Liquid-liquid phase separation in polysulfone/solvent/water systems

The cloud point curves for polysulfone (PSf)/solvent/water systems were determined by a titration method. A small amount of water was needed to induce liquid-liquid demixing and the temperature effect was small. From numerical calculations, it was found that the binary interaction parameters for the PSf/solvent/water system enlarges the homogeneous region in the phase diagram with a smaller nonsolvent-polymer interaction parameter χ₁₃, a greater nonsolvent-solvent interaction parameter χ₁₂, and a smaller solvent-polymer interaction parameter χ₂₃ and the effect of polymer molecular weight was negligible except in the range of low molecular weight. The phase diagrams, calculated with constant χ₁₂ that was chosen from the concentration-dependent interaction parameter g₁₂ value of the concentration range, were similar to the results obtained with g₁₂. The slope of the tie lines indicated that demixing of the ternary system occurred at relatively similar nonsolvent concentration in both phases. A value of 2.7 for the water-PSf interaction parameter was obtained by fitting the experimental cloud point curve with the calculated binodal lines. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65: 2643–2653, 1997     

Mechanical strength and origin of the strengthening effect of tantalum in superhard W0.5Ta0.5B monoboride

Tungsten borides with excellent mechanical properties have been recognized as a class of ultrahard compounds in various industrial applications. Here, motivated by the recent experimental work, a quantitative comparison analysis on the structure, mechanical strength, and electronic structure of the tantalum-strengthened superhard W_0.5Ta_0.5B monoboride and its parent material WB has been studied by first-principles calculations. Excellent agreements of the calculated lattice parameters and simulated X-ray diffraction between present results and experimental data have confirmed the crystal structure of the synthesized W_0.5Ta_0.5B. Compared to the WB, the calculated stress-strain curves show an enhanced shear strength and improved ductility of W_0.5Ta_0.5B on (100) and (010) crystal planes, originating from the reduction of antibonding states between the W-e_g states which enables strenuous sliding of metal bilayer in W_0.5Ta_0.5B. Furthermore, the lattice instability of W_0.5Ta_0.5B under large shear strain with an intriguing sequential bond-breaking mode that is derived from the first breaking of zigzag B chains and the subsequent collapsing of WB₇ and TaB₇ polyhedrons by simultaneously breaking of B–W and B–Ta bonds. These findings shed strong light on the strengthening mechanism of W_0.5Ta_0.5B and the design for novel ultra-incompressible and superhard solids in transition metal monoborides.     

Cloud point curves for poly(vinyl methyl ether) and monodisperse polystyrene mixtures

The compatibility behaviour of poly(vinyl methyl ether) (PVME) and monodisperse polystyrene (PS) is studied for solution cast films. The molecular weight of monodisperse PS ranges from 2100 to 2 000 000 whereas the PVME used is polydisperse and has weight-average molecular weight of 51 500. When cast from toluene solution, the mixtures undergo phase separation at elevated temperatures. The cloud point curves move to markedly lower temperatures with increasing molecular weight which is similar to the lower critical solution temperature (LCST) behaviour for polymer solutions. They move to lower temperatures until the molecular weight of PS reaches about 51 500.The effect of molecular weight distribution on the cloud point of equal amounts of PS and PVME mixtures simulated by mixing two monodisperse polystyrenes of different molecular weight for PS part is accurately predicted by using weight-average molecular weight for PS in this range. However, if the molecular weight of PS exceeds about 110 000 the molecular weight dependence of cloud point temperature is reversed and the prediction for polydisperse polymer by using weight-average molecular weight fails. This phenomenon is discussed from several viewpoints including the possibility of the effect of chain entanglement.Mixtures of PVME and PS of M_w = 20 400 were also cast from an ‘incompatible’ solvent, trichloroethylene. Compatibility is found to be dependent on composition and even phase-separated samples show at least one cloud point, indicating at least partial mixing of the two polymers. Finally, it is demonstrated that crosslinking of compatible films can be achieved by electron irradiation to form true interpenetrating networks. The cloud point temperatures are increased drastically after crosslinking.     

Phase equilibria of poly(methyl methacrylate) in supercritical mixtures of carbon dioxide and chlorodifluoromethane

Phase behavior data are presented for poly(methyl methacrylate) (PMMA: M_w= 15,000, 120,000) in supercritical solvent mixtures of carbon dioxide (CO₂) and chlorodifluoromethane (HCFC-22). Experimental cloud point curves, which were the phase boundaries between single and liquid-liquid phases, were measured by using a high-pressure equilibrium apparatus equipped with a variable-volume view cell at various CO₂ compositions up to about 63 wt% (on a polymer-free basis) and at temperatures up to about 100 °C. The cloud point curves exhibited the characteristics of a lower critical solution temperature phase behavior. As the CO₂ content in the solvent mixture increased, the cloud point pressure at a fixed temperature increased significantly. Addition of CO₂ to HCFC-22 caused a lowering of the dissolving power of the mixed solvent due to the decrease of the solvent polarity. The cloud point pressure increased with increasing the molecular weight of PMMA.     

Selective cracking of natural gasoline over HZSM-5 zeolite

This work presents a study on the catalytic cracking of natural gasoline (extracted from natural gas) over HZSM-5 zeolite. A factorial planning was carried out to evaluate the effect of temperature and W/F ratio on the cracking of natural gasoline, analyzing their effects on conversion and product distribution using an analysis based on surface response methodology. The process was optimized focusing on the maximization of the mass fractions and the production of specific products such as ethene, propene and butanes. The results have shown that the maximum selectivity and hourly mass production of ethene is obtained at high temperature (450 °C) and low catalyst weight to flow rate ratio (W/F) (7.2 to 8.2 g_cat h/mol). Maximum selectivity of propene is obtained at 350 °C and 7.0 g_cat h/mol, while the best condition for maximum mass production is found at 421 °C and 5.7 g_cat h/mol. The highest mass production of butanes is favored by high temperature (450 °C) and mid range W/F ratios (12.1 g_cat h/mol), while the highest selectivity is found at low temperature (350 °C).     

The effect of airflow pattern on filter breakthrough in physical adsorption

The Wheeler-Jonas (WJ) model for prediction of the protection capacity of organic vapor filters under a fixed airflow was extended to breathing-simulation, pulsating flow. Breakthrough curves of dimethyl-methyl phosphonate (DMMP) and decane were measured under fixed flow and sinusoidal flow. A linear dependence of ln(C_X/C₀) on the breakthrough time (t_B) was observed in all the experiments, indicating that the concepts of critical bed weight (W_C) and dynamic adsorption capacity (W_E) as defined by the WJ model are applicable to pulsating flow as well. W_E was found to be almost unchanged by the flow pattern, whereas W_C was considerably larger (by 7-44%) at pulsating flow compared to fixed flow with the same average rate. Thus, shifting from fixed flow to pulsating flow may shorten t_B significantly. The effect of the flow type on t_B increases with the ratio of the critical weight to the total bed weight. For a high protection level (C₀/C_X=60?000), the protection capacity of personal NBC canisters was reduced by up to 15% upon shifting from fixed to pulsating flow.     

Pulsed electron beam irradiation of dilute aqueous poly(vinyl methyl ether) solutions

A dilute aqueous solution of the temperature-sensitive polymer, poly(vinyl methyl ether) (PVME), was irradiated by a pulsed electron beam in a closed-loop system. At temperatures, below the lower critical solution temperature (LCST), intramolecular crosslinked macromolecules, nanogels, were formed. With increasing radiation dose D the molecular weights M_w increase, whereas the dimensions (radius of gyration R_g, hydrodynamic radius R_h) of the formed nanogels decrease. The structure of the PVME nanogels was analyzed by field emission scanning electron microscopy (FESEM) and globular structures with d=(10-30) nm were observed. The phase-transition temperature of the nanogels, as determined by cloud point measurements, decreases from T_cr=36 °C (non-irradiated polymer) to T_cr=29 °C (c_p=12.5 mM, D=15 kGy), because of the formation of additional crosslinks and an increase in molecular weights. The same behavior was observed for a pre-irradiated PVME (γ-irradiation) with higher molecular weight due to intermolecular crosslinks. After pulsed electron beam irradiation the molecular weight again slightly increases whereas the dimension decreases. Above D=1 kGy the calculated ρ-parameter (ρ=R_g/R_h) is in the range of ρ=0.5-0.6 that corresponds to freely draining globular structures.     

The adhesion of amorphous polystyrene surfaces below Tg

The bonding of polystyrene (PS) surfaces below T_g was investigated by two different fracture tests: the lap-shear joint method and the cantilever beam method. Adhesion energy values obtained by the two methods are in agreement and develop with (time)^1/2, at temperatures as low as T_g−16 °C. Even if the double cantilever method is the most common test found in the literature for adhesions above T_g, for low adhesion values, below T_g, the lap-shear joint geometry is more appropriate. Moreover, when the glass transition temperature is used as a reference temperature, polydisperse and monodisperse PS adhesion energy curves are superposable, suggesting that the auto-adhesion is not significantly favored by the presence of numerous chain ends at the surface (due to the low molecular weight chains provided by the polydisperse PS).     

Dissolution du nickel en transpassivité—I. Méthode de calcul

The behaviour of nickel has been studied in 0,5 M H₂SO₄, 25°C, in the transpassive region. The experimental polarisation curves were determined by a potentiostatic technique over the potential range 1000–1800 mV/sce; the dissolution curves were determined directly by the nickel weight loss per unit time at each potential; the oxygen discharge curves were determined by the volume of gas evolved per unit time. In the transpassive region several reactions occur simultaneously. In order to explain the nickel dissolution, and the depassivation of the metal, in this potential range, a model of the mechanism is proposed, and the method of calculation described. The experimental current voltage curves are compared and are in good accord with the calculated curves.     

Brownian dynamics simulations of bead-rod-chain in simple shear flow and elongational flow

We have simulated a dilute polymer solution under simple shear and elongational flows using the bead-rod-chain model, by incorporating intra-chain hydrodynamic interaction and excluded volume effects. Configurational properties and rheological quantities were calculated. For the simple shear flow, shear rate dependencies of chain's size, shape, and rotation were monitored. Shear-thinning was observed at all shear rates. In addition, the critical strain rate, ε_c, at which the polymer undergoes a coil-stretch transition under elongational flow, was investigated. The slope at the inflection point of the log(R_g²) vs log(ε) curve increased as the chain length increased, indicating a possible first order transition, in agreement with theories and experiments.     

Rheological characterization of 4,4′-bis(diethylamino)benzophenone-functionalized polybutadienes with low and medium vinyl content

An experimental study was undertaken to investigate the thermomechanical properties of a certain epoxy/amine configuration. The basic structure of all the epoxies was the same—DGEBA—and the curing agent used was PACM 20. By varying the epoxy prepolymer molecular weight and the stoichiometry between epoxy and amine, a range of different epoxy networks were produced. Glass transition temperatures were evaluated by using differentil scanning calorimetry (DSC). Modulus values as well as an alternative T_g determination were provided by dynamic mechanical analysis (DMA). Coefficients of thermal expansion were obtained from thermomechanical analysis (TMA). The tensile tests conducted at room and elevated temperatures provided additional modulus data along with the yield point, tensile strength, and elongation at break data. Property vs. stoichiometry curves exhibited a maximum for the glass transition temperature and the over the T_g modulus at the stoichiometric point. On the other hand, the under T_g modulus showed a minimum at the stoichiometric point. The results of the yield strength show remarkable similarity with the results of the modulus. Strength and elongation at break do not show clear trends, but a much different behavior is exhibited between room and elevated temperatures. © 1996 John Wiley & Sons, Inc.     

Kinetics of demixing and remixing in poly(ethylene oxide)/poly(ether sulphone) blends as studied by modulated temperature differential scanning calorimetry

The phase diagram of a poly(ethylene oxide)/poly(ether sulphone) blend is constructed using modulated temperature DSC and optical microscopy. It includes the glass transition (T_g), melting point (T_m) and cloud point temperature (T_cl), which define the regions of interest in this work: demixing above T_cl, followed by remixing in the miscible melt region between T_cl and T_m or T_g. The width of T_g has to be taken into account to understand partial vitrification effects during phase separation at high temperatures. Dynamic rheometry provides complementary information about partial vitrification and devitrification. Excess contributions due to demixing and remixing on the time-scale of the modulation appear in the heat capacity in both non-isothermal and quasi-isothermal conditions. This apparent heat capacity signal is used to study the effects of composition of the blend and molecular weight of the constituents on the demixing and remixing kinetics and on the interrelation between both. Partial vitrification during demixing is important in this respect. Step-wise quasi-isothermal measurements show a time-independent excess heat capacity in between T_cl and the interference of vitrification.     

Study of the kinetics of sewage sludge pyrolysis using DSC and TGA

Thermogravimetry and differential scanning calorimetry were used to examine, in nitrogen, four sewage sludges at modest heating rates. A mechanism consisting of two independent reactions was derived for an undigested sludge. The parameters are: n₁ = 10, E₁ = 130 kJ mol^?1, A₁ = 1 × 10¹⁸s^?1, n₂ = 15, E₂ = 250 kJ mol^?1, A₂ = 1 × 10²⁵s^?1, with initial weight fractions of W₁ = 0.20, W₂ = 0.43, the remainder being non-reactive ash. Analysis was performed by comparing Friedman multiple-heating-rate analyses of experimental and model curves in an iterative manner. DSC experiments provide an understanding of the pyrolytic reaction mechanism and heat transfer in the thermogravimetric analyser. DSC analysis of sewage sludge was sensitive to decomposition reactions of small quantities of organic salts in the sludge. Sewage sludge could be a profitable pyrolysis feedstock if mixed with municipal solid waste.     

A theory for the diffusion of large molecules in amorphous polymers above Tg

A general theory of diffusion of large molecules in rubbery amorphous polymers is of interest for the scientific understanding and with regard to material design and process optimization. A broadly applicable model would be useful in developing controlled transport of plasticizers and other additives through polymeric substances. A diffusion model is presented which has been developed for large molecular penetrants above the T_g of the amorphous polymer allowing for required increase in redistribution of the free volume of the polymer structure, as well as the penetrant size and shape. Applicability of the model is demonstrated by comparing theoretically developed diffusion curves for DNOP and DNDP in PVC vs. their weight fractions at 82°C and 91°C. These theoretically derived plots are compared with experimental D vs. w₁ curves for these systems generated at lower temperature.     

Theoretical evaluation of kp and λ in g.p.c.: A criterion to define ideal reference systems

From a thermodynamic point of view, theoretical expressions for the distribution coefficient of solute-gel interactions in g.p.c., k_p, and the preferential sorption coefficient, λ, have been developed. In the light of these expressions, a criterion to define an ideal reference system in which solutes are separated solely by steric exclusion, k_p = 1, has been reached. This criterion implies that g^°₁₂ + g^°₁₃ = 1 and λ = 0 (g^°_ij being the Flory-Huggins interaction parameters). From literature data of k_p, λ values have been calculated. These values vary with molecular weight as in the case of solvent(1)/solvent(2)/polymer(3) systems.     

Eutectoid WxC embedded WS2 nanosheets as a hybrid composite anode for lithium-ion batteries

A novel eutectoid structure, W_xC-embedded WS₂ nanosheets hybrids composite, was developed by hydrothermal reaction followed by a carbonization process. The fabricated WS₂–W_xC hybrid nanosheets electrode was used for lithium-ion batteries as an anode material, and demonstrated the specific capacity of 272 mA h·g^?1 at 0.01 A g^?1 with enhanced rate competence and cycling behavior when compared with individual WS₂ and W₂C electrode. While the large interlayer spacing in WS₂ facilitates rapid Li⁺ transport, the extremely high electronic conductivity of W_xC provides a highly conductive electron transfer pathway, which facilitates fast and reversible (de)lithiation reactions during charging and discharging. Further, these outcomes point the way for developing future eutectoid hybrid systems for advanced energy-storage applications.     

Producing critical exponents from gelation for various photoinitiator concentrations; a photo differential scanning calorimetric study

Photoinitiated radical polymerization of an 80 wt% epoxy diacrylate (EA) and 20 wt% tripropyleneglycoldiacrylate (TPGDA) mixture with various 2-Mercaptothioxanthone (TX-SH) photoinitiator concentrations was studied by using photo-differential scanning calorimetric (Photo-DSC) technique. Photopolymerization reactions were carried out under the same conditions of temperature and light intensity. It was observed that all conversion curves during gelation at various photoinitiator concentration present nice sigmoidal behavior which suggests application of the percolation model. The critical time, where polymerization reaches the maximum rate (Rp_max) is called the glass transition point (t_g). The gel fraction exponents, β were produced from the conversion curves around t_g. The observed critical exponents were found to be around 0.55, predicting that the gel system obeys the percolation model. Rp_max and final conversion (C_s) values were found to be increased as the photoinitiator concentration was increased. On the other hand t_g values decreased as photoinitiator concentration was increased, indicating higher TX-SH concentration causes early glass transition during radical polymerization.     

Determination of molecular weight and molecular sizes of polymers by high temperature gel permeation chromatography with a static and dynamic laser light scattering detector

Flow-mode static and dynamic laser light scattering (SLS/DLS) studies of polymers, including polystyrene, polyethylene, polypropylene and poly(dimethylsiloxane) (PDMS), in 1,2,4-trichlorobenzene (TCB) at 150 °C were performed on a high temperature gel permeation chromatography (GPC) coupled with a SLS/DLS detector. Both absolute molecular weight (M) and molecular sizes (radius of gyration, R_g and hydrodynamic radius, R_h) of polymers eluting from the GPC columns were obtained simultaneously. The conformation of different polymers in TCB at 150 °C were discussed according to the scaling relationships between R_g, R_h and M and the ρ-ratio (ρ=R_g/R_h). Flow-mode DLS results of PDMS were verified by batch-mode DLS study of the same sample. The presented technique was proved to be a convenient and quick method to study the shape and conformation of polymers in solution at high temperature. However, the flow-mode DLS was only applicable for high molecular weight polymers with a higher refractive index increment such as PDMS.     

Simultaneous determination of three Flory–Huggins interaction parameters in polymer/solvent/nonsolvent systems by viscosity and cloud point measurements

We propose an integral procedure to simultaneously obtain the three Flory–Huggins interaction parameters in polymer(3)/solvent(2)/nonsolvent(1) systems by carrying out only viscosity and cloud point measurements. First, the interaction parameter between polymer and solvent, χ₂₃, is obtained by applying the Rudin's model which has been well established for various polymer solutions. Then the solvent/nonsolvent interaction parameter χ₁₂ and polymer/nonsolvent interaction parameter χ₁₃ are calculated by combining the Rudin's model and the modified Flory–Huggins free energy for the ternary system, which leads to a group of candidate values of χ₁₂ and χ₁₃. Finally, these values are selected by the best agreement between the calculated binodal curves and the measured cloud points of the ternary system. This procedure has been successfully applied to PES/NMP/H₂O, PAN/DMSO/H₂O, and PAN/DMF/H₂O systems. All the values obtained are comparable to those reported previously. The procedure is simple and easy to follow, with no requirement of complex equipments.     

Studies on the miscibility and phase structure in blends of poly(epichlorohydrin) and poly(vinyl acetate)

The miscibility of atactic poly(epichlorohydrin) (aPECH) with poly(vinyl acetate) (PVAc) was examined under two different conditions: (i) in dilute solution, using vicometeric measurements and (ii) as cast films, using differential scanning calorimetric (DSC) and FT-infrared spectroscopy. Phase separation on heating, i.e. lower critical solution temperature (LCST) behavior of the aPECH/PVAc blends was examined by the measurement of transmitted light intensity against temperature. From viscosity measurements, the Krigbaum-Wall polymer-polymer interaction (ΔB) was evaluated. The DSC results show that the aPECH/PVAc blends are miscible as evidenced by the observation of a single composition-dependent glass-transition temperature (T_g) which is well described by the Couchman and Gordon Taylor models. The Flory-Huggins interaction parameter (χ₁₂) calculated from the T_g-method was negative and equal to −0.01, indicating a relatively low interaction strength. The FT-IR results match very well with those of DSC. The cloud point phenomenon is thermodynamically driven but phase separation, once taken place, is diffusion controlled in normal accessible time.