Concurrent Crystallization Research Articles

Crystallization Behaviors of Polypropylene/Polyamide-6 Blends Modified by a Maleated Thermoplastic Elastomer

In this paper, with the maleated thermoplastic elastomer (TPEg) as a compatibilizer of polypropylene (PP)/polyamide-6 (PA6) blends, the effects of interfacial compatibilization on the crystallization behaviors of the blends were investigated by differential scanning calorimetry (DSC). Compared with the binary blend, the addition of TPEg significantly affected the crystallization behaviors of the individual component in the blends, especially those of PA6. The presence of TPEg weakened the nucleating role of the PA6 on the PP matrix. The increasing concentration of the compatibilizer caused a decrease in crystallization temperature (Tc) and crystallization enthalpy (ΔHc) associated with PA6. When TPEg amount was added up to 24 wt %, the crystallization of PA6 at its bulk Tc was almost completely suppressed and the complete concurrent crystallization of PA6 and PP matrix took place. In the case of PP/PA6/TPEg blend containing 24 wt % TPEg, it was found that the disappearance of bulk PA6 crystallization peak was independent of the cooling rate and annealing time, but can be introduced by self-nucleation experiments. The above fractionated crystallization phenomenon was attributed to the reduction in PA6 particle size due to compatibilization role of added TPEg, which leads to the lack of the active heterogeneities. The crystalline morphologies under isothermal conditions were also observed by polarized optical microscopy (POM).

A differential scanning calorimetry study of the crystallization kinetics of tristearin‐tripalmitin mixtures

AbstractA comprehensive study of the isothermal crystallization kinetics of tripalmitin‐tristearin mixtures was carried out using DSC, with data fitted to the Avrami equation. Polymorphs were identified by subsequent melting of samples in the differential scanning calorimeter, with additional confirmatory information obtained from wide‐angle X‐ray diffraction. It was found that α‐, β′‐, and β‐forms require small (<1.0°C), moderate (3.5–8.5°C), and large (9.0–13.0°C) amounts of subcooling below their respective polymorph melting temperatures for nucleation to occur. Concurrent crystallization of β and β′ polymorphs was not observed. The β polymorphs exhibited sharper heat flow exotherms than β′, due to the higher crystallization driving forces experienced. Analysis of apparent induction times shows that the activation free energy of nucleation for the β‐form is significantly higher than for the β′‐form. Samples rich in either species crystallized faster (both shorter apparent induction times and sharper peaks) than samples with equivalent compositions. Driving‐force arguments do not fully explain this behavior, strongly suggesting that mass transfer resistances (greatest for equivalent compositions) have a significant effect on kinetics. Multiple crystallization events were observed for 50–80% tristearin samples between 56 and 60°C and were attributed to a demixing of tripalmitin‐rich and tristearin‐rich β phases, in line with established phase diagrams.

Sintered sanidine glass-ceramics from industrial wastes

Glass obtained from melting a mixture of industrial wastes (panel glass from dismantled cathode ray tubes, mining residues from feldspar excavation and lime from fume abatement systems of the glass industry) has been employed for the production of sanidine-based glass-ceramics. The glass-ceramics were developed by a sintering treatment with concurrent crystallization, from fine powders (<37 μm), at a relatively low temperature (880 °C). The enhanced nucleating activity of glass surfaces likely promoted the formation of sanidine, hardly found in glass-ceramics, as the main crystal phase. Due to the achieved mechanical properties (bending strength of about 120 MPa, Vickers’ microhardness exceeding 7 GPa) and aesthetic appearance, resulting from a compact and homogeneous microstructure, the obtained sanidine glass-ceramics may find applications as construction materials.

Sintered glass-ceramics from mixtures of wastes

Panel glass from dismantled cathode ray tubes, mining residues from feldspar excavation and lime from fume abatement systems of the glass industry have been employed as raw materials for several glass compositions. The prepared glasses were ground into fine powders and subjected to sintering treatments at low temperatures (880–930 °C), with concurrent crystallization, thus obtaining sintered glass-ceramics. The mechanical properties (for example, bending strength exceeding 100 MPa) and the aesthetic appearance of the materials, together with the simplicity of the manufacturing method, are promising for applications in the building industry.

Depletion regions in the aluminum-induced layer exchange process crystallizing amorphous Si

Annealing of aluminum/amorphous silicon bilayers below the eutectic temperature of the aluminum/silicon system leads to an exchange of the layer positions and a concurrent crystallization of silicon (aluminum-induced layer exchange). This letter discusses a model for the self-limited suppression of nucleation during the process. This characteristic feature is the reason why large grain sizes can be obtained. In our experiments, we combine nucleation caused by supersaturation and undercooling. Si depletion regions around existing grains are made visible. These experiments give direct proof of the idea that the suppression of nucleation occurs by Si depletion in the aluminum-induced layer exchange process.

Aluminum-induced crystallization of amorphous silicon: Influence of temperature profiles

Annealing of amorphous silicon/aluminum layer stacks below the eutectic temperature leads to a layer exchange and concurrent crystallization of silicon. The resulting polycrystalline silicon layers are of great interest for large area thin film devices. This paper reports about the influence of annealing temperature profiles on the layer exchange process. It is shown that temperature profiles have pronounced influence on the nucleation process. The obtained results are helpful for a better understanding of the physics of the layer exchange process and for an improvement of the layer exchange process by allowing for a reduction of the process time with a simultaneous increase in grain size. It is shown in particular that the characteristic suppression of nucleation is caused by Si-depletion around the growing Si-grains.

Suppression of nucleation during the aluminum-induced layer exchange process

AbstractFormation of polycrystalline silicon (poly-Si) thin films on inexpensive glass substrates is of great interest for large area electronic devices. Large grain sizes are desirable to reduce grain boundary effects. In the aluminum-induced layer exchange process Al/a-Si bi-layers exchange their positions with a concurrent crystallization of the amorphous Si (a-Si) in a simple annealing step. The process is characterized by the self regulated suppression of nucleation by existing grains resulting in large grain sizes above 10 μm. This paper elucidates the process within the Al Si phase diagram. The change in Si concentration within the Al is shown to cause the nucleation suppression.

The Lower Zone–Critical Zone Transition of the Bushveld Complex: a Quantitative Textural Study

The Lower Zone---Critical Zone boundary of the Bushveld Complex is an intrusion-wide, major stratigraphic transition from ultramafic harzburgite and pyroxenite in the Lower Zone to increasingly plagioclase-richpyroxenitesandnoritesintheCriticalZone.Quantitative textural and compositional data for 29 samples through this transition show the following: Lower Zone orthopyroxene grains are larger, have higher aspect ratios, are better foliated and have a lower trapped liquid component than those of the Critical Zone. The larger grain size of the Lower Zone results in crystal size distribution plots that are rotated to lower slopes and intercepts relative to those in the Critical Zone. Although all rocks show differing amountsoffoliation, mineral lineations are weak to absent. These data are consistent with significant compaction-driven recrystallization in the study section. Numerical modeling of concurrent compaction and crystallization provides a quantitative model of how the Lower Zone---Critical Zone transition may have formed: plagioclase is rare in the Lower Zone because compaction removes interstitial liquid before it reaches plagioclasesaturation.However,asthecrystalpilegrows,plagioclase saturation is reached in the interstitial liquid before compaction is complete in more evolved pyroxenites, producing more abundant but still modest amounts of plagioclase characteristic of the Lower Critical Zone. It is concluded that both the textures and the modal mineralogy are largely controlled by compaction and compactiondriven recrystallization; primary magmatic textures are not preserved.

A rheological study of the phase transition in thermoplastic polyurethanes. Critical gel behavior and microstructure development

The phase transition behavior of a commercial thermoplastic polyurethane (TPU) is investigated by rheological techniques. Oscillatory shear flow experiments are performed during both the isothermal annealing following a rapid cooling from the melt state, and the subsequent heating ramp (remelting). Rheological measurements reveal that the microphase separation between hard and soft segments and the concurrent crystallization of the hard phase domains produce a sol-to-gel type of transition. We show that the microstructure of TPUs at the critical gel point is strictly related to the applied thermal history. In particular, the critical gel properties during the cooling down transition are found to be quantitatively different from those measured during the reverse, heating up process. When coupled to differential scanning calorimetry measurements under the same thermal history, the rheological technique is shown to be a useful tool to determine the mechanisms of microstructure evolution.

Thermal and structural study on densification of rare-earth-doped PbO–SiO 2 aerogels

An attempt to prepare Ce- and La-doped SiO 2–PbO glasses for applications as scintillators was realised via aerogel route. The synthesis has been optimised in order to obtain large massive samples required for this application. The effect of Pb in the silica matrix as a co-dopant for rare-earths atoms is discussed: in particular, an increase of rare-earth solubility has been observed. Both 207 Pb and 29 Si magic angle spinning (MAS) NMR were used to investigate local order and connectivity. The densification of aerogels, investigated by means of DSC, TGA, and X-ray powder diffraction (XRPD), led to glass–ceramic samples because concurrent crystallisation occurred.

Melt mixing and crystallization under Theistareykir, northeast Iceland

Analysis of the compositions of crystals and melt inclusions from a suite of 40 gabbroic and wehrlitic nodules in a single eruptive body provides a record of concurrent mixing and crystallization of melts under NE Iceland. The crystals in the nodules have a similar range of compositions to those found as phenocrysts in the flow, and many of the nodules may have been generated by crystallization of a magma with a similar composition to that of the host flow. While plagioclase is only present in nodules where the average forsterite content of olivines is &lt;Fo88.5, clinopyroxene is found in nodules with average olivine forsterite contents of up to Fo91.6. The order of crystallization ol → ol + cpx → ol + cpx + plg is consistent with generation of the nodules at pressures &gt;0.8 GPa and is in agreement with estimates of crystallization pressures for the host basalt. The relationship between the compositional variability of melt inclusions and the forsterite content of the host olivine is revealed by REE analyses of over 120 melt inclusions. The degree of variability in REE concentrations and REE/Yb ratios decreases with falling forsterite content of the host olivine, as expected if melt mixing and fractional crystallization are operating together. The standard deviation of the REEs falls by a factor of ∼4 between Fo90 and Fo87. This change in olivine composition can be produced by crystallization of 20% which occurs on cooling of ∼50°C. The relative rates of mixing, cooling and crystallization may provide constraints upon the dynamics of magma bodies. The oxygen isotopic composition of olivines from the nodules and phenocrysts is highly variable (δ18O from 3.3–5.2 per mil) and shows little correlation with the forsterite content of the olivine. The full range of oxygen isotope variation is present in olivines with Fo89–90, and the low δ18O signal is associated with melts of high Mg# and La/Yb. Such geochemical relationships cannot be produced by assimilation of low Mg# crustal materials alone, and may reflect oxygen isotopic variation within the mantle source. The geochemistry of the melt inclusions and their host crystals can be accounted for by fractional melting of a mantle source with variable composition, followed by concurrent mixing and crystallization beneath the Moho.

Plasticizer effect on the melting and crystallization behavior of polyvinyl alcohol

The melting and crystallization behavior of polyvinyl alcohol (PVA) were examined as a function of plasticizer amount. The melting temperature ( T m) of PVA decreased with increasing the amount of glycerin. The effect of a plasticizer rapidly diminished when the phase separation of glycerin in PVA occurred. In addition, the crystallization peak temperature ( T c) of a fully hydrolysed PVA was reduced, and the maximum crystallization rate ( K max) was retarded, and the crystallite size distribution ( ω 1/2) was widened. However, the crystallization behavior ( T c, K max, ω 1/2) of a partially hydrolysed PVA could be disregarded compared with a fully hydrolysed PVA. The thermal history did not affect the crystallization behavior of a fully hydrolysed PVA, but largely affected that of a partially hydrolysed PVA. In a fully hydrolysed PVA / a partially hydrolysed PVA blend system, two T m peaks appeared, and the concurrent crystallization occurred.

Isothermal sintering with concurrent crystallization of polydispersed soda–lime–silica glass beads

The Clusters model, previously developed to describe the viscous sintering kinetics of polydispersed powders undergoing simultaneous surface crystallization, was tested in isothermal sintering experiments of devitrifying spherical glass particle compacts. The experiments were conducted at different temperatures to vary the relative kinetics of sintering and crystallization. The viscosity, glass–vapor surface energy, particle size distribution, average number of neighbors around each particle, particle to sample size ratio, and the effect of pre-existing surface crystals on the sintering kinetics were determined and taken into account in the calculations. The model describes the experimental sintering kinetics for a series of temperatures for two different size distributions and predicts the saturation of densification due to pre-existing surface crystals and surface crystallization. The calculations and experimental results confirmed that higher temperatures benefit sintering over crystallization.

Non-isothermal sintering with concurrent crystallization of polydispersed soda–lime–silica glass beads

We test the Clusters model for non-isothermal sintering of crystallizing polydispersed particles against experimental data obtained under different conditions. We demonstrate that the model predicts the densification curves for two distinct size distributions of devitrifying soda–lime–silica glass having spherical particles. For each system a minimum heating rate can be found, so that for higher rates the compacts always reach maximum density, while for lower heating rates crystallization inhibits densification. In the latter case, the residual porosity increases with decreasing heating rate. We discuss the results taking into account several complicating factors, such as pre-existing surface crystals, the average number of necks per particle, compositional shifts due to crystallization, temperature gradients and degassing during sintering. Finally, we discuss the physical and processing parameters that determine whether sintering will be favorable over crystallization.

Glass sintering with concurrent crystallization

We critically review and discuss the main glass-sintering models: Frenkel, Mackenzie–Shuttleworth, Scherer and the recently developed Clusters model, and focus on the problem of sintering with concurrent crystallization. The Clusters model is tested under various practical conditions. Isothermal tests are carried out on a widely polydispersed alumino-borosilicate (ABS) glass having jagged particles, which is stable against devitrification, and on a soda–lime–silica (SLS) glass with a narrow spherical particle distribution, which crystallizes easily. The algorithm for non-isothermal processes is also tested with two distinct systems: the same ABS glass and a narrow-sized cordierite glass, which is devitrification-prone. In addition to physical parameters such as viscosity, surface tension, particle-size distribution, crystal growth rate and number of nucleation sites, microscopic-particle-packing data are introduced into the model and it is demonstrated that the evolution of both density and pore size distribution can be reasonably predicted. All the results are discussed taking into account the assumptions made in the derivations and other complicating factors, such as irregular particle shape, compositional shifts due to crystallization, temperature gradients and degassing during sintering. Finally, we discuss the physical and processing parameters that determine whether sintering will be favorable over crystallization. We demonstrate that the Clusters model and related algorithm provide a powerful simulation tool to design the isothermal or non-isothermal densification of devitrifying or stable glass compacts with any particle-size distribution, thus minimizing the number of time-consuming laboratory experiments.

Practical enantioselective synthesis of endothelin antagonist S-1255 by dynamic resolution of 4-methoxychromene-3-carboxylic acid intermediate.

A practical multikilogram-scale synthesis of enantiomerically pure S-1255 (1), a potent and orally active ET(A) receptor antagonist, is described. Utilizing readily available starting materials and reagents, the entire sequence of reactions starting from 2,5-dihydroxyacetophenone 8 proceeded under mild conditions to give 1 in an excellent chemical yield (8 steps, 41% overall yield) and in a high enantiopurity (98% ee). The crucial step of the synthesis is a dynamic resolution of key intermediate 16. (R)-Methoxy acid (R)-16 having 97-99% ee was obtained in 83-84% yield from racemic 16 as a crystalline (1S,2R)-(+)-norephedrine or (+)-cinchonine salt by the dynamic resolution comprising concurrent crystallization and in situ racemization. A mechanism of the dynamic resolution through a ring-opened zwitterionic intermediate is discussed. In the final synthetic step, an effective carbon-carbon bond formation between the C4 carbon and the p-anisyl group was accomplished by a conjugate addition-elimination reaction of Grignard reagent 3 to (R)-16 to give 1 having 98% ee. Owing to high efficiencies of functional group transformations, carbon-carbon bond formations, and the dynamic resolution, the synthesis required no chromatographic purification and was amenable to a multikilogram-scale preparation. Several kilograms of 1 for clinical trials were successfully prepared by this process.

Plasticity and structural instability in a bulk metallic glass deformed in the supercooled liquid region

The deformation behavior of a bulk amorphous Zr–10Al–5Ti–17.9Cu–14.6Ni alloy was characterized in the supercooled liquid region. The alloy was observed to exhibit Newtonian behavior at low strain rates but to become non-Newtonian at high strain rates. Structures of the amorphous material, both before and after deformation, were examined using X-ray diffraction and high-resolution electron microscopy. Experimental results showed the presence of nanocrystallites in the deformed samples, suggesting that the non-Newtonian behavior was associated with the concurrent crystallization of the amorphous structure during deformation; that is, a mixed crystalline-plus-amorphous structure was being tested. A mechanistic model based upon structural evolution has been developed to interpret the observed non-Newtonian behavior.

Extended Plasticity in the Supercooled Liquid Region of Bulk Metallic Glasses

Bulk metallic glasses have good mechanical formability in viscous states. The good formability offers a great advantage of fabricating near-net-shape structural components. Whereas large tensile ductility has been observed in metallic glasses in the supercooled liquid region, the exact deformation mechanism, and particularly whether such alloys deform by Newtonian viscous flow (m = 1, where m is the strain rate sensitivity exponent) or not, remains a controversial issue. In this paper, existing data are analyzed and the apparent controversy is discussed. Results obtained from an amorphous alloy (composition: Zr-10Al-5Ti-17.9Cu-14.6Ni in at%) are presented. Dynamic structural evolution is characterized using electron microscopy and focus X-ray. It is demonstrated that, despite the fact that the deformation was carried out in the supercooled liquid range, concurrent crystallization of the amorphous structure occurred. As a result, a non-Newtonian behavior was observed.

Effect of P(lLA-co-?CL) on the compatibility and crystallization behavior of PCL/PLLA blends

This article describes the compatibility of two semicrystalline polymers, poly(e-caprolactone) (PCL) and poly(l-lactic acid) (PLLA). The compatibility of the PCL/PLLA blends was enhanced by the compatibilizing effect of the poly(l,l-lactide-co-e-caprolactone) [P(lLA-co-eCL)]. A discussion details the effect of the concentration of the compatibilizing agent, the copolymer of l,l-lactide and e-caprolactone of a 50/50 mol ratio [P(lLA-co-eCL)], on the compatibility and the crystallization behavior of the blends of PCL and PLLA. It was found that the addition of P(lLA-co-eCL) could suppress the crystallization of PLLA at its Tc and induced the concurrent crystallization of PLLA and PCL. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 226–231, 2000

Blending of immiscible polymers in a mixing zone of a twin screw extruder—effects of compatibilization

AbstractThe dependence of the morphology development of physical as well as of reactive compatibilized polypropylene/polyamide 6 (PP/PA6) blends in a mixing zone of a co‐rotating twin screw extruder on blend composition and screw rotational speed was investigated. A special process analytical set‐up based on a co‐rotating twin screw extruder was used, which allowed melt sampling from different positions along the operating extruder in time periods less than 10 seconds. It has been shown that the disperse particle sizes in physical blends depend crucially on the blend composition because of the increasing influence of coalescence with an increasing concentration of the disperse phase. Furthermore, the morphology of physical PP/PA6 blends depends strongly on their rheological properties. In contrast, the influence of the screw rotational speed on the morphology is minor. The resulting particle size in a mixing zone is achieved already after a short screw length. The particle size of compatibilized blends is significantly smaller than in physical blends because of the better conditions for drop break‐up and the suppression of coalescence effects. Due to this, compatibilization has a stronger influence on the blend morphology than a variation of process or rheological conditions with physical blends. Furthermore, the compatibilization leads to a concurrent crystallization of the PA6 phase with the PP phase.