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     

Crystallization of polylactic acid under in situ deformation during nonsolvent‐induced phase separation

In this study, we investigate polylactic acid (PLA) crystallization under in situ biaxial extension in a nonsolvent‐induced phase separation foaming process. Our ternary system consists of PLA, dichloromethane (DCM) as solvent and hexane as nonsolvent. For the first time, the formation of a shish‐kebab crystalline morphology is observed in such a solution‐based foaming process in certain solid–liquid phase separated systems. The formation of shish‐kebabs is described based on the coil‐stretch transition concept. The rapid biaxial deformation caused by macropore growth uniaxially stretches the long chains that are tied with at least two single crystals which eventually leads to the formation of shish structures throughout the polymer‐rich phase. The kebab lamellae then form perpendicularly on the shish cores. The scanning electron microscopy (SEM) observations and our interpretation of the crystallization phenomena are confirmed by differential scanning calorimetry (DSC) analysis. The observation of various crystalline morphologies, particularly shish‐kebabs, and the elucidation of their formation mechanisms contribute to the understanding of phase separation and pore growth as well as crystallization in such polymer–solvent–nonsolvent systems. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 1055–1062     

Phase separation and polymer crystallization in a poly(4‐methyl‐1‐pentene)–dioctylsebacate–dimethylphthalate system via thermally induced phase separation

The effects of the polymer concentration and quenching temperature on the phase separation, the membrane morphology and polymer crystallization behavior in a poly(4‐methyl‐1‐pentene) (TPX)‐dioctylsebacate (DOS)‐dimethylphthalate (DMP) system via thermally induced phase separation were studied with a pseudobinary phase diagram, with the weight ratio of DOS:DMP = 1:1. SEM was used to observe the membrane morphology and structure, whereas the TPX crystallization behavior was studied with DSC and WAXD. Liquid‐liquid phase separation occurred, although quenching under the crystallization temperature. As the quenching temperature decreased, the pore size decreased, with better connected pore structure formed. The membranes quenched at 333 and 363 K showed good cellular structures, with an average pore size of about 2.3μm, whereas the pores of the membranes quenched at 393 and 423 K were not well formed, with some lamellar crystals on the inner side. The diluent assisted the mobility of the polymer chain, which improved the polymer crystallization. Dual‐melting‐peak behavior occurred for all the samples studied here. As the quenching temperature increased, the first peak of the melting trace moved to a higher temperature, whereas the second one stayed almost the same. The flexibility of the TPX main chain was restricted by the side groups, which allowed liquid‐liquid phase separation to occur first when quenched below the equilibrium crystallization temperature. This allowed primary and secondary crystallization, which was responsible for the dual‐melting‐peak behavior. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 153–161, 2007     

Chemically induced phase separation in the preparation of porous epoxy monolith

A methodology for preparing porous epoxy monolith via chemically induced phase separation was proposed. The starting system was a mixture of an epoxy precursor, diglycidyl ether of bisphenol‐A (DGEBA), a curing agent, 4,4′‐diaminodiphenylmethane (DDM), and a thermoplastic polymer, polypropylene carbonate (PPC). As DGEBA was cured with DDM, the system became phase‐separated having PPC particles dispersed in epoxy matrix. After PPC particles were removed by thermal degradation, a porous structure was obtained. The phase separation mechanism was determined by the initial composition and illustrated by a pseudophase diagram. The pore size increased with increasing the concentration of PPC and raising the curing temperature. The intermediate and final morphologies of the system were studied using optical and scanning electron microscopy, respectively. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2010     

Selection criteria for solvent and coagulation medium in view of macrovoid formation in the wet phase inversion process

Sponge‐like and finger‐like structures are two distinct membrane structures commonly observed in membranes produced by the wet immersion process. An index Φ calculated solely from solubility parameters was defined as an indicator of the membrane structure. The Φ values of four polymers, poly(methyl metharylate), polysulfone, cellulose acetate, and poly(vinylidene fluoride), in various solvent‐nonsolvent pairs were calculated and compared with the corresponding membrane structures. It was found that the finger‐like structure often occurred at higher Φ values. Although the Φ value represents mostly the thermodynamics nature of a system, as an index for prior selection of solvent‐nonsolvent pairs for a particular polymer, a general rule of thumb was developed. Taking 15% polymer concentration and 300 μm casting thickness as a referential casting condition, selecting solvent‐nonsolvent pairs with Φ values higher than 0.25 is suggested, when a finger‐like structure is desired. The polymer concentration in the casting solution and the casting thickness will also affect the membrane structure. If a higher polymer concentration needs to be used, selection of a polymer‐solvent‐nonsolvent system with a Φ value much higher than 0.25 is suggested, or keeping the casting thickness lower than 300 μm to obtain a finger‐like membrane structure. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1495–1502, 1999     

Scaling behavior of nonisothermal phase separation

The phase separation process in a critical mixture of polydimethylsiloxane and polyethylmethylsiloxane (PDMS/PEMS, a system with an upper critical solution temperature) was investigated by time-resolved light scattering during continuous quenches from the one-phase into the two-phase region. Continuous quenches were realized by cooling ramps with different cooling rates kappa. Phase separation kinetics is studied by means of the temporal evolution of the scattering vector qm and the intensity Im at the scattering peak. The curves qm(t) for different cooling rates can be shifted onto a single mastercurve. The curves Im(t) show similar behavior. As shift factors, a characteristic length Lc and a characteristic time tc are introduced. Both characteristic quantities depend on the cooling rate through power laws: Lc approximately kappa(-delta) and tc approximately kappa(-rho). Scaling behavior in isothermal critical demixing is well known. There the temporal evolutions of qm and Im for different quench depths DeltaT can be scaled with the correlation length xi and the interdiffusion coefficient D, both depending on DeltaT through critical power laws. We show in this paper that the cooling rate scaling in nonisothermal demixing is a consequence of the quench depth scaling in the isothermal case. The exponents delta and rho are related to the critical exponents nu and nu* of xi and D, respectively. The structure growth during nonisothermal demixing can be described with a semiempirical model based on the hydrodynamic coarsening mechanism well known in the isothermal case. In very late stages of nonisothermal phase separation a secondary scattering maximum appears. This is due to secondary demixing. We explain the onset of secondary demixing by a competition between interdiffusion and coarsening.     

Phase separation by continuous quenching: similarities between cooling experiments in polymer blends and reaction-induced phase separation in modified thermosets

Small angle light scattering (SALS) has been applied to study the phase separation kinetics in a binary polymer mixture of poly(ethyl methyl siloxane) (PEMS) and poly(dimethyl siloxane) (PDMS). The phase separation was induced by cooling an initially homogeneous mixture with well defined cooling rates. The results have been compared to time resolved SALS and microscopy in the course of reaction-induced phase separation in mixtures of an epoxy resin and polysulfone (PSU). For the critical PEMS/PDMS mixture with an upper critical point it was found in a continuous quenching experiment that the time evolution of the scattered light intensity I(q,t) scales with the cooling rate. The similarity to the scaling behavior of I(q,t) in isothermal experiments after fast quenches (scaled by the quench depth) is discussed. A secondary phase separation was found and has been explained by the competition between the growth of the two phase structure during cooling and the mutual diffusion without the assumption of gelation or vitrification. For the epoxy/PSU mixture with 15% PSU, after the appearance of a bicontinuous structure a secondary phase separation was observed. Mixtures with higher PSU-contents formed epoxy-rich droplets in the PSU-rich matrix by nucleation and growth mechanism. The frustration of the structure growth can be explained by approaching vitrification of one or both phases. The similarity between continuous cooling experiments in blends and the reaction-induced phase separation have been discussed in the generalized χN vs. composition phase diagram (N: degree of polymerization, χ: Flory-Huggms interaction parameter).     

Preparation of polypropylene microspheres for selective laser sintering via thermal‐induced phase separation: Roles of liquid–liquid phase separation and crystallization

Although selective laser sintering (SLS) has been widely applied in many fields, more research work is needed to develop proper polymer microspheres for SLS. Thermal‐induced phase separation (TIPS) is a facile way but rarely reported to prepare the polymer microspheres. The roles of liquid–liquid phase separation (LLPS) and crystallization in the TIPS process are not clear. In this study, proper polypropylene (PP) microspheres for SLS are successfully prepared via TIPS with xylene. The diameters and morphologies of these PP microspheres can be regulated easily by changing the PP concentration and the quench temperature. The large undercooling drives the solution into the metastable LLPS region and produces PP microspheres with smooth surfaces. The PP crystallization occurs both on the LLPS interface and inside the polymer‐rich phase when the solution is quenched to a temperature near the binodal line, and the tiny bent lamellae are formed on the microsphere surface. At higher temperature only PP crystallization occurs, which results in the formation of PP particles consisting of packed lamellae. The PP microspheres prepared here are suitable for SLS and promote the development of SLS potentially. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 320–329     

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.     

Computer simulation of phase separation under a double temperature quench

The authors numerically study a two-step quench process in an asymmetric binary mixture. The mixture is first quenched to an unstable state in the two-phase region. After a large phase-separated structure is formed, the authors again quench the system deeper. The second quench induces the formation of small secondary droplets inside the large domains created by the first quench. The authors characterize this secondary droplet growth in terms of the temperature of the first quench as well as the depth of the second one.     

Effect of nano‐size nodular structure induced by CNT‐promoted phase separation on the fabrication of superhydrophobic polyvinyl chloride films

The nonsolvent‐induced phase separation (NIPS) method was employed to fabricate the porous films based on polyvinyl chloride loaded with carbon nanotubes (CNTs). The combinational addition of CNTs and a proper nonsolvent (ethanol) resulted in a porous surface layer with the nano‐size nodular structure possessing an exact superhydrophobic behavior (water contact angle [WCA] = 157° and sliding angle [SA] <5°). The size of PVC nodules at the surface layer varies in the range of 200 to 800 nm depending on the nonsolvent concentrations, and polymer molecular weight. The effects of various nonsolvent concentrations as well as PVC molecular weight on the surface properties of the films were also investigated. Morphological and roughness analyses revealed the pronounced role of PVC molecular weight on the size of nodules, and the structural uniformity of the surface morphology based on the thermodynamic parameters such as relaxation time and dynamic of polymer chains. The concurrent use of CNTs and nonsolvent led to promote the NIPS process due to the nucleating role of CNTs, which were localized within the polymer‐rich phase leading to an ultra‐fine and packed nodular surface structure. Transmission electron microscopy results also proved the very well dispersion quality of CNTs. Glass transition temperature of PVC was also assessed, and the results were correlated to the associating ability of CNTs with polymer chains during the phase separation process. Overall, the promising potential of CNT/ethanol combination on the surface porosity and hydrophobicity of PVC nanocomposite films was revealed in this study, which could further extend its application window.     

Control of droplet size of polymer–diluent blends through thermally induced phase separation

The effects of process conditions and molecular structure of polymer and diluent on the droplet size of membranes formed by thermally induced phase separatiom (TIPS) process were examined. The observed upper critical solution temperature–type phase boundaries of nylon‐12 blended with poly(ethylene glycol) (PEG) and nylon 12 diluted with poly(ethylene glycol) dimethyl ether (PEGDE) and their interaction energy densities calculated using the Flory–Huggins theory suggest that the nylon‐12/PEGDE blends are less stable than the nylon‐12–PEG blends. Infrared spectra confirmed that the difference in phase stability might come from specific interactions of the hydroxyl terminal groups of PEG with the amide groups from nylon‐12, which are not be feasible in the nylon‐12–PEGDE blends. The phase stability of diluent PEG blended with various nylons that are different in the number of methyl groups in the repeat unit was ranked in the order of: nylon‐6–PEG blend < nylon‐12–PEG blend < nylon‐11–PEG blend. We also noted that the phase‐separated droplets grew by both coalescence and the Oswald ripening process after the onset of phase separation. As a result, the cubic exponent of average droplet radius (R³) plotted against time satisfied the linear relationship. As the blends became less stable, the droplet growth rate increased and larger equilibrium droplets formed at a constant quenching depth. The TIPS membranes with desired pore structure could be prepared by controlling the molecular structure of components as well as by varying processing conditions such as quenching depth and annealing time. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 3042–3052, 2000     

Critical parameters controlling the properties of monolithic poly(lactic acid) foams prepared by thermally induced phase separation

Monolithic poly(lactic acid) (PLA) foams were produced by thermally induced phase separation. PLA solutions with concentrations 8–22 wt % were prepared in tetrahydrofuran/methanol (THF/MeOH) solvent/nonsolvent mixtures at 55 °C. Homogenous solutions were quenched at ?20 °C to induce phase separation and gelation. Resulting gels were mechanically stabilized by solvent exchange. Subsequent supercritical CO₂ drying yielded monolithic PLA foams. Crystal structure and degree of crystallinity of the foams were obtained by x‐ray diffractometry and differential scanning calorimetry. Morphologies were determined by scanning electron microscopy. Tuning the PLA concentration and THF/MeOH ratio enabled preparation of monolithic PLA foams. Depending on the experimental conditions various morphologies, such as: interconnected networks, thin platelets, lamellar stacks, axialites, and spherulites were formed. Monoliths obtained were highly crystalline. By changing the PLA concentration monoliths with controlled average pore sizes (170–1440 nm) and porosities (80–90%) were produced. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 98–108     

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.     

Evidence of mechanisms occurring in thermally induced phase separation of polymeric systems

Thermally induced phase separation is a fabrication technique for porous polymeric structures. By means of easy‐to‐tune processing parameters, such as system composition and demixing temperature, a vast latitude of average pore dimensions, pore size distributions, and morphologies can be obtained. The relation between demixing temperature and morphology was demonstrated via cloud point curve measurement and foams fabrication with controlled thermal protocols, for the model system poly‐l ‐lactide–dioxane–water. The morphologies obtained at a temperature lower than cloud point showed a closed‐pore architecture, suggesting a “nucleation‐and‐growth” separation mechanism, which produced larger pores at higher holding times. Conversely, the porous structures attained when holding the sample above the cloud point exhibited open pores with dimensions independent of time, denoting a phase separation occurring during sample freezing. Finally, the influence of the cooling rate on final morphology was investigated, showing a clear correlation with microstructure and pore size. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 979–983     

Phase separation kinetics of polyelectrolyte solutions

The kinetics of phase separation of aqueous solutions of sodium-poly(styrene sulfonate) (NaPSS) containing barium chloride (BaCl(2)) is studied by static and dynamic light scattering. We report a novel mechanism of phase separation, where an enrichment of polymer aggregates of well-defined size occurs in the very early stage of nucleation, which is then followed by a growth process in the formation of the new phase. In the latter stage, the polymer aggregates formed in the early stage act as the templating nuclei. Even in the homogeneous phase at higher temperatures above the upper critical phase boundary, polymer aggregates are present in agreement with previously reported results. Upon rapidly cooling the system below the phase boundary, the number concentration of the aggregates increases first by maintaining their size to be relatively monodisperse, before the growth process takes over at later times. The size and fractal dimension of aggregates in the homogeneous phase and the early nucleation stage of phase separation and the dependence of nucleation time and growth rate on quench depth and salt concentration are determined. The hydrodynamic radius (R(H)) of the unaggregated chains is of the order of 1-10 nm depending on the molecular weight of NaPSS, while R(H) of aggregates is of the order of 100 nm independent of the molecular weight of NaPSS. Unaggregated chains follow good solution behavior with a fractal dimension of 5/3 while the fractal dimension of aggregates is larger than 3.5 suggesting the branched nature of aggregates. Nucleation time is sensitive to quench depth and salt concentration. Increasing a quench depth or increasing BaCl(2) concentration shortens the nucleation time. After the nucleation time, during the growth period, the size of aggregates grows linearly with time, with growth rate being higher for deeper quench depths and higher BaCl(2) concentrations. The mechanism of phase separation of aqueous solutions of NaPSS and BaCl(2) is seen to proceed by utilizing the already-existing aggregates to nucleate the new phase, in marked contrast to hitherto known results on phase separation in uncharged polymer systems.     

Formation of porous flat membrane by phase separation with supercritical CO2

Microporous polystyrene membranes were prepared by the phase separation process using the supercritical CO₂ as a nonsolvent for the polymer solution. The thin polymer solution in a laboratory dish was located inside a cell and the supercritical CO₂ was introduced to induce the phase separation. The dry flat microporous membranes were obtained without collapse of the structure after the CO₂ pressure was diminished. Effects of the experimental conditions such as the CO₂ pressure, the polymer concentration and the temperature on the average pore size and membrane porosity were investigated.     

Effect of crystallization and liquid–liquid phase separation on phase‐separation kinetics in poly(ethylene‐co‐vinyl alcohol)/glycerol solution

Liquid–liquid thermally induced phase separation of the polymer‐diluent system of poly(ethylene‐co‐vinyl alcohol) (EVOH)‐glycerol was examined under light scattering. For EVOH with an ethylene content of 38 mol % (EVOH38), maxima of the scattered light intensity were observed that indicated that phase separation occurred by the spinodal decomposition (SD). The growth of the structures formed by the general liquid–liquid phase separation obeyed a power‐law scaling relationship in SD. For EVOH with an ethylene content of 32 mol % (EVOH32), the liquid–liquid phase separation resulted from the polymer crystallization. In this case, the structure growth showed the characteristic behavior in which the crystalline particles were initially formed, and then the droplets formed by the liquid–liquid phase separation induced by the crystallization grew rapidly. Furthermore, the growth of the droplet by the phase separation was followed by an optical microscope measurement at a constant cooling rate. The phase‐separated structure formed after the crystallization can grow faster than that formed by the normal liquid–liquid phase separation. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 194–201, 2003     

Dipolar motions and phase transitions in a side‐chain polysiloxane liquid crystal. A study by thermally stimulated depolarization currents

The relaxation mechanisms present in a side‐chain liquid crystalline polymer have been studied by Thermally Stimulated Depolarization Currents (t.s.d.c.), in a wide temperature range covering the glassy state, the glass transition region, and the liquid crystalline phase. The thermal sampling procedure was used to decompose the complex relaxations into its narrowly distributed components. Three relaxation mechanisms were observed in this polymer: a relaxation below the glass transition temperature that is broad and extends from −150°C up to −110°C, the glass transition relaxation whose maximum intensity appears at ∼20°C, and a relaxation above the glass transition temperature, in the liquid crystalline phase. The attribution of these relaxations at the molecular level is discussed. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 227–235, 1999     

Neutron scattering and monte carlo determination of the variation of the critical nucleus size with quench depth

We have used a combination of neutron scattering experiments and Monte Carlo simulations to study the initial stages of first-order phase transitions. We focus on quenches wherein the nascent phase is formed by homogeneous nucleation, and we approach the spinodal, i.e., the quench depth at which the original phase becomes unstable. In this regime, we show how critical nuclei sizes are determined from neutron scattering structure factors. Prevailing thought is that the size of the critical nucleus should increase with increasing quench depth and diverge at the spinodal. To the contrary, our experiments and simulations indicate that the critical nucleus size decreases monotonically as quench depth is increased and is finite at the spinodal.