Phase Separation In Melts Research Articles

Synergistic effect on TiO2 doped poly (vinyl alcohol-co-ethylene) nanofibrous film for filtration and photocatalytic degradation of methylene blue

Abstract Nanofiber based materials are common media for wastewater treatment but often limited with the efficiency and cost-effective in practical application. In this work, TiO2 nanoparticles were evenly immobilized in the poly (vinyl alcohol-co-ethylene) nanofibrous scaffold by a novel facile method based on the melt phase separation technique. Methylene blue (MB) was chosen as a model pollutant to evaluate the synergistic effect on the hybrid film. The experimental results showed that the hybrid film with embedded TiO2 nanoparticles exhibits highly removal efficiency of MB under UV irradiation, reaching to 97.3% within 150 min, and could be further enhanced by coupling with a dye-enriched filtration process. Meanwhile, this excellent photocatalytic activity allow the hybrid film to be regenerated rapidly and repeated used with no significant decrease in its filtration performance. Thus, this promising filtration capability, highly photocatalytic degradation activity and well repetitive use performance would make the hybrid film as an ideal candidate to effective removal of organic dyes from wastewater.

Message in a bottle: Spontaneous phase separation of hydrous Vesuvius melt even at low decompression rates

Violent explosive volcanic eruptions are destructive and threaten millions of people and infrastructure. Ejected ash, pumice and gases are end-products of volcanic factories sourced deep within magma chambers and conduits. The different production stages are not directly observable. However, experimental simulation reveals different stages of dynamic volcanic processes. The starting point of explosive volcanic eruptions is determined by the phase separation of an H2O fluid from a supersaturated hydrous silicate melt. The number of formed H2O fluid vesicles per unit volume of silicate melt (VND) is a basic property that controls the efficiency of fluid-melt separation, ascent velocity and finally explosive volcanism. We performed decompression experiments at superliquidus temperatures to simulate phase separation of a single phase hydrous silicate melt during ascent with AD79 Vesuvius white pumice composition using decompression rates of 0.024–1.7 MPa⋅s−1. The white pumice buried Herculaneum and Pompeii and is representative of other catastrophic phonolitic and trachytic explosive eruptions like the violent 39 ka Campi Flegrei and the 1815 AD Tambora eruption. Here we report a high log⁡VND of 5.2 (in mm−3) that is independent from decompression rate within the investigated range. Even at a decompression rate of 0.024 MPa⋅s−1 the formation of a high VND inevitably causes rapid degassing due to short H2O diffusion distances from the melt into fluid vesicles. A decompression rate meter based on nucleation theory, which is commonly used to estimate magma ascent velocity during volcanic eruptions using VND of volcanic ejecta, cannot be adapted to explain our experimentally determined decompression rate independent VND. Alternatively, decompression induced H2O–silicate melt phase separation may be described by diffusion controlled spinodal decomposition where maximum supersaturation is reached. This process occurs spontaneously and free of activation energy if hydrous melt is driven into thermodynamic instability where the second derivative of free energy of mixing to the H2O content is ≤0. However, the decompression rate independent VND has profound consequences for the dynamics of natural polyphase hydrous magma phase separation. Even at low ascent rates spontaneous hydrous melt phase separation facilitates rapid density decrease accompanied by sudden increase of magma buoyancy triggering explosive eruptions.

The evolution of immiscible silicate and fluoride melts: Implications for REE ore-genesis

The Mid-Proterozoic peralkaline Strange Lake pluton (Québec-Labrador, Canada) exhibits extreme enrichment in high field strength elements (HFSE), including the rare earth elements (REE), particularly in pegmatites. On the basis of a study of melt inclusions, we proposed recently that fluoride–silicate melt immiscibility played an important and perhaps dominant role in concentrating the REE within the pluton. Here we present further evidence for silicate–fluoride immiscibility at Strange Lake from a sample of the hypersolvus granite, which contains an inclusion composed largely of REE and HFSE minerals.The inclusion (∼5cm in diameter) comprises a narrow rim containing chevkinite-(Ce) and zircon in a fluorite matrix, a core of fluorbritholite-(Ce) and bastnäsite-(Ce) and a transition zone between the rim and the core consisting of a fine-grained intergrowth of bastnäsite-(Ce), gagarinite-(Y) and fluorite. We propose that the inclusion formed as a result of silicate–fluoride immiscibility, which occurred early in the emplacement history of the Strange Lake pluton, and that it represents the fluoride melt. After separation of the two melts, the boundary between them acted as a locus of crystallisation, where crystals formed repeatedly due to heterogeneous (surface catalysed) nucleation. Zircon crystallised shortly after melt phase separation, and was followed by the growth of perthite together with arfvedsonite and quartz. As a result, the silicate melt surrounding the fluoride inclusion became enriched in volatiles that facilitated crystallisation of progressively larger crystals in the inclusion; large crystals of arfvedsonite and perthite were succeeded by even larger crystals of quartz. Massive crystallisation of chevkinite-(Ce) followed, forming the rim of the inclusion. The fluoride melt, which constituted the matrix to the silicate minerals and chevkinite-(Ce), crystallised after chevkinite-(Ce), forming fluorbritholite-(Ce) and fluorite.Aqueous fluid exsolved from the silicate melt and altered the inclusion, replacing fluorbritholite-(Ce) with fluocerite-(Ce) and then bastnäsite-(Ce). This was followed by the formation of a fine-grained intergrowth of bastnäsite-(Ce), gagarinite-(Y) and fluorite at the expense of the earlier bastnäsite-(Ce). Chevkinite-(Ce) was not affected. Zircon, however, was replaced by anhydrous zirconosilicates and, in turn, by hydrous zirconosilicates.The inclusion represents the first macroscopic example of silicate–fluoride immiscibility in nature. We propose that globules of the fluoride melt were initially dispersed within the silicate melt and preserved only rarely in unaltered hypersolvus granite. They accumulated in the residual melt (and therefore in pegmatites) scavenging REE, Ca and F from the silicate melt and, with rare exceptions, were later destroyed by fluids. The latter process contributed significantly to the enrichment of the host rocks in REE, Ca and F.

Interplay between a Strong Memory Effect of Crystallization and Liquid–Liquid Phase Separation in Melts of Broadly Distributed Ethylene–1-Alkene Copolymers

Narrow metallocene-made random ethylene copolymers display a strong memory effect of crystallization above their equilibrium melting point akin to the melt memory effect observed in model ethylene–1-butene copolymers. The onset temperature for self-nucleation or surviving self-seeds displays a bell shape with increasing comonomer content with a maximum at ∼2 mol % branches. Self-seeds do not survive at temperatures above the equilibrium melting point for homopolymers and copolymers either with very low branching or with a branching content >4.5 mol %. The self-seeds are associated with clusters of ethylene sequences that remain in the melt in close proximity and accelerate a subsequent crystallization, as observed by higher crystallization peak temperatures and higher nucleation density. Contrasting this behavior, commercial ethylene–1-alkene copolymers with a broad, bimodal comonomer distribution display an inversion of the crystallization rate in a range of melt temperatures where narrow copolymers show...

Simulation of phase separation in melts of reacting multiblock copolymers

The dissipative particle dynamics method is employed to conduct the computer simulation of the phase separation occurring in a melt of a polymer containing monomer units of two types under the conditions of reversible polycondensation or interchain exchange. The chemical reactions are simulated via the Monte Carlo method. It is shown that growth in the Flory-Huggins parameter leads to macrophase separation, irrespective of the rate and type of the reactions and the spatial structure of an initial system. Relative changes in the probabilities of elementary reactions between units of different types shift the phase-transition point. Different levels of refinement are considered for description of the stationary states being developed. By the example of interchain exchange, it is shown that the structure of the polymer melt in the initial state can substantially affect the dynamics of the phase separation.

Simulation of phase separation in melts of regular and random multiblock copolymers

The dissipative particle dynamics method and parallel computing are employed for computer simulation of phase separation in melts of regular and random (Markovian) AB multiblock copolymers of symmetric compositions. For the first time, it is shown that, as the Flory-Huggins parameter χ is increased, lamellar microstructures are formed in all these systems. At preset block length M, the microstructures arise in regular copolymers at lower χ values than those in random copolymers, while the formed lamellas are narrower and have smaller amounts of defects. At χM > 100, a superstrong segregation regime develops in the regular copolymers. In random copolymer melts, bicontinuous structures are observed for a long time; in the long run, they are likewise transformed into lamellar structures whose imperfection substantially increases with M.

Preparation Principle and Crystal Growth Kinetics of Loaded Nanocrystal TiO<sub>2</sub>

Nanocrystal TiO2 photocatalystic materials were prepared by melt-phase separation technique, and its preparation principle was discussed in this paper. Anatase nanometer TiO2 could be obtained by heat-treatment at 550 °C for 10 min, which is well combined with porous glass carrier and the particles size is about 25 nm. The growth kinetics of TiO2 crystalline analysis results showed that the growth activation energy Q1 was about 63.27 KJ/mol (<600 °C), and Q2 was about 22.78 KJ/mol (>600 °C).TiO2 crystalline growth closely related to glass phase separation. TiO2 particles grew quickly with the glass phase separation size increase, and then the particles growth rate became slow because of being limited by glass phase separation structure.

Microphase separation in regular and random сopolymer melts by DPD simulations

Phase separation in melts of regular and random multiblock AB copolymers of equimolar composition and various mean block length was modeled using parallel dissipative particle dynamics simulations. Regular copolymers form well-shaped lamellas, strong and super-strong segregation regimes being identified. Microstructure formation in random copolymers with short blocks takes place at higher χ values and results in rough lamellas, the period of which is larger in comparison with the regular multiblocks. For random copolymers, increasing the mean block length deteriorates lamellar morphology and finally results in the formation of a bicontinuous structure. In a large simulation box, multidomain morphologies are observed.

Phase separation in melts of the ZrO2-Al2O3 system

The morphology of the quenched and slowly crystallized samples in the ZrO2-Al2O3 system is investigated in the composition range 25–70 wt % ZrO2. It is revealed that, irrespective of the cooling rate, the samples contain large baddeleyite (or corundum) crystals, eutectic mixtures, and characteristic regions of intergrown elongated baddeleyite and corundum grains with micron sizes. These regions have the same phase composition at any initial ratio between zirconium and aluminum oxides and at any cooling rates of the melt. A hypothesis is put forward that these regions are products of the decomposition of ZrO2 · 2Al2O3 associates.

Effects of Heat Treatment Temperature on the Photocatalytic Properties of TiO<sub>2</sub> Prepared by Melt-Phase Separation Process

The principle of melt-phase separation process for preparation of nanoparticle TiO2 is based on the adjustment of glass phase-separation structure. TiO2 grain size can be controlled by changing heat treatment temperature. The sample was composed of Na2O-B2O3-SiO2-TiO2 glass system (melted at 1573K). A photocatalytic nanoparticle TiO2 which was loaded on porous glass can be obtained by heat treatment of glass system at various temperatures and then acid washing. The results have shown that the glass phase-separation structure size matches crystallization rate and an optimal photodegradation property of TiO2 is achieved. It was found that the suitable heat treatment condition was at 848K for 12h, although its performance slightly changes with the composition variations.

Crystal Morphology Control by Melt Phase Separation in Biodegradable Polymer Blends

AbstractThe effect of lower critical solution temperature (LCST) phase separation on the crystallization of poly(ε-caprolactone) PCL in PCL/poly(D,L-lactide) (PDLA) blends is studied by simultaneous small-angle x-ray scattering (SAXS) and wide-angle x-ray scattering (WAXS). Phase separation is induced by controlled temperature jumps into the LCST (two-phase) region, which is above the melting temperature (60 °C) of PCL and the glass transition temperature (50 °C) of PDLA. We have followed the nanoscale structural changes (< 100 nm) during subsequent crystallization at 45 °C of critical (0.36 PCL) and off-critical (0.50 PCL) blend compositions in both one-phase and two-phase melts. The spherulite morphology (1-100 μm) is examined with optical microscopy. When crystallization follows LCST phase separation, the shape, size and distribution of the spherulites depends on the extent of melt phase separation. In our x-ray measurements, the WAXS crystallinity of PCL is less than 40 % for the temperature range of interest. We perform a correlation function and intensity model analysis of our SAXS data to obtain morphological variables that characterize the intraspherulitic morphology. These morphological variables are relatively constant during crystallization and are also independent of melt phase separation. On the other hand, the ultimate crystallinity and the crystallization rate depend on the extent of melt phase separation.

Macromorphology of polypropylene homopolymer tacticity mixtures

The macromorphology of isotactic/atactic (iPP/aPP) and isotactic/syndiotactic (iPP/sPP) polypropylene mixtures is examined by optical microscopy. The spherulitic macrostructure of equimolecular weight [weight-average molecular weight (Mw) = 200k] iPP/aPP blends is volume-filling to very high aPP concentrations when the crystallization temperature is 130 °C. Similar spherulitic macrostructures (spherulite size and volume-filling nature) are observed for iPP homopolymer and a 50/50 iPP/aPP blend at low crystallization temperatures (115–135 °C). At higher crystallization temperatures (140–145 °C), a equimolecular weight (Mw = 200k) 50/50 iPP/aPP blend exhibits nodular texture that blurs the spherulitic boundaries. Double temperature jump experiments show that the nodular texture is due to melt phase separation that develops prior to crystallization. The upper critical solution temperature (UCST) of a 50/50 iPP/aPP blend (Mw = 200k) lies below 155 °C, and the blend is miscible at conventional melt processing temperatures. The UCST behavior is controlled by the blend molecular weight and aPP microstructure. aPP microstructures containing increased isospecific sequencing (although still noncrystalline) exhibit a reduced tendency for phase separation in 50/50 mixtures (Mw = 200k) and the absence of nodular texture at low undercoolings (140–145 °C). Equimolecular weight (Mw = 200k) 50/50 iPP/sPP mixtures exhibit phase-separated texture at all crystallization temperatures. The size scale of the phase-separated texture decreases with decreasing crystallization temperature because of a competition between crystallization and phase separation from a melt initially well mixed from the initial solution blending process. Extended melt annealing experiments show that the 50/50 iPP/sPP mixture (Mw = 200k) is immiscible in the melt at conventional melt processing temperatures. The iPP/sPP pair shows a much stronger tendency for phase separation than the iPP/aPP polymer pair. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1947–1964, 2000

Liquid-Liquid Phase Separation of Melts and Glasses in Ferric Ferrous Oxide-Silica System

ABSTRACTThe existence of liquid-liquid miscibility gap in ferric ferrous oxide-silica system has been reported, however, the phase separation phenomena and the derived morphology of the phase separated glasses are uncertain. In this study, the melt-quenched samples of 5 Fe3O4-95 SiO2 and 15 Fe3O4-85 SiO2 (mol%) were prepared by melting at 2300°C or 2200°C (expected to be above miscibility gap), and subsequently at 1800°C or 1750°C (in immiscible region) by use of infrared image furnace and quenching at the rate of ≈102 K/sec. The glassy materials exhibited phase separation having discrete spherical particles or interconnected structure due to the composition, melting temperature and time. Also, the segregation of Fe component occurred during melting, which was caused by the difference of specific gravity of components in the melt.

Liquid-liquid phase separation in melts of blends of linear with branched polyethylenes: morphological exploration of the phase diagram

The liquid-liquid phase separation in blends of linear and branched polyethylenes has been observed using a morphological technique. The results confirm our previous conclusion that the phase-separated region of the phase diagram has a closed loop shape. We have also been able to demonstrate that the phase separation occurs at the same temperature on heating and cooling and hence argue that it represents an equilibrium phase boundary. The process of phase separation has been studied by observing changes in the shape and sizes of the phase-separated regions with time. In some cases, we argue that it is possible to distinguish between morphologies caused by liquid-liquid phase separation which occurred through nucleation and spinodal decomposition.