Light scattering behavior and the kinetics of pressure-induced phase separation in solutions of poly(ε-caprolactone) in acetone + CO2 binary fluid mixtures

The miscibility and the kinetics of pressure-induced phase separation in solutions of poly(ε-caprolactone) (PCL) in acetone + CO₂ binary fluid mixtures have been studied at pressures up to 28 MPa and temperatures up to 410 K using a unique high pressure view-cell equipped with a dual set of pistons and dual set of sapphire windows. One set of the windows separated by 25.4 mm allows the assessment of the phase state and is used to monitor the transmitted light intensities. The second set of windows separated by 50 μm is used to monitor the scattered light intensities over a wide range of scattering vector q (from 0.35 to 4 μm⁻¹) which allows the assessment of the mechanism of phase separation. Investigations have been carried out for a wide range of polymer concentrations, from 2.0 to 34.9 wt%, while holding the acetone-to-CO₂ (wt:wt) ratio in each solution at a constant value of 2:1. The dual set of pistons that are employed which are synchronized and motorized create a churn-like action in the cell insuring effective mixing, even at the high polymer concentrations by translating the cell content across a magnetically-coupled rotating mixer impeller. The piston actions assure also that the solution is effectively introduced into the narrow gap between the scattering windows, and refreshed. The solutions at off-critical concentrations undergo pressure-induced phase separation via nucleation and growth mechanism which shows circular symmetric patterns in their light scattering patterns. For these solutions, the Debye–Bueche type scattering function was used to analyze the domain size of the new phase that forms and develops after a pressure quench. The phase separation in solutions at or near the critical polymer concentrations (9.0–15.0 wt%) proceeds via spinodal decomposition which is characterized by the formation and evolution of the spinodal ring patterns corresponding to a maximum in the angular variation of the scattered light intensities. The results in early stage of the spinodal decomposition were described by the linearized Cahn theory. The variation of the scattered light intensity maximum I_m and its location in scattering vectors q_m with time in the later stage of the spinodal decomposition obey power-law scaling according to I_m ∼ t^β and q_m ∼ t^−α. The results for the 9.0 and 12.0 wt% solutions show that β/α changes its value from β/α > 3 to β/α ≈ 3 with time, indicating the progression of the spinodal decomposition from intermediate to late stage.     

Liquid-liquid phase separation in a polyethylene blend monitored by crystallization kinetics and crystal-decorated phase morphologies

A series of polyethylene (PE) blends consisting of a linear high density polyethylene (HDPE) and a linear low density polyethylene (LLDPE) with an octane-chain branch density of 120/1000 carbon was prepared at different concentrations. The two components of this set of blends possessed isorefractive indices, thus, making it difficult to detect their liquid-liquid phase separation via scattering techniques. Above the experimentally observed melting temperature of HDPE, T_m = 133 °C, this series of blends can be considered to be in the liquid state. The LLDPE crystallization temperature was below 50 °C; therefore, above 80 °C and below the melting temperature of HDPE, a series of crystalline-amorphous PE blends could be created. A specifically designed two-step isothermal experimental procedure was utilized to monitor the liquid-liquid phase separation of this set of blends. The first step was to quench the system from temperatures of known miscibility and isothermally anneal them at a temperature higher than the equilibrium melting temperature of the HDPE for the purpose of allowing the phase morphology to develop from liquid-liquid phase separation. The second step was to quench the system to a temperature at which the HDPE could rapidly crystallize. The time for developing 50% of the total crystallinity (t_1/2) was used to monitor the crystallization kinetics. Because phase separation results in HDPE-rich domains where the crystallization rates are increased, this technique provided an experimental measure to identify the binodal curve of the liquid-liquid phase separation for the system indicated by faster t_1/2. The annealing temperature in the first step that exhibits an onset of the decrease in t_1/2 is the temperature of the binodal point for that blend composition. In addition, the HDPE-rich domains crystallized to form spherulites which decorate the phase-separated morphology. Therefore, the crystal dispersion indicates whether the phase separation followed a nucleation-and-growth process or a spinodal decomposition process. These crystal-decorated morphologies enabled the spinodal curve to be experimentally determined for the first time in this set of blends.     

Viscoelastic effects on the phase separation in thermoplastics modified cyanate ester resin

A high temperature thermosetting bisphenol-A dicyanate, BADCy was blended with a thermoplastic poly(ether imide) (PEI). The phase separation behavior of the blend was investigated by scanning electron microscopy (SEM) and time resolved light scattering (TRLS). It was found by SEM that the blend with 20 and 25 wt% PEI had a phase inversion structure. The results of TRLS displayed clearly that the phase separation took place according to a spinodal decomposition (SD) mechanism and the evolution of both scattering vector q_m and the maximum scattering intensity I_m followed Maxwell-type relaxation equation. The temperature-dependent relaxation time τ for the blends can be described by the Williams-Landel-Ferry equation. It demonstrated experimentally that the phase separation behaviors in PEI/BADCy blends were affected by viscoelastic effect.     

Phase separation and dewetting in polystyrene/poly(vinyl methyl ether) blend thin films in a wide thickness range

Phase separation and dewetting processes of blend thin films of polystyrene (PS) and poly(vinyl methyl ether) (PVME) in two phase region have been studied in a wide film thickness range from 65 μm to 42 nm (∼2.5R_g, R_g being radius of gyration of a polymer) using optical microscope (OM), atomic force microscope (AFM) and small-angle light scattering (LS). It was found that both phase separation and dewetting processes depend on the film thickness and were classified into four thickness regions. In the first region above ∼15 μm the spinodal decomposition (SD) type phase separation occurs in a similar manner to bulk and no dewetting is observed. This region can be regarded as bulk. In the second region between ∼15 and ∼1 μm, the SD type phase separation proceeds in the early stage while the characteristic wavelength of the SD decreases with the film thickness. In the late stage dewetting is induced by the phase separation. In the third region between ∼1 μm and ∼200 nm the dewetting is observed even in the early stage. The dewetting morphology is very irregular and no definite characteristic wavelength is observed. It is expected that the irregular morphology is induced by mixing up the characteristic wavelengths of the phase separation and the dewetting. In the fourth region below ∼200 nm the dewetting occurs after a long incubation time with a characteristic wavelength, which decreases with the film thickness. It is considered that the layered structure is formed in the thin film during the incubation period and triggers the dewetting through the capillary fluctuation mechanism or the composition fluctuation one.     

Attenuation of concentration fluctuations after a quench to a temperature in the single-phase region

Attenuation of concentration fluctuations in a polystyrene/poly(2-chlorostyrene) blend after a quench from a temperature in the spinodal region to one in the single-phase region was studied by using the time-resolved light scattering technique and a scanning electron microscope. The characteristic wave number q_m, where the maximum of the scattered light intensity was located, move towards smaller values during the attenuation process. The corresponding growth of the wave length of the dominant concentration fluctuation was clearly observed by the electron microscope. The scattered light intensities at small wave numbers were found to increase before they attenuated. In the later period the power-law behavior q_m∼t^−0.3 was observed. The exponent was smaller than the value (0.5) obtained on the basis of the non-linear theory by Akcasu and collaborators.     

Effects of cooling temperature and aging treatment on the morphology of nano‐ and micro‐porous poly(ethylene‐co‐vinyl alcohol) membranes by thermal induced phase separation method

Poly(ethylene‐co‐vinyl alcohol) (EVOH 32) / 1,3‐propanediol mixtures are processed by thermally induced phase separation for the formation of porous membranes. The crystallization line was determined both by the cloud‐point and DSC methods. Two precursor solution compositions, four quench temperatures and various aging times were explored. It is found possible to generate both polymer‐crystallization controlled morphologies (for high quenches and/or sufficiently aged dopes), especially globular microporous ones, and novel nano‐scale porous morphologies dominated by intra‐binodal phase separation (for low quenches and limited or no precursor solution aging). Structural characterization of the membranes was accomplished via application of scanning electron microscopy and wide angle X‐ray diffraction. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40374.     

Effects of bath temperature on the morphology and performance of EVOH membranes prepared by the cold‐solvent induced phase separation (CIPS) method

The development and characteristics of porous EVOH membranes by cold‐solvent induced phase separation (CIPS) process were investigated. Binary dopes of 1,3‐propandiol/EVOH prepared at 80 °C were immersed in 1,3‐propandiol at a lower temperature to engender polymer precipitation. The quench temperature affects phase separation modes, and hence structure and performance of resulting CIPS membranes. When the bath temperature was set below the crystallization line and above the binodal (e.g. 45 °C), the formed membrane was dominated by a packing of semicrystalline EVOH globules. When the bath was set at a temperature just below the spinodal (e.g. 20 °C), spinodal decomposition (SD) dominated the precipitation process to give a lacy‐like bicontinuous structure; yet there is also a clear imprint from polymer crystallization. When the bath temperature was set deeply within the spinodal dome (e.g. 5 °C), polymer crystallization affected only little the SD‐derived bicontinuous morphology. Water permeation flux, wettability, tensile strength, and ultra‐filtration experiments of the membranes were conducted. The results indicated that those properties were closely correlated with the porosity level, pore size, and membrane morphology. Moreover, X‐ray diffraction and DSC analyses indicated that the formed membranes had a crystallinity of 38 to 42%, consistent with the literature data. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44553.     

Evolution of phase morphology in compatibilized polymer blends at constant quench depths: Complementary studies by light scattering and transmission electron microscopy

The effect of added block copolymer on the phase separation and morphology evolution in a partially miscible blend of polystyrene and polybutadiene near the critical composition is studied by temperature jump light scattering (TJLS) and transmission electron microscopy (TEM). As block copolymer is added, the phase boundary is shifted to lower temperatures and the phase separation process is slowed dramatically. Since the quench depth greatly affects the rate of phase separation in any blend system, we have used equivalent quench depths by adjusting for the shift in the phase boundary as block copolymer is added. The morphology evolution of these ternary blends was studied by preparing TEM specimens at equivalent shallow quench depths (ΔT = 1.6°C) and allowing each blend mixture to coarsen for the time required to reach a specific constant size, or q-value, using the TJLS data on the kinetics of phase separation. The q-range selected was q ～ 0.003–0.005 nm^?1, which corresponds to a spacing of 1–2 μm in real space. The combination of light scattering and microscopy techniques more rigorously describes the compatibilization process in these complex ternary systems.     

Crystallization and phase separation kinetics in blends of linear low-density polyethylene copolymers

We have systematically studied the crystallization and liquid-liquid phase separation (LLPS) kinetics in statistical copolymer blends of poly(ethylene-co-hexene) (PEH) and poly(ethylene-co-butene) (PEB) using primarily optical microscopy. The PEH/PEB blends exhibit upper critical solution temperature (UCST) in the melt and crystallization temperature below the UCST. The time evolution of the characteristic morphology for both crystallization and LLPS is recorded for blends at various compositions and following a quench from initial homogenous melts at high temperature to various lower temperatures. The crystallization kinetics is measured as the linear growth rate of the super structural crystals, whereas the LLPS kinetics is measured as the linear growth rate of the characteristic length of the late-stage spinodal decomposition. The composition dependence crystallization kinetics, G, shows very different characteristics at low and high isothermal crystallization temperature. Below 116 °C, G decreases with increasing PEB content in the blend, implying primarily the composition effect on materials transport; whereas at above 116 °C, G shows a minimum at about the critical composition for LLPS, implying the influence of the LLPS. On the other hand, LLPS kinetics at 130 °C is relatively invariant at different compositions in the two-phase regime. The length scale at which domains are kinetically pinned, however, depends strongly on the composition. In a blend near critical composition, a kinetics crossover is shown to separate the crystallization dominant and phase separation dominant morphology as isothermal temperature increases.     

Effects of spinodal decomposition on mechanical properties of a polyolefin blend from high to low strain rates

The effects of spinodal decomposition, a typical type of liquid-liquid phase separation (LLPS), on the mechanical properties of a pretreated statistical copolymer blend of poly(ethylene-co-hexene) (PEH) and poly(ethylene-co-butene) (PEB) were characterized by tensile testing under different strain rates. An important finding was that the strain rate and the crystallization temperature had to be considered as independent variables in analyzing the effects of spinodal decomposition on the tensile behaviors. At the high strain rate, the stress-strain curves kept irrespective of LLPS time, in which the interfacial relaxation between phase domains could not be detected, except the case crystallizing at 120 °C for 10 min. This was explained in terms of the distribution of the crystals elaborated by differential scanning calorimetry (DSC) results. However, when a relatively low strain rate was employed, a clear deterioration of tensile properties with LLPS proceeding was observed for the cases with low crystallization temperature because of its detection ability for large scale structural information, such as the phase boundary; unexpectedly, the effect of LLPS on the tensile properties was found to disappear in the high crystallization temperature cases which was due to the cooperative functions of the phase boundary and the internal structures of the phase domains. These abundant results provided a novel and indispensable instruction for the processing of polymer blends from the theoretical viewpoint.     

Morphology development in photopolymerization-induced phase separated mixtures of UV-curable thiol-ene adhesive and low molecular weight solvents

The influence of photopolymerization rate, solvent quality, and processing parameters on the photopolymerization-induced phase separated morphology of mixtures of thiol-ene based optical adhesive with mixed solvents of diglyme and water or acetone and isopropanol is described. Upon exposure to UV radiation (∼50 mW/cm², 365 nm) for periods of 10-90 s, homogeneous solutions of 5-10 wt% NOA65 and NOA81 adhesive formed phase separated structures with characteristic sizes ranging from 400 nm to 10 μm, with increased photopolymerization rates leading to smaller feature sizes. In the systems containing diglyme and water, morphologies formed by phase separation at a lower degree of photopolymerization were characteristic of spinodal decomposition, while morphologies formed by phase separation at a higher degree of photopolymerization exhibited characteristics of viscoelastic phase separation. In the systems containing acetone and isopropanol, interactions between evaporation and photopolymerization-induced phase separation led to the development of more complicated morphologies, including three-dimensional sparse networks. These morphologies provide a combination of connectivity and low overall volume fraction that can significantly enhance the performance of many multi-functional structures.     

Spinodal decomposition as a probe to measure the effects on molecular motion in poly(styrene-co-acrylonitrile) and poly(methyl methacrylate) blends after mixing with a low molar mass liquid crystal or commercial lubricant

The effects on molecular motion observed through early stage phase separation via spinodal decomposition, in melt mixed poly(styrene-co-acrylonitrile) (SAN) containing 25% by weight of acrylonitrile (AN) and poly(methyl methacrylate) (PMMA) (20/80 wt%) blends after adding two low molar mass liquid crystals (CBC33 and CBC53) and two lubricants (GMS and zinc stearate) were investigated using light scattering techniques. The samples were assessed in terms of the apparent diffusion coefficient (D_app) obtained from observation of phase separation in the blends. The early stages of phase separation as observed by light scattering were dominated by diffusion processes and approximately conformed to the Cahn-Hilliard linearised theory. The major effect of liquid crystal (LC) was to increase the molecular mobility of the blends. The LC generally increased the Cahn-Hilliard apparent diffusion coefficient, D_app, of the blend when added with concentrations as low as 0.2 wt%. GMS and zinc stearate can also improve the mobility of the blend but to a lesser extent and the effect does not increase at higher concentration. On the other hand, the more LC added, the higher the mobility. In all systems the second derivative of the Gibbs free energy becomes zero at the same temperature. The improved mobilities therefore seem to arise from changes in dynamics rather than thermodynamic effects.     

Morphology and properties control on rubber-epoxy alloy systems

Morphology and properties of polymer alloys can be controlled by thermody-namlcally reversible (structure freeze-in) or irreversible (structure lock-in) processes by simultaneously manipulating miscibility, mechanisms of phase separation, glass transition temperature (structural relaxation), and cure kinetics of polymer systems. Using phase diagrams consisting of binodal and spinodal curves, the morphology of epoxy/CTBN (carboxyl-terminated butadiene acryloni-trile copolymer) systems can be controlled by the mechanism of nucleation and growth or by spinodal decomposition via simultaneously manipulating the kinetic processes of phase separation and curing reactions. We have found that the particle size of the rubber reinforcement in epoxies is affected by the mechanisms of phase separation. Phase separation by nucleation and growth gives larger rubber particles than the corresponding phase separation by spinodal decomposition. This contrast in the morphology development is the consequence of controlling phase separation through chemorheological behavior. Medication of the phase separation kinetics in epoxy/CTBN systems was extremely effective at altering both morphology and properties of these alloys. This technique offers a means to shift the glass transition temperature of the rubber-rich phase drastically without reducing the glass transition temperature of the epoxy-rich phase significantly. Such control over morphology is the key to ultimately controlling material properties. This morphology manipulation allows us to tailor the mechanical properties of alloy systems.     

Phase separation of poly (methyl methacrylate) / poly (styrene-co-acrylonitrile) blends in the presence of silica nanoparticles

The effects of silica nanoparticles on the phase separation of poly (methyl methacrylate)/poly (styrene-co-acrylonitrile) (PMMA/SAN) blends are studied by the rheological method. The binodal temperatures of near-critical compositions were obtained by the gel-like behavior during spinodal decomposition, which is a character of polymer blends with co-continuous morphology. The shifted Cole–Cole plot method was introduced to determine the binodal temperatures of off-critical compositions based on the appearance of shoulder-like transition in the terminal regime of blends with droplet morphology. Such method is found also applicable in nanoparticle filled polymer blends. Moreover, a new method to determine the spinodal temperature from Fredrickson-Larson mean field theory was suggested, where the concentration fluctuation's contribution to the storage modulus is used instead of the whole dynamic moduli. This method was also successfully extended to nanoparticle filled polymer blend. The influences of the concentration and the average diameter of silica particles on the phase separation temperature were studied. It was found that the small amount of the silica nanoparticles in PMMA/SAN blends will significantly change the phase diagram, which is related to the selective location of silica in PMMA. The comparisons with thermodynamic theory of particle-filled polymer blends are also discussed.     

Analysis of unsaturated polyester-polyurethane interpenetrating polymer networks II: Phase separation behavior

The phase separation behavior of unsaturated polyester (UPE)-polyurethane (PU) interpenetrating polymer networks (IPNs) was investigated by light scattering measurements during simultaneous polymerization. The scattered light intensity change with time showed the formation of the dispersed domains, and the average domain correlation length could be calculated from the angle of maximum scattering intensify. It was noted that the dominant phase separation process was spinodal decomposition due to fast reaction. The morphology observed by the transmission electron micrographs for various process conditions showed similar results as obtained from the light scattering experiment.     

Extension of a compressible lattice model to CO2 + cosolvent + polymer systems

We have recently proposed a compressible lattice model for CO₂ + polymer systems in which CO₂ forms complexes with one or more functional groups in the polymer. Furthermore, we have shown that this model is able to simultaneously correlate phase equilibria, sorption behavior, and glass transition temperatures in such systems. In the present work, we extend the model to ternary CO₂ + cosolvent + polymer systems and demonstrate that cloud point behavior in CO₂ + dimethyl ether + poly (?-caprolactone), CO₂ + dimethyl ether + poly (isopropyl acrylate), and CO₂ + dimethyl ether + poly (isodecyl acrylate) systems can be predicted using parameters obtained from binary data. Our results also suggest that dimethyl ether may form weak complexes with poly (?-caprolactone), poly (isopropyl acrylate), and poly (isodecyl acrylate).     

Later-stage spinodal decomposition in polymer solution under high pressure—analyses of qm and Im

Later-stage spinodal decomposition (SD) of polymer solutions (polypropylene/trichlorofluoromethane) induced by pressure-jump was examined in situ as a function of pressure P by using time-resolved light scattering method with the cell designed for high pressure and high temperature. The time-evolution of the magnitude of scattering vector q_m(t,P) at maximum scattered intensity and the maximum scattered intensity I_m(t,P) were analyzed in order to characterize the coarsening processes of the later-stage SD, where t refers to time after the onset of pressure-jump. The changes in q_m(t,P) and I_m(t,P) with t at different P's were found to fall onto the respective master curves on the reduced plots, indicating that the scaling postulate is valid not only for the coarsening behaviors at different temperatures but for those at different P's.     

Studies on phase separation rate in porous polyimide membrane formation by immersion precipitation

Phase separation rate during porous membrane formation by immersion precipitation was investigated by light scattering in a polyimide/N‐Methylpyrrolidone (NMP)/water system. In the light scattering measurement, plots of scattered intensity against scattered angle showed maxima in all cases, which indicated that phase separation occurred by a spinodal decomposition (SD). Characteristic properties of the early stage of SD, such as an apparent diffusion coefficient D_app and an interphase periodic distance Λ, were obtained. The growth process of Λ was also followed by light scattering. The growth rate had the same tendency as D_app when water content in the nonsolvent bath and the polymer concentration in the cast solution were changed. The pore size of the final membrane increased with decreasing water content, which was opposite to the tendency of Λ growth rate. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 292–296, 2003     

HYDRODYNAMIC EFFECTS IN THE PHASE SEPARATION OF BINARY POLYMER MIXTURES

The phenomenon of phase separation by spinodal decomposition was studied for polymer blends made by compositional quenching. The modified Cahn-Hilliard theory of phase separation was extended to include hydrodynamics, with a volumetric body force, due to concentration gradients, that induced convective flows. This force influenced the morphology and the growth rate of the average domain size. Unlike the conventional treatment of flows driven by surface tension, the velocity and pressure fields were treated as continuous functions of spatial position.

Numerical solutions for the phase separation in a binary mixture were obtained for a three-dimensional system with periodic boundary conditions. For near critical quenches with similar volume fractions for the two components, cocontinuity was destroyed by the hydrodynamics, giving discrete domains. The breakup in interconnectivity is believed to be a universal phenomenon. The domain growth rate followed a power law, r → τⁿ. The growth exponent depended on the dimensionless viscosity group, ξ = (R_g T/v_s) (Km/μD_AB) and ranged from n = 0.32 ± 0.006 for ξ J = 0 (no hydrodynamic effects) to n ∼ 1 for ξ = 1. For off-critical quenches in which a dispersed phase would be formed by diffusion alone, the scaling exponent showed little enhancement. The simulations accurately predicted the particle size formed in the early stages of spinodal decomposition.     

Acceleration or retardation to crystallization if liquid-liquid phase separation occurs: Studies on a polyolefin blend by SAXS/WAXD, DSC and TEM

Blends of statistical copolymers containing ethylene/hexene (PEH) and ethylene/butene (PEB) exhibited the behavior of upper critical solution temperature (UCST). The interplay between the early and intermediate stage liquid-liquid phase separation (LLPS) and crystallization of the PEH/PEB 50/50 blend was studied by time-resolved simultaneous small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) techniques. Samples were treated by two different quench procedures: in single quench, the sample was directly quenched from 160 °C to isothermal crystallization temperature of 114 °C; while in double quench, the sample was firstly quenched to 130 °C for 20 min annealing, where LLPS occurred, and then to 114 °C. It was found that in the early stage of crystallization, the integrated values of Iq² and crystallinity, X_c, in the double quench procedure were consistently higher than those in the single quench procedure, which could be attributed to accelerated nucleation induced by enhanced concentration fluctuations and interfacial tension. In the late stage of crystallization, some morphological parameters were found to crossover and then reverse, which could be explained by retardation of lamellar growth due to phase separation formed during the double quench procedure. This phenomenon was also confirmed by DSC measurements in blends of different compositions at varying isothermal crystallization temperatures. The crystal lamellar thickness determined by SAXS showed a good agreement with TEM observation. Results indicated that the early stage LLPS in the PEH/PEB blend prior to crystallization indeed dictated the resulting lamellar structures, including the average size of lamellar stack and the stack distribution. There seemed to be little variation of lamellar thickness and long period between the two quenching procedures (i.e., single quench versus double quench).