, and electron microscopy was used to characterize the structures of membranes produced. The morphological characteristics of the Teflon^® PFA membranes have been explained on the basis of equilibrium driving forces for liquid-liquid and solid-liquid phase separations.     

Entanglement and phase separation in hyperbranched polymer/linear polymer/solvent ternary blends

Hyperbranched polyethylene (HBPE)/linear polystyrene (PS)/chloroform (CF) solution was selected as a model system to investigate the effect of branching structure on entanglement and phase separation behavior in semi-dilute ternary polymer solutions. All the HBPE materials in this work were found to have similar chain architectures and the critical molecular weight was estimated to be 81.2 kDa. The results obtained by elastic light scattering and intrinsic fluorescence methods suggested that all ternary solutions exhibited UCST transition behavior upon cooling. Also, it was found that the increase in the molecular weight of PS led to increase in the phase separation rate, consistent with de Gennes prediction. However, the increase of molecular weight of HBPE did not monotonously reduce the compatibility of polymer components and the phase separation rate in ternary blends is as follows: medium molecular weight HBPE (HBPE-M) > high molecular weight HBPE (HBPE-H) > low molecular weight HBPE (HBPE-L). This abnormal behavior can be explained by the fact that, (i) for HBPE-L, no entanglements between HBPE chains occurred and the branching effect can be ignored, and (ii) for HBPE-M and HBPE-H, entanglement of HBPE chains can be formed, and the dilution of branches on entanglement of backbones should be taken into consideration, that is, the shorter the branches of HBPE, the higher the possibility of interpenetration of HBPE backbones between neighboring molecules and, consequently, the faster aggregation of HBPE during phase separation. Furthermore, a simple model based on decomposition reaction was proposed to quantitatively describe the phase separation kinetics and the apparent activation energies of phase separation were calculated to be −150.3 and −52.3 kJ/mol for HBPE-M/PS/CF and HBPE-H/PS/CF systems, respectively.     

Monte Carlo simulation of phase separation kinetics in a concentrated polymer solution

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Kinetics of phase separation by spinodal decomposition in a liquid-crystalline polymer solution

Time-resolved light scattering studies have been undertaken for elucidating the dynamics of phase separation in aqueous HPC (hydroxypropyl cellulose) liquid-crystalline solutions. The HPC/water system phase separates during heating and returns to a single phase upon cooling. The phase diagram of thermally induced phase separation was subsequently established on the basis of cloud point measurements. For kinetic studies, T (temperature) jump experiments of 10 per cent aqueous HPC solutions were undertaken. Phase separation occurs in accordance with the spinodal decomposition mechanism. At low T jumps or in reverse quenched experiments, the scattering maximum remains invariant as predicted by the linearized Cahn-Hilliard theory. However, at large T jumps, the SD is dominated by non-linear behaviour in which scattering peaks move to low scattering angles. The latter process has been identified to be a coarsening mechanism associated with the coalescence of phase separated domains driven by a surface tension. A reduced plot has been established with dimensionless variables Q and t. It was found that the scaling law is not valid over the entire spinodal process. The time evolution of the scattering profiles of 10 per cent HPC solutions, following a Tjump to 49°C, is tested with the scaling law of Furukawa. It seems that the kinetics of phase separation at 10 per cent solution resemble the behaviour of off-critical mixture.     

Kinetics of phase separation by spinodal decomposition in a liquid-crystalline polymer solution

Abstract

Time-resolved light scattering studies have been undertaken for elucidating the dynamics of phase separation in aqueous HPC (hydroxypropyl cellulose) liquid-crystalline solutions. The HPC/water system phase separates during heating and returns to a single phase upon cooling. The phase diagram of thermally induced phase separation was subsequently established on the basis of cloud point measurements. For kinetic studies, T (temperature) jump experiments of 10 per cent aqueous HPC solutions were undertaken. Phase separation occurs in accordance with the spinodal decomposition mechanism. At low T jumps or in reverse quenched experiments, the scattering maximum remains invariant as predicted by the linearized Cahn-Hilliard theory. However, at large T jumps, the SD is dominated by non-linear behaviour in which scattering peaks move to low scattering angles. The latter process has been identified to be a coarsening mechanism associated with the coalescence of phase separated domains driven by a surface tension. A reduced plot has been established with dimensionless variables Q and t. It was found that the scaling law is not valid over the entire spinodal process. The time evolution of the scattering profiles of 10 per cent HPC solutions, following a Tjump to 49°C, is tested with the scaling law of Furukawa. It seems that the kinetics of phase separation at 10 per cent solution resemble the behaviour of off-critical mixture.     

Kinetics of thermally induced phase separation in ternary polymer solutions. I. Modeling of phase separation dynamics

Simulations based on Cahn–Hilliard spinodal decomposition theory for phase separation in thermally quenched polymer/solvent/nonsolvent systems are presented. Two common membrane‐forming systems are studied, cellulose acetate [CA]/acetone/water, and poly(ethersulfone) [PES]/dimethylsulfoxide [DMSO]/water. The effects of initial polymer and nonsolvent composition on the structure‐formation dynamics are elucidated, and growth rates at specific points within the ternary phase diagram are quantified. Predicted pore growth rate curves exhibit a relative maximum with nonsolvent composition. For shallow quenches (lower nonsolvent content) near a phase boundary, the pore growth rate increases with increasing quench depth, whereas for deep quenches, where the composition of the polymer‐rich phase approaches that of a glass, the pore growth rate decreases with increasing quench depth. With increasing initial polymer concentration, the overall rate of structure growth is lowered and the growth rate maximum shifts to higher nonsolvent compositions. This behavior appears to be a universal phenomenon in quenched polymer solutions which can undergo a glass transition, and is a result of an interplay between thermodynamic and kinetic driving forces. These results suggest a mechanism for the locking‐in of the two‐phase structure that occurs during nonsolvent‐induced phase inversion. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1449–1460, 1999     

Three-dimensional numerical simulations of lamellar structure via two-step surface-directed phase separation in polymer blend films

Lamellar structure via two-step surface-directed phase separation in polymer blend films is numerically investigated in three-dimensional (3D) space, which is more physically appropriate for the experimental situation than that in two-dimensional (2D) space [L.-T. Yan and X. M. Xie, J. Chem. Phys. 128, 034901 (2008)]. The 3D phase morphology and its evolution dynamics in both critical and off-critical conditions have been studied. The wetting layer formation mechanism during the second quench has been concerned. The effects of noise on the ordered phase structures have also been examined. The simulated results in 3D space give a more certain evidence that the lamellar structure can be induced by the surface or interface when the system is in the equilibration state with very shallow quench depth first and then imposed on a further quench depth in the unstable region of the phase diagram. It is found that the lamellar structure can also be induced in the polymer blends with off-critical condition. The simulated results demonstrate that the formation of the lamellar structure can present two basic processes and obey logarithmic growth law at the initial and metaphase stages. The results also show that a stronger thermal noise corresponds to a smaller region with the lamellar structure.     

Preparation of PVDF/PEO-PPO-PEO blend microporous membranes for lithium ion batteries via thermally induced phase separation process

Microporous poly(vinylidene fluoride)/polyethylene oxide-co-polypropylene oxide-co-polyethylene oxide (PVDF/PEO-PPO-PEO, or PVDF/F127) blend membranes were prepared via thermally induced phase separation (TIPS) process using sulfolane as the diluent. Then they were soaked in a liquid electrolyte to form polymer electrolytes. The effects of F127 weight fraction on the morphology, crystallinity and porosity of the blend membranes were studied. It was found that both electrolyte uptake of blend membranes and ionic conductivity of corresponding polymer electrolytes increased with the increase of F127 weight fraction. The maximum ionic conductivity was found to reach 2.94 ± 0.02 × 10⁻³ S/cm at 20 °C. Electrochemical stability window was stable up to 4.7 V (vs. Li⁺/Li). The testing results indicated that the PVDF/F127 blend membranes prepared via TIPS process can be used as the polymer microporous matrices of polymer electrolytes for lithium ion batteries.     

Diffusion and phase separation in polymer solution during asymmetric membrane formation

Most of the commercially available polymeric membranes are prepared by the phase inversion process. In this process a thermodynamically stable polymer solution is brought to phase separation by immersing the solution in a surplus of nonsolvent, followed by an exchange of solvent and nonsolvent. The ultimate membrane structure is the result of an interplay of mass transfer and phase separation. Asymmetric membranes as well as symmetrical porous membranes can be obtained. Two types of demixing processes (l-l phase separation and formation of aggregates) can be distinguished by the kinetics of phase separation, as the formation of aggregates is supposed to be a slower process than l-l demixing. Because it is impossible to measure the composition changes during the demixing processes experimentally, a theoretical analysis has to be applied. A suitable formalism to calculate the diffusion induced composition changes in the immersed casting solution, as a function of thermodynamic and hydrodynamic interaction parameters will be described. With this theory it can be shown that two distinctly different mechanisms of membrane formation may occur resulting in two different types of membranes. One type has a relatively thick toplayer and mostly exhibits reverse osmosis, gas separation and pervaporation properties; the other type results in a porous type of membrane, which will exhibit ultra- and microfiltration properties. Model calculations are in agreement with light transmission experiments on membrane forming systems. Therefore, it could be concluded that the elucidation of the diffusion behavior in the immersed polymer film is the key to better understanding of membrane formation by means of immersion precipitation.     

Preparation of epoxy monoliths via chemically induced phase separation

Epoxy porous monoliths were prepared from a commercial epoxy resin, D.E.R. 331, that cured with a tertiary amine, 2,4,6-tris-(dimethylaminomethyl) phenol, in the presence of a solvent, diisobutyl ketone (DIBK). During the curing process, polymers were formed and a decrease in its solubility in DIBK; the solution thus phase-separated, usually referred to as chemically induced phase separation. The phase separation formed interconnected polymer-poor phase that then became interconnected pores after the removal of DIBK. By varying the content of DIBK from 32 to 40 vol.%, epoxy monoliths with interconnected pores were prepared, with surface pore size ranging from 0.20 to 2.33 μm, overall porosity from 0.41 to 0.60, and ethanol permeability from 10 to 4,717 L/(m²?h^?1?bar^?1). The glass transition temperatures of the epoxy monoliths, measured with differential scanning calorimetry, were all higher than 100 °C, and temperatures of 5 % weight loss, analyzed by thermal gravimetry, were higher than 350 °C, evidencing the monoliths’ high thermal stability. Also, the monolith morphology was found to be strongly related to the reaction mechanism of polymerization. The results indicate that the mechanism of chain initiation and propagation associated with the tertiary amine can effectively form monoliths with interconnected pores, which cannot be easily prepared with a stepwise polymerization mechanism associated with using primary amine as the curing agent.     

Effects of polymer chain length and stiffness on phase separation dynamics of semidilute polymer solution

Three-dimensional dissipative particle dynamics (DPD) simulations were performed to investigate the phase separation dynamics of semidilute polymer solutions with different polymer chain length and stiffness. For the polymer solution composed of shorter and more flexible chains, a crossover of the domain growth exponent from 1/3 to 2/3 was observed during the course of phase separation, indicating that the growth mechanism altered from diffusion to interfacial-tension driven flow. When the chain flexibility was kept the same but the chain was lengthened to allow for the chain entanglement to occur, the growth exponent changed to 1/4 in the diffusion-dominating coarsening regime while the growth exponent remained 2/3 in the flow-dominating regime. When the chain length was kept short but the stiffness was increased, the growth exponent became 1/6 in the diffusion-dominating regime and little effect was observed in the flow-dominating coarsening regime. The slow down of the phase separation dynamics in the diffusion-dominating coarsening could be explained by that the polymer chains could only perform wormlike movement when chain entanglements occurred or when the chain motion was limited by chain stiffness during phase separation. Moreover, when both the effects of chain length and stiffness were enhanced, polymer networks composed of longer and stiffer chains appeared and imposed an energy barrier for phase separation to occur. As a result, the polymer solution with stiffer and longer chains required a larger quench depth to initiate the phase separation and caused the delay in crossover of the coarsening mechanism from diffusion to flow.     

The onset of three-phase separation in ternary polymer solutions

A theoretical study is made of three-phase separation in a ternary solution consisting of a pure solvent and two homologous monodisperse polymers with relative chain lengths P₁ and P₂. The Flory—Huggins interaction parameter X is assumed to depend linearly on the volume fraction ? of the polymer mixture: χ = χ₀(T) + k(T)_?. For a given P₁ the ratio of P₂ to P₁ for the onset of three-phase separation increases with an increase in k, first gradually and then sharply. Loci of critical points for this condition on the χ₀ vs. ? diagram are determined for a series of fixed values of P₁ and their significance is discussed.     

Reaction-induced phase separation in a semiinterpenetrating network of reactive ternary blends

Morphology and reaction mechanisms were probed on a model reactive ternary blend system of polycarbonate (PC), poly(methyl methacrylate) (PMMA), and diglycidylether of bisphenol-A (DGEBA) epoxy by using differential scanning calorimetry (DSC), scanning electron microscopy (SEM), and Fourier transform infrared spectroscopy (FTIR). Molecular interactions/reactions between the components in the blends after heating treatments are discussed. It was found that reactions took place among the components and that an interpenetrating network was built. The possible reaction mechanisms and the resulting structures after the heating treatments were probed. In the blends, PC and DGEBA reacted to form a network, while PMMA remained free. The semiinterpenetration, however, did not result in a network interlocked into a homogeneous state. The single T_g of the heated ternary DGEBA/PC/PMMA blends actually did not reflect a homogeneous interpenetrating network. Due to relatively small PMMA domains, the ternary blend network exhibited a single T_g. Upon etching the PMMA domains from the blend by acetone, a clearly interpenetrating network of reacted PC and epoxy was exposed and confirmed. The reactions leading to such a morphology are discussed with experimental evidence. © 1996 John Wiley & Sons, Inc.     

Coalescence-induced coalescence and dimensional crossover during the phase separation in ternary surfactant/polymer/water mixtures

We studied the separation process in the ternary mixtures of nonionic surfactant (C(12)E(6), hexaethylene glycol monododecyl ether), polymer (PEG = poly(ethylene glycol)), and water. The separation process of PEG/water rich domains from the surfactant rich matrix was observed by the optical microscopy. From the morphological analysis, we determined the size of the domains as a function of time. On this basis we identified a dominating mechanisms of domains growth, that is the coalescence-induced coalescence mechanism. The coalescence (collision) event of two droplets induces a flow or a change of concentration distribution around droplets which pushes other droplets together inducing further growth. We also observed the evaporation-condensation (Lifshitz-Slyozov) mechanism of growth, but it did not affect the growth of large domains appreciably. We determined two regimes of the coalescence-induced coalescence associated with the dimensionality of the system. When the domains were smaller or comparable in size to the sample thickness we observe a three-dimensional growth. When the domains became larger than the sample thickness, a two-dimensional growth was observed. In the first regime, the size of the domains, L(t), grew linearly with t, while in the second regime, L(t) approximately t(0.3). In the binary, surfactant/water system, water domains grew by the geometrical coalescence-induced coalescence as L(t) approximately t in three dimensions.     

Surface-directed phase separation via a two-step quench process in binary polymer mixture films with asymmetry compositions

Surface-directed phase separation via a two-step quench process in asymmetry polymer mixtures is numerically investigated by coupling the Flory-Huggins-de Gennes equation with the Cahn-Hilliard-Cook equation. Two distinct situations, i.e., the minority component is preferred by the surface and the majority component is preferred by the surface, are discussed, respectively. The morphology and evolution dynamics of the phase structure, especially the secondary domain structure, are analyzed. The wetting layer formation mechanisms during the two-step quench process are examined. The simulated results demonstrate that different secondary domain structures in these two situations can be induced by the second quench with deeper quench depth, which can be used to tailor phase morphology. It is also found that, in the second quench process, the evolution of the wetting layer thickness can cross over to a faster growth when the preferential component is the minority component. In this situation, the formation mechanism of the wetting layer will change and is eventually determined by the second quench depth. However, when the preferential component is the majority component, a deeper second quench depth corresponds to a slower growth of the wetting layer thickness. The chemical potential is calculated to explain the difference regarding the growth dynamics of the wetting layer thickness between these both situations.     

Exact phase boundary of an amphiphile-rich phase in a ternary solution on a Bethe lattice

A model ternary solution is considered in which the bonds of a three-coordinate Bethe lattice are covered by rodlike molecules of types AA, BB, and AB, the AB-type molecule representing an amphiphile. The phase boundary in temperature-composition space of an ordered amphiphile-rich phase is calculated exactly.     

Controlling morphological and electro-optical properties via the phase separation in polymer/liquid-crystal composite materials

ABSTRACT

The polymer/liquid-crystal composite materials have been extensively studied for their potential applications. Various optical devices based on this composite material have been proposed and realised. The device performance is highly dependent on the phase separation of this composite material. Here, we investigate the photopolymerisation-induced phase separation in this composite material. Depending on the mass ratios between the polymer and the liquid crystal, the phase separation can be well controlled and subsequently affect the morphological and electro-optical properties. At a fixed ratio, we can realise either phase-separated composite films or conventional polymer-dispersed liquid crystal films with completely different optical properties. By carefully controlling the exposure conditions, the morphologies and electro-optical properties have been studied and optimised in details. With in-depth studies and optimisation, the photopolymerisation-induced phase separation technique could be utilised to realise many different optical functions based on the polymer/liquid-crystal composite materials.     

Kinetics of thermally induced phase separation in ternary polymer solutions. II. Comparison of theory and experiment

Light‐scattering measurements and spinodal decomposition modeling have been used to quantify the kinetics of pore growth in thermally quenched polymer‐solvent–nonsolvent [poly(methyl methacrylate) (PMMA)/1‐methyl‐2‐pyrrolidinone (NMP)/glycerin] solutions. Solutions of fixed composition were quenched to a series of temperatures and light‐scattering measurements and model calculations were performed to determine the temperature dependence of the pore growth rate. Both the experimental results and the model calculations show that the growth rate exhibits a maximum at an intermediate quench temperature that is related to an interplay between the thermodynamic and transport effects that govern pore growth. A similar growth‐rate maximum is also observed when a series of solutions of varying nonsolvent composition are all quenched to the same temperature. The relevance of these experiments to the dynamics of pore growth and the eventual locking‐in of the two‐phase structure that forms during nonsolvent‐induced phase inversion is discussed. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1461–1467, 1999     

Exploitation of a microporous organic polymer as a stationary phase for capillary gas chromatography

Microporous organic polymers (MOPs) have emerged as a new class of functional porous materials with unique characteristics and potential uses in diverse areas. However, the field of MOPs for gas chromatographic (GC) separations has not been well explored. Herein, a MOP namely KAPs-1 was dynamic coated onto a capillary column for the first time. The fabricated column exhibited a nonpolar nature and the column efficiency for n-dodecane was up to 7769 plates m⁻¹. The KAPs-1 coated column showed high GC separation performance for a series of volatile organic compounds (VOCs) including the challenging ethylbenzene and xylene isomers, which could not be resolved at baseline on the commercial 5% phenyl polysiloxane stationary phase. Moreover, the relative standard deviations for five replicate determinations of the studied analytes were 0.0–0.6%, 0.9–3.2%, 1.1–5.9%, 0.8–3.7% for retention time, peak area, peak height and peak width, respectively. To investigate the interaction between some analytes and the stationary phase, thermodynamic and kinetic parameters were also evaluated. The results of this study show it is very promising to utilize MOPs as stationary phases for capillary GC.