Shear-induced phase separation and crystallization in semidilute solution of ultrahigh molecular weight polyethylene: Phase diagram in the parameter space of temperature and shear rate

In the previous papers, we elucidated enhancement of concentration fluctuations, phase separation, and crystallization induced by steady state or step-up shear flow, as observed by shear small-angle light scattering, optical microscopy, and birefringence, for a semidilute solution of ultrahigh molecular weight polyethylene in paraffin as an athermal solvent. However the studies were done only at a given temperature of 124 °C, which is higher than the nominal melting temperature of the quiescent solution T_nm (115–119 °C). It is crucial to extend the studies over a wider temperature range in order to generalize shear-induced phase behavior of the solution. Thus in this work we constructed a kind of phase diagram in the parameter space of temperature (T) and shear rate (). The temperature range covered was higher than T_nm, so that the phase diagram is strictly concerned with shear-induced phase behavior (i.e., without shear the solution is homogeneous and in a single-phase state). The diagram identified Regimes I–III in the T– space as will be detailed in the text. In constructing the phase diagram we found the following new points also. (i) The critical shear rate _cx which defines the boundary between Regimes I and II was independent of T. (ii) Regime III identified previously through the dependence of the integrated scattered intensity only at a particular temperature T = 124 °C was further separated into two regimes of III_a and III_b below and above a critical temperature (147 °C), respectively, through the observation of the dependence as a function of T: In Regime III_a, the sheared solution developed the optically anisotropic fibrous structures, indicative of the shear-induced crystallization triggered by the shear-induced concentration fluctuations in Regime II; In Regime III_b, the solution is so stable that it did not show a trend of the shear-induced crystallization even at the highest shear rates accessible in this experiment, but it only showed the shear-induced phase separation. (iii) The critical shear rates _c,streak and _cz, which define respectively the boundary between Regimes II and III_a and that between Regimes II and III_b, are sensitive to temperature.     

Phase separation and phase inversion of polyurethane networks

A series of polyurethane networks were prepared from MDI (4,4¹-diphenyl methane diisocyanate), ethylene glycol and a polyoxyethylene-tipped polyoxypropylene triol. The phase separation and phase inversion phenomena of these polyurethane networks were investigated by differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA) and measurement of their tensile properties. The DSC and DMA data indicate that the segmented copolyurethanes possess a two-phase morphology comprising soft and hard segments. It can be found from DSC data that the polyether soft segments exhibit a T_g (glass transition temperature) of –60 °C, and the aromatic hard segments display a T_g of about 128 °C. Two T_gs corresponding to the comprised segments can also be found by DMA for some segmented polyurethanes. Varying the content of aromatic hard segments over the range from 0 to 80 wt% changes the material behavior from a soft rubber through a highly extensible elastomer to a brittle semi-ductile glassy material. Based on the property-composition plots, phase inversion appears to occur at a hard segment content of about 50 wt%.     

Time-resolved light scattering of the phase separation in polymer-dispersed liquid crystals formed by photo-polymerization induced phase separation

Time-resolved light scattering is utilized to monitor the phase separation of photo-initiated polymer-dispersed liquid crystals. At the lowest cure intensities studied, the system undergoes spinodal decomposition and the results are analyzed with Cahn-Hilliard theory. As the cure intensity increases, the rate of phase separation increases such that the early stages of spinodal decomposition are no longer observable. These systems are analyzed using the Debye-Bueche model, which provides the time evolution of the number and size of LC domains. These results indicate that an increase in cure beam intensity initially increases the rate of domain growth, but this effect is overwhelmed by the fast vitrification and cross-linking that can occur at highest cure beam intensities.     

Phase separation of diamine chain-extended poly(urethane) copolymers: FTIR spectroscopy and phase transitions

As part of our continuing effort to understand microphase separation of poly(urethane urea) block copolymers, FTIR spectroscopy and thermal techniques (DSC and DMA) were used to investigate the phase behavior of two series of MDI-polytetramethylene oxide soft segment copolymers, chain-extended with ethylene diamine or a diamine mixture. Due to the complex nature and multiple absorbances in the carbonyl and N-H regions of the FTIR spectra, quantitative analysis was not possible. However, qualitative trends could be discerned, and the spectral changes were found to be in excellent agreement with our previous quantitative analysis of the same copolymers using small-angle X-ray scattering. DSC and DMA experiments both indicate that the soft phase T_g decreases with increasing hard segment content. This is contrary to increased hard segment mixing in the soft phase, but can be rationalized by taking into consideration soft segment crystallinity and the concentration of ‘lone’ MDI units in the soft phase.     

Rheology and phase separation of polymer blends with weak dynamic asymmetry

Rheological methods have been frequently used to study the phase separation behavior of partially miscible polymer blends. Usually the binodal temperature can be determined from the failure of time-temperature superposition (TTS) principle in isothermal experiments, or the deviation of the storage modulus from the apparent extrapolation of modulus in miscible regime in non-isothermal experiments. However, these methods are shown in this work to be not widely applicable even in blends with weak dynamic asymmetry due to the thermo-rheological complexity. A rheological model which is an integration of the double reptation model and the self-concentration model is found to describe the linear viscoelasticity of miscible blends quite satisfactorily, from which it is possible to follow the contribution from the miscible blends even in the two phase regime. Then, the binodal temperature is readily defined as the deviation of experimental data from such model prediction for miscible blends. Such method is successfully applied in a model polymer blend (poly(methyl methacrylate)/poly(styrene-co-maleic anhydride), PMMA/SMA) with weak dynamic asymmetry.     

Synthetic hydrogels 2. Polymerization induced phase separation in acrylamide systems

Acrylamide hydrogels were synthesized in the presence of various non-solvents for linear polyacrylamide to examine phase separation during polymerization. The process was found to be dependent upon the segmental volume, the chemical structure, and the concentration of the non-solvent. The concept of conversion-phase diagram for linear polymer is introduced and used qualitatively to understand polymerization induced phase separation (PIPS), and to predict the onset of PIPS during hydrogel synthesis.     

Kinetics of phase separation and crystallization in poly(ethylene-ran-hexene) and poly(ethylene-ran-octene)

The kinetics of phase separation and crystallization in the blends of poly(ethylene-ran-hexene) (PEH) and poly(ethylene-ran-octene) (PEOC) at several compositions were studied using phase contrast optical microscopy and time-resolved simultaneous small-angle X-ray scattering and wide-angle X-ray diffraction. The phase contrast optical microscopy showed the interconnected bicontinuous structure during phase separation process, which is characteristic of a spinodal decomposition. During isothermal crystallization, the average lamellar spacing increases with time for blends at all concentrations. The crystallinity and crystal growth rate depend on the PEH concentration. At dilute PEH concentrations, crystallization of PEH chains is difficult because they are surrounded by many non-crystallizable PEOC chains. On the other hand, at higher PEH concentrations, crystallization processes are similar to pure PEH. For example, the spherulitic growth rates are similar for a PEH/PEOC=50/50 blend and pure PEH.     

Thermodynamic analysis of phase separation in an epoxy/polystyrene mixture

The phase separation process of an epoxy prepolymer based on diglycidyl ether of bisphenol A (DGEBA) with a thermoplastic polystyrene (PS) was thermodynamically studied in the frame of the Flory-Huggins theory. The thermodynamic treatment was carried out in two steps: first analysing the phase separation in cloud point conditions, and second analysing the advance of the phase separation for two compositions of 2 and 10% in volume of PS. The effect of the polydispersity of thermoplastic on phase separation was also studied. The polydispersity of PS produces a displacement of the threshold temperature to lower thermoplastic volume fraction (between 2 and 3%) and higher temperature value and the fact that the shadow curve and coexistence curves do not superimpose with the cloud point curve. Theoretical calculations of molecular weight distributions of PS at different degrees of phase separation were realized and different average molecular properties were obtained in each separated phase.     

Interplay of crystallization and liquid-liquid phase separation in polyolefin blends: A thermal history dependence study

The kinetic interplay between crystallization and liquid-liquid phase separation (LLPS) in random copolymer blends of poly(ethylene-ran-hexene) (PEH) and poly(ethylene-ran-butene) (PEB) has been studied using optical microscopy. Morphologies of blends gone through three different thermal histories are compared: (1) single-quench (SQ), a homogeneous melt quickly cooled to isothermal crystallization temperatures (T_cry), (2) double-quench (DQ), a homogeneous melt quickly cooled to an intermediate temperature (T_lps) between binodal and equilibrium melting temperature (T_m⁰) and stored for a period of time and then cooled to T_cry, and (3) cyclic-quench (CQ), a homogeneous melt quickly cooled to T_lps and stored for a period of time, then gone through four cycles of crystallization and remelting. Comparing DQ morphologies to SQ ones, both crystal growth rate and nucleation density in the former are affected by prior LLPS. A scaling argument has been provided to partially account for the observed phenomena. In CQ, characteristic lengths of secondary features induced by crystallization depend strongly on the overall PEH composition, whereas are insensitive to temperature cycling. The contrast of large domains becomes more prominent upon cyclic crystallization and remelting. On the other hand, primary LLPS domains coarsen with CQ while loosing the contrast.     

One step liquid phase heterogeneous synthesis of phenytoin over MgAl calcined hydrotalcites

Heterogeneous liquid phase synthesis of phenytoin (5,5-diphenylhydantoin) was carried out over MgAl calcined hydrotalcites for the first time under environmental friendly conditions. The catalytic activity results showed very high conversion (80–95%) and selectivity (90–95%) of the desired product phenytoin over MgAl calcined hydrotalcites. The calcined hydrotalcites can be recycled without further loss in the activity and the possible mechanism of the reaction is also proposed.     

pH-triggered phase inversion and separation of hydrophobised bacterial cellulose stabilised Pickering emulsions

The pH-triggered transitional phase behaviour of Pickering emulsions stabilised by hydrophobised bacterial cellulose (BC) is reported in this work. Neat BC was esterified with acetic (C₂–), hexanoic (C₆–) and dodecanoic (C₁₂–) acids, respectively. We observed that C₆– and C₁₂–BC stabilised emulsions exhibited a pH-triggered reversible transitional phase separation. Water-in-toluene emulsions containing of 60 vol.% dispersed phase stabilised by C₆– and C₁₂–BC were produced at pH 5. Lowering the pH of the aqueous phase to 1 did not affect the emulsion type. Increasing the pH to 14, however, caused the emulsions to phase separate. This phase separation was caused by electrostatic repulsion between modified BC due to dissociable acidic surface groups at high pH, which lowered the surface coverage of the water droplets by modified BC. When the pH was re-adjusted to 1 again, w/o emulsions re-formed for C₆– and C₁₂–BC stabilised emulsions. C₂–BC stabilised emulsions, on the other hand, underwent an irreversible pH-triggered transitional phase separation and inversion. This difference in phase behaviour between C₂–BC and C₆–/C₁₂–BC was attributed to the hydrolysis of the ester bonds of C₂–BC at high pH. This hypothesis is in good agreement with the measured degree of surface substitution (DSS) of modified BC after the pH-triggered experiments. The DSS of C₂–BC decreased by 20% whilst the DSS remained constant for C₆– and C₁₂–BC.     

Effect of polymer-filler surface interactions on the phase separation in polymer blends

For the blends of chlorinated polyethylene and copolymer of ethylene with vinyl acetate, the effect of the introducing filler (fumed silica) on the phase behavior of the blends was investigated. It was found that introducing filler in polymer blends depending on its amount lead either to the increase or to the decrease in the temperature of phase separation. At the filler concentration where both components transit into the state of a border layers, the phase separation temperature increases. This effect was explained by the change of the total thermodynamic interaction parameter in the ternary system polymer-polymer-filler. At lower concentration of a filler, the possible effect is the redistribution of the blend components according to their molecular masses between filler surface (in the border layer) and in the bulk that may diminish the phase separation temperature.Effect of the filler on the phase behavior was explained by the simultaneous action of two mechanisms: by changing the thermodynamics of interaction near the surface due to selective adsorption of one of the components and by the redistribution of components according to their molecular masses between the boundary region (near the surface) and in the matrix.The measurements of the kinetics of phase separation and calculation of the parameters of the activation energy are in agreement with proposed mechanisms.     

Morphology control of various aromatic polyimides by using phase separation during polymerization

Morphology control of various aromatic polyimides representative as poly(4,4′-oxydiphenylene pyromelliteimide) was examined by using the phase separation during solution polymerization. Polymerizations of aromatic dianhydrides and aromatic diamines to the polyimides were carried out in poor solvents at 240-330 °C for 6 h with no stirring. Polymerization concentrations were from 0.25% to 3.0%. The polyimides were obtained as yellow precipitates. Two categorized morphologies were created, which were particles and crystals exhibiting lath-like and plate-like habits. These morphologies of polyimides could be selectively controlled by the polymerization conditions. The higher concentrations, less miscible solvents and lower temperatures were preferable to yield the particles via liquid-liquid phase separation. On the contrary, the lower concentration, miscible solvents and higher temperature were desirable to yield the crystals. The polyimide precipitates showed high crystallinity and possessed excellent thermal stability at which the 10 wt% loss temperatures in N₂ were in the range of 590-694 °C.     

Preparation of monodispersed porous polyacrylamide microspheres via phase separation in microchannels

We report on the formation of polyacrylamide (PAM)/polyethylene glycol (PEG) core/shell droplets in a microchannel via the polymerization-induced phase separation of an acrylamide (AM)/PEG aqueous system. Monodispersed porous PAM microspheres were prepared from the PAM/PEG core/shell droplets, and we examined the effects of experimental parameters on the phase separation process and on the particle size and pore structure of the resulting PAM microspheres. PAM microspheres could be readily obtained with adjustable particle sizes and porosities by altering the PEG and crosslinker contents and by using PEG with different molecular weights. The relation between the swelling value and porosity is correlated.     

Fluctuation-assisted nucleation in the phase separation/crystallization of iPP/OBC blends

This work mainly focused on the nucleation behavior in iPP/OBC (isotactic polypropylene/polyolefin block copolymers) blends with two distinct OBCs. The influence of composition and molecular structure of the OBC component on the crystallization kinetics of the blends was investigated systematically with the aim to better understand the interplay between the two coupled phase transitions in the blends: macrophase separation and crystallization. The isothermal crystallization kinetics showed component and composition dependence in iPP/OBC blends. All the blends in the studied range have enhanced nucleation ability of iPP than the pure iPP under identical conditions. Furthermore, the distinct macrophase separation morphology resulting from the different compatibility between the various OBCs and iPP caused remarkable diversity between the blends: the nuclei density is qualitatively higher (or the nucleation rate is qualitatively faster) in the more compatible blends, and this enhancement of nucleation can be depressed by imposing a macrophase separation process before crystallization. The crystal nuclei from the phase separated matrix were preferentially formed at the interface of the phase domains, and then grew toward and into the iPP-rich phase. It is postulated that the increased nuclei density and/or nucleation rate followed the fluctuation-assisted nucleation mechanism: the enhanced concentration fluctuation at the interfacial area created by the spinodal decomposition played an important role in the nucleation behavior of iPP/OBC blends. The decreased interface areas with increased domain sizes after deeper phase separation, coupled with a more depressed concentration fluctuation, are responsible for lower nuclei density after long time annealing for phase separation.     

泡沫分离法制备聚苯乙烯多孔微球

以一种新型、简单、高效的溶剂挥发法制备不同孔形态和粒径分布的聚苯乙烯微球。该方法利用机械搅拌和升温过程中溶剂挥发产生的泡沫,将油相液滴夹带进入泡沫相,使溶剂在气相中迅速挥发,诱导聚合物与非良溶剂发生相分离成孔。结果表明：随着聚合物与致孔剂用量比减小,微球结构形态由多孔演变到中空结构;聚乙烯醇（PVA）质量分数由1%增大至3%时,微球的平均粒径由52μm±23μm减小至23μm±20μm及转速由300r/min增大至700r/min时,微球粒径由107μm±40μm减小到45μm±20μm,但由于产生的泡沫量和泡沫形态不同影响了溶剂的挥发过程,故得到微球的多孔形态不同,增大PVA浓度得到的微球表面孔数目较少、孔径较大,而增大转速得到的微球孔数目较多。此外,该方法在油水相比≥1时,在泡沫中也能得到稳定规则的多孔微球,而传统的在水相中引发相分离的方法因水相无法完全分散油相,故无法成球。     

Ordered porous polymer films via phase separation in humidity environment

A transition of morphology from island-like structure to disordered and ordered holes on the surface of polystyrene (PS) and poly(2-vinylpyridine) (PVP) blend films were observed with the increase of humidity. At appropriate weight ratio of PS/PVP and PS molecular weight, when humidity reached to a critical value, the hexagonal arrays of holes formed for PS/PVP blend films due to ‘breath figures’ stabilized by PVP with its strong hygroscopic characteristics during phase separation.     

The salting-out effect and phase separation in aqueous solutions of electrolytes and poly(ethylene glycol)

The compositions of coexisting phases have been determined for aqueous two-phase systems containing poly(ethylene glycol) (PEG) of nominal molecular weight 8000 and a series of eight electrolytes: NaOH, Na₂CO₃, Na₂SO₄, Na₂HPO₄, Na₃PO₄, (NH₄)₂SO₄, MgSO₄, ZnSO₄. The salting-out effects of the electrolytes on the polymer were obtained by fitting a Setschenow-type equation to the tie line data to derive a salting-out coefficient for each electrolyte. For the sodium salts the relative effectiveness of the anions in forming biphasic mixtures follows the Hofmeister series. The relative salting-out effectiveness of the sulphate salts, however, is probably influenced by complexation of the cations by PEG. The values of the salting-out coefficients reflect the extent of preferential hydration of the ethylene oxide units in the polymer chains.     

Role of water structure on phase separation in polyelectrolyte-polyethyleneglycol based aqueous two-phase systems

Partitioning of proteins in aqueous two-phase systems (ATPS) has emerged as one of the important downstream processing techniques in bioprocess technology. The phase separation behavior of polyelectrolyte-polyethyleneglycol (PEG) based ATPS have been studied to elucidate the mechanism controlling phase behavior. The effect of various inorganic salt additives revealed the importance of water structure as a major factor controlling phase separation in these systems. Nitrate and potassium (water structure breaking ions) elevated the binodial line while sulphate, phosphate and sodium (water structure making ions) depressed the binodial line in both polyacrylic acid-PEG as well as polyethylenimine-PEG based ATPS. The effect of increase in concentration of either of the constituent polymers in both systems (at constant salt concentration) always led to a greater propensity towards phase separation. These results point to a mechanism in which salt-assisted polymer-modified water structure interactions play a central role in phase separation in ATPS.     

Structural and morphological development in poly(ethylene-co-hexene) and poly(ethylene-co-butylene) blends due to the competition between liquid-liquid phase separation and crystallization

Isothermal crystallization behavior of poly(ethylene-co-hexene) (PEH) and the 50/50 blend (H50) of PEH with amorphous poly(ethylene-co-butylene) (PEB) was studied by time-resolved synchrotron simultaneous small-angle X-ray scattering/wide-angle X-ray diffraction (SAXS/WAXD) techniques and optical microscopy (OM). The X-ray study revealed the changes of structural and morphological variables such as the scattering invariant, crystallinity and lamellar long period, et al. In H50, the lamellar morphology was found to be dependent on competition between liquid-liquid phase separation (LLPS) and crystallization. At high temperature, LLPS becomes dominating, resulted in crystallization of PEH with minimal influence of PEB. At low temperature, LLPS is suppressed, PEB component shows obvious influence on PEH crystallization, PEB is thought to be partially included into PEH lamellar stacks and PEH-PEB co-crystallization is unlikely, however, possible. Optical microscopy was used to monitor crystal nucleation and growth rates in PEH and H50, providing complementary information about the effect of temperature on LLPS and crystallization. Real-space lamellar morphologies in PEH and H50 were characterized by atomic force microscopy (AFM), PEH exhibited sheaf-like spherulites while H50 exhibited hedrites. Overall, the competition between LLPS and crystallization in H50 blend influences the structural and morphological development. Controlling the interplay between LLPS and crystallization of PEH/PEB blends, it is possible to control the structure and morphology as practically needed.