Showing papers on "Crystallization published in 2007"

Polymorphous Crystallization and Multiple Melting Behavior of Poly(l-lactide): Molecular Weight Dependence

Abstract: The effect of molecular weight (MW) on the polymorphous crystallization and melting behavior of poly(l-lactide) (PLLA) were systemically studied by differential scanning calorimetry (DSC), polarized optical microscopy (POM), wide-angle X-ray diffraction (WAXD), and time-resolved Fourier transform infrared (FTIR) spectroscopy. It was found that the polymorphism of PLLA is not influenced much by MW, and the α‘- and α-form crystals are produced at low and high crystallization temperature (Tc), respectively, regardless of the MW. However, MW significantly affects the crystallization kinetics, and the crystallization rate reduces greatly with MW increasing. Moreover, the Tc- and MW-dependent melting behavior of PLLA was clarified with combining the DSC and FTIR results. It was found that the α‘- to α-crystalline phase transition occurs prior to the dominant melting in both the low- and high-MW PLLA crystallized at low Tc. Unlike the high-MW PLLA, in low-MW PLLA crystallized at low Tc, the α‘-form crystals only...

Crystallization and Melting Behavior of Poly (l-lactic Acid)

Abstract: Effects of the crystallization temperature on the crystal structure and its melting behavior of poly (l-lactic acid) (PLLA) have been investigated by means of wide-angle (WAXS) and small-angle (SAXS) X-ray scattering, optical microscopy, and differential scanning calorimetory (DSC). PLLA was found to crystallize as the α form when the crystallization temperature Tc was higher than 120 °C, while significant change in lattice parameters was seen for Tc's below 120 °C. The ratio of the a- and b-axis lengths begins to decrease with Tc below 120 °C and is 31/2 below 90 °C, which suggests a new crystalline form with hexagonal packing, namely, the α‘ form. The possible reason for α‘ formation is discussed. High-temperature WAXS and SAXS measurements showed that α‘ crystal transforms into ordered a form during heating. The transition takes place at 150 °C without a decrease in scattering intensity and without heating rate dependence. The mechanism for the transition is discussed.

Kinetics of Monodisperse Iron Oxide Nanocrystal Formation by “Heating-Up” Process

Abstract: We studied the kinetics of the formation of iron oxide nanocrystals obtained from the solution-phase thermal decomposition of iron−oleate complex via the “heating-up” process. To obtain detailed information on the thermal decomposition process and the formation of iron oxide nanocrystals in the solution, we performed a thermogravimetric-mass spectrometric analysis (TG-MS) and in-situ magnetic measurements using SQUID. The TG-MS results showed that iron−oleate complex was decomposed at around 320 °C. The in-situ SQUID data revealed that the thermal decomposition of iron−oleate complex generates intermediate species, which seem to act as monomers for the iron oxide nanocrystals. Extensive studies on the nucleation and growth process using size exclusion chromatography, the crystallization yield data, and TEM showed that the sudden increase in the number concentration of the nanocrystals (burst of nucleation) is followed by the rapid narrowing of the size distribution (size focusing). We constructed a theore...

Morphology and Phase Segregation of Spin-Casted Films of Polyfluorene/PCBM Blends

Abstract: In this study the morphology of spin-casted films of polymers blended with [6,6]-phenyl C61-butyric acid methyl ester (PCBM) has been studied. It was found that the lateral structure formation in the films is favored by rapid solvent evaporation and strong polymer−PCBM repulsion. The formation of homogeneous films is favored by slow evaporation and weak polymer−PCBM repulsion. The effect of solvent evaporation rate is the opposite of what is found for spin-casting polymer−polymer blends. The results can be explained by the kinetics of phase separation and the phase behavior involving limited solubility and crystallization of PCBM.

Identification of active biomolecules in the high-yield synthesis of single-crystalline gold nanoplates in algal solutions.

Abstract: In this work, single-crystalline gold nanoplates were produced by treating an aqueous solution of chloroauric acid with the extract of the unicellular green alga Chlorella vulgaris at room temperature. The results suggest proteins as the primary biomolecules involved in providing the dual function of Au(III) reduction and the size- and shape-controlled synthesis of the nanogold crystals. A protein with a molecular weight of approximately 28 kDa was isolated and purified by reversed-phase HPLC; this protein tested positive for the reduction of chloroauric acid in aqueous solution. The isolated protein (named gold shape-directing protein, or GSP for convenience) was then used to produce gold nanoplates with distinctive triangular and hexagonal shapes in high yields (approximately 90 %). The kinetics of the reduction reaction could be manipulated through changes in the GSP concentration to produce plates with lateral sizes ranging from nanometers to micrometers. The growth of gold nanoplates in the GSP solution with time was monitored by microscopic and spectroscopic techniques, thereby allowing the detection of several key intermediates in the growth process.

Catalytic efficiency of iron(III) oxides in decomposition of hydrogen peroxide: competition between the surface area and crystallinity of nanoparticles.

Abstract: Various iron(III) oxide catalysts were prepared by controlled decomposition of a narrow layer (ca. 1 mm) of iron(II) oxalate dihydrate, FeC2O4·2H2O, in air at the minimum conversion temperature of 175 °C. This thermally induced solid-state process allows for simple synthesis of amorphous Fe2O3 nanoparticles and their controlled one-step crystallization to hematite (α-Fe2O3). Thus, nanopowders differing in surface area and particle crystallinity can be produced depending on the reaction time. The phase composition of iron(III) oxides was monitored by XRD and 57Fe Mossbauer spectroscopy including in-field measurements, providing information on the relative contents of amorphous and crystalline phases. The gradual changes in particle size and surface area accompanying crystallization were evaluated by HRTEM and BET analysis, respectively. The catalytic efficiency of the synthesized nanoparticles was tested by tracking the decomposition of hydrogen peroxide. The obtained kinetic data gave an unconventional no...

Effect of Molecular Weight on the Structure and Morphology of Oriented Thin Films of Regioregular Poly(3-hexylthiophene) Grown by Directional Epitaxial Solidification†

Abstract: Regioregular head-to-tail (HT)-coupled poly(3-hexylthiophene-2,5-diyl) (P3HT) with a weight-average molecular weight (Mw) in the 7.3–69.6 kDa range is crystallized by directional epitaxial solidification in 1,3,5-trichlorobenzene (TCB) to yield highly oriented thin films. An oriented and periodic lamellar structure consisting of crystalline lamellae separated by amorphous interlamellar zones is evidenced by atomic force microscopy (AFM) and transmission electron microscopy (TEM). Both the overall crystallinity as well as the orientation of the crystalline lamellae decrease significantly with increasing Mw. The total lamellar periodicity is close to the length of “fully extended” chains for Mw = 7.3 kDa (polystyrene-equivalent molecular weight, eq. PS) and it saturates to a value of ca. (25–28) ± 2 nm for Mw ≥ 18.8 kDa (eq. PS). This behavior is attributed to a transition from an oligomeric-like system, for which P3HT chains are essentially in a fully extended all-trans conformation and do not fold, to a semicrystalline system that involves a periodic alternation of crystalline lamellae separated by extended amorphous interlamellar zones, which harbor chain folds, chain ends, and tie molecules. For P3HT with Mw of ca. 7.3 kDa (eq. PS), epitaxial crystallization on TCB allows for the growth of both “edge-on” and “flat-on” oriented crystalline lamellae on the TCB substrate. The orientation of the lamellae is attributed to 1D epitaxy. Because of the large size of the “flat-on” crystalline lamellae, a characteristic single-crystal electron diffraction pattern corresponding to the [001] zone was obtained by selected area electron diffraction (SAED), indicating that P3HT crystallizes in a monoclinic unit cell with a = 16.0 A, b = 7.8 A, c = 7.8 A, and γ = 93.5°.

Effect of Solubility and Nucleating Duality of N,N'-Dicyclohexyl-2,6-naphthalenedicarboxamide on the Supermolecular Structure of Isotactic Polypropylene

Abstract: N,N‘-Dicyclohexyl-2,6-naphthalenedicarboxamide (NJS) is an efficient nucleating agent to prepare isotactic polypropylene (iPP) samples rich in the β-modification (β-iPP). However, NJS is not a selective β-nucleating agent, as the related samples always contain both α and β modification of iPP. Depending on the final temperature of heating (Tf), NJS may be partially or completely dissolved in the iPP melt. The dual nucleation ability of NJS was studied by polarized light microscopy (PLM) and calorimetry (DSC). It was established that the lateral surface of the needle crystals of NJS acts as α-nucleating agent. Because of the solubility and dual nucleating ability of NJS, a wide variety of supermolecular structures may form in its presence. The feature of the structure formed during the crystallization of iPP depends on the concentration of the nucleating agent, on the end temperature of heating, and on the thermal conditions during cooling and crystallization. In this study we observed an αβ-transcrystalli...

Suspensions of Silica Particles Grafted with Concentrated Polymer Brush: Effects of Graft Chain Length on Brush Layer Thickness and Colloidal Crystallization

Abstract: We previously reported that monodisperse silica particles (SiPs) afforded with a high-density brush of poly(methyl methacrylate) (PMMA) and suspended in a good solvent for PMMA formed a colloidal crystal in a certain concentration range (Macromolecules 2006, 39, 1245). Here we investigated similar hybrid particles with respect to the influence of graft chain length Lc on their hydrodynamic diameter Dh in dilute suspension and on colloidal crystallization in more concentrated suspension. The average radius r0 of SiPs was 65 nm, and the surface density σ0 of PMMA grafts at the SiP surface was about 0.7 chains/nm2 (about 36 000 chains per particle). The hydrodynamic thickness of the swollen brush layer h (=Dh/2 − r0) was qualitatively interpretable by a modified Daoud−Cotton-type scaling model. Namely, for short graft chains, h obeyed the universal relation, h[1 + (h/2r0)] ∼ Lcσ01/2, applicable to concentrated polymer brushes on flat as well as spherical surfaces, and for chains longer than a critical length...

Amorphous iron(III) oxide--a review.

Abstract: The syntheses of amorphous Fe(2)O(3) nanoparticles of varying size and morphology, their magnetic properties, crystallization mechanism, and applications are reviewed herein. The synthetic routes are classified according to the nature of the sample (powders, nanocomposites, films, coated particles). The contributions of various experimental techniques to the characterization of an amorphous Fe(2)O(3) phase are considered in this review, including some key experimental markers, allowing its distinction from nanocrystalline "X-ray amorphous" polymorphs (maghemite, hematite). We discuss the thermally induced crystallization mechanisms depending on transformation temperature, atmosphere, and the size of the amorphous particles that predetermine the structure of the primarily formed crystalline polymorph. The controversial description of the magnetic behavior, including an interpretation of the low-temperature and in-field Mossbauer spectra, is analyzed.

Amorphous state and delayed ice formation in sucrose solutions

Abstract: Summary Phase transitions of amorphous sucrose and sucrose solutions (20–100% sucrose) were studied using differential scanning calorimetry, and related to viscosity and delayed ice formation. Glass transition temperature (Tg) was decreased by increasing water content. Ice formation and concurrent freeze concentration of the unfrozen solution increased apparent Tg. Tg could be predicted weight fractions and Tg values of components. Williams–Landel-Ferry (WLF) relation could be used to characterize temperature dependence of viscosity above Tg. Crystallization of water above Tg was time dependent, and annealing of solutions with less than 80% sucrose at –35°C led to a maximally freeze-concentrated state with onset of glass transition at –46°C, and onset of ice melting at -34°C. The state diagram established with experimental and predicted Tg values is useful for characterization of thermal phenomena and physical state at various water contents.

Thermodynamics and Kinetics of Bulk Metallic Glass

Abstract: Bulk metallic glasses (BMGs) are multicomponent alloys with typically three to five components with large atomic size mismatch and a composition close to a deep eutectic. Packing in BMG liquids is very dense, with a low content of free volume resulting in viscosities that are several orders of magnitude higher than in pure metal melts. The dense packing accomplished by structural and chemical atomic ordering also brings the BMG-forming liquid energetically and entropically closer to its corresponding crystalline state. These factors lead to slow crystallization kinetics and consequentially to high glass-forming ability. This article highlights the thermodynamic and kinetic properties of BMGs and their contributions to extraordinarily high glass-forming ability. Some possible links with mechanical properties are also suggested.

Continuous structural evolution of calcium carbonate particles: a unifying model of copolymer-mediated crystallization

Abstract: Two double-hydrophilic block copolymers, each comprising a nonionic block and an anionic block comprising pendent aromatic sulfonate groups, were used as additives to modify the crystallization of CaCO3. Marked morphological changes in the CaCO3 particles were observed depending on the reaction conditions used. A poly(ethylene oxide)-b-poly(sodium 4-styrenesulfonate) diblock copolymer was particularly versatile in effecting a morphological change in calcite particles, and a continuous structural transition in the product particles from polycrystalline to mesocrystal to single crystal was observed with variation in the calcium concentration. The existence of this structural sequence provides unique insight into the mechanism of polymer-mediated crystallization. We propose that it reflects continuity in the crystallization mechanism itself, spanning the limits from nonoriented aggregation of nanoparticles to classical ion-by-ion growth. The various pathways to polycrystalline, mesocrystal, and single-crysta...

Transition of Ionic Liquid [bmim][PF6] from Liquid to High-Melting-Point Crystal When Confined in Multiwalled Carbon Nanotubes

Abstract: In this communication, we report our finding on the transition of ionic liquid [bmim][PF6] from the liquid state to high-melting-point crystal when confined in multiwalled carbon nanotubes. This novel crystallization behavior is explained by the nanosized effect of carbon nanotubes on the structure of ionic liquid.

Magma Oceans and Primordial Mantle Differentiation

Abstract: Crystallization and chemical differentiation of the early Earth were governed by a combination of various processes Within the uncertainties of physical parameters, the two end-member models, equilibrium crystallization and fractional crystallization, are both possible In terms of crystal size, the boundary between these models is ~ 1 mm Analysis of nucleation and crystal growth in the convecting magma ocean suggests that the crystals in the magma ocean are approximately this size The equilibrium model is preferred because it seems to satisfy better the geochemical constraints According to this model, at the early stages, crystallization proceeds from bottom up without any substantial crystal–melt segregation In a vigorously convecting magma ocean, the thermal profile is approximately adiabatic and the crystal fraction decreases gradually with depth The basic physical reason for equilibrium crystallization is a fast convective cooling during which the solid and the liquid phases had a very limited time window for crystal–melt segregation, about 1000 years Chemical differentiation due to crystal–melt segregation is more significant at low pressures corresponding – with large uncertainties – to the upper mantle It is caused by several factors When the temperature drops below liquidus everywhere, the nucleation–growth–dissolution cycle of crystals changes to continuous crystal growth This increases the crystal size to ~ 1 cm indicating the beginning of crystal–melt segregation When the crystal fraction increases to about 60% near the surface, the convective heat transport is controlled by solid-state creep, which is many orders of magnitude slower Crystal–melt segregation occurs via melt percolation, which is consistent with the geochemical constraints Crystallization of the remaining partially molten layers takes about 10 7 –10 9 years This stage merges with the subsequent planetary evolution controlled by mantle convection and radiogenic heating The long lifetime of the shallow magma ocean suggests that this might be the place where liquid iron equilibrates with the mantle before it sinks down to the core

Solubility-driven thin film structures of regioregular poly(3-hexyl thiophene) using volatile solvents

Abstract: Using atomic force microscopy and grazing-incidence x-ray diffraction, the authors found that the use of volatile CH2Cl2 not only offers an advantage in minimizing regioregular poly(3-hexyl thiophene) (RR P3HT) film deposition time, but also directs the desirable parallel orientation of π-π stacking planes of RR P3HT with respect to solid substrates, for both spin and drop castings. The substrate temperature effects have been investigated to support claims that the substrate-induced crystallization of RR P3HT preseeds the parallel oriented crystals prior to spin casting, when the warm CH2Cl2 solution is loaded on a “cold” substrate held at room temperature.