Showing papers on "Crystallization published in 2009"

Nucleation of crystals from solution: classical and two-step models.

Abstract: Crystallization is vital to many processes occurring in nature and in the chemical, pharmaceutical, and food industries. Notably, crystallization is an attractive isolation step for manufacturing because this single process combines both particle formation and purification. Almost all of the products based on fine chemicals, such as dyes, explosives, and photographic materials, require crystallization in their manufacture, and more than 90% of all pharmaceutical products contain bioactive drug substances and excipients in the crystalline solid state. Hence control over the crystallization process allows manufacturers to obtain products with desired and reproducible properties. We judge the quality of a crystalline product based on four main properties: size, purity, morphology, and crystal structure. The pharmaceutical industry in particular requires production of the desired crystal form (polymorph) to assure the bioavailability and stability of the drug substance. In solution crystallization, nucleation...

Estimation of Drug–Polymer Miscibility and Solubility in Amorphous Solid Dispersions Using Experimentally Determined Interaction Parameters

Abstract: The amorphous form of a drug may provide enhanced solubility, dissolution rate, and bioavailability but will also potentially crystallize over time. Miscible polymeric additives provide a means to increase physical stability. Understanding the miscibility of drug–polymer systems is of interest to optimize the formulation of such systems. The purpose of this work was to develop experimental models which allow for more quantitative estimates of the thermodynamics of mixing amorphous drugs with glassy polymers. The thermodynamics of mixing several amorphous drugs with amorphous polymers was estimated by coupling solution theory with experimental data. The entropy of mixing was estimated using Flory–Huggins lattice theory. The enthalpy of mixing and any deviations from the entropy as predicted by Flory–Huggins lattice theory were estimated using two separate experimental techniques; (1) melting point depression of the crystalline drug in the presence of the amorphous polymer was measured using differential scanning calorimetry and (2) determination of the solubility of the drug in 1-ethyl-2-pyrrolidone. The estimated activity coefficient was used to calculate the free energy of mixing of the drugs in the polymers and the corresponding solubility. Mixtures previously reported as miscible showed various degrees of melting point depression while systems reported as immiscible or partially miscible showed little or no melting point depression. The solubility of several compounds in 1-ethyl-2-pyrrolidone predicts that most drugs have a rather low solubility in poly(vinylpyrrolidone). Miscibility of various drugs with polymers can be explored by coupling solution theories with experimental data. These approximations provide insight into the physical stability of drug–polymer mixtures and the thermodynamic driving force for crystallization.

Phase diagram of P3HT/PCBM blends and its implication for the stability of morphology.

Abstract: In this work, the phase diagram of poly(3-hexyl thiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) blends is measured by means of standard and modulated temperature differential scanning calorimetry. Blends were made by solvent-casting from chlorobenzene, as blends cast from toluene or 1,2-dichlorobenzene prove to retain effects of phase segregation during casting, hindering the determination of the phase diagram. The film morphology of P3HT/PCBM blends cast from chlorobenzene results from a dual crystallization behavior, in which the crystallization of each component is hindered by the other component. A single glass transition is observed for all compositions. The glass transition temperature (Tg) increases with increasing concentration of PCBM: from 12.1 degrees C for pure P3HT to 131.2 degrees C for pure PCBM. The observed Tg defines the operating window for the thermal annealing and explains the long-term instability of both the morphology and the photovoltaic performance of the P3HT/PCBM solar cells.

Confined Crystallization of Polyethylene Oxide in Nanolayer Assemblies

Abstract: The design and fabrication of ultrathin polymer layers are of increasing importance because of the rapid development of nanoscience and nanotechnology. Confined, two-dimensional crystallization of polymers presents challenges and opportunities due to the long-chain, covalently bonded nature of the macromolecule. Using an innovative layer-multiplying coextrusion process to obtain assemblies with thousands of polymer nanolayers, we discovered a morphology that emerges as confined polyethylene oxide (PEO) layers are made progressively thinner. When the thickness is confined to 20 nanometers, the PEO crystallizes as single, high-aspect-ratio lamellae that resemble single crystals. Unexpectedly, the crystallization habit imparts two orders of magnitude reduction in the gas permeability.

Time‐Dependent Morphology Evolution by Annealing Processes on Polymer:Fullerene Blend Solar Cells

Abstract: Changes in the nanoscale morphologies of the blend films of poly (3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), for high-performance bulk-heterojunction (BHJ) solar cells, are compared and investigated for two annealing treatments with different morphology evolution time scales, having special consideration for the diffusion and aggregation of PCBM molecules. An annealing condition with relatively fast diffusion and aggregation of the PCBM molecules during P3HT crystallization results in poor BHJ morphology because of prevention of the formation of the more elongated P3HT crystals. However, an annealing condition, accelerating PCBM diffusion after the formation of a well-ordered morphology, results in a relatively stable morphology with less destruction of crystalline P3HT. Based on these results, an effective strategy for determining an optimized annealing treatment is suggested that considers the effect of relative kinetics on the crystallization of the components for a blend film with a new BHJ materials pair, upon which BHJ solar cells are based.

Effect of surfactant surface coverage on formation of solid lipid nanoparticles (SLN).

Abstract: The effect of surfactant surface coverage on formation and stability of Tween 20 stabilized tripalmitin solid lipid nanoparticles (SLN) was investigated. A lipid phase (10% w/w tripalmitin) and an aqueous phase (2% w/w Tween 20, 10 mM phosphate buffer, pH 7) were heated to 75 degrees C and then homogenized using a microfluidizer. The resulting oil-in-water emulsion was kept at a temperature (37 degrees C) above the crystallization temperature of the tripalmitin to prevent solidification of emulsion droplets, and additional surfactant at various concentrations (0-5% w/w Tween 20) was added. Droplets were then cooled to 5 degrees C to initiate crystallization and stored at 20 degrees C for 24 h. Particle size and/or aggregation were examined visually and by light scattering, and crystallization behavior was examined by differential scanning calorimetry (DSC). Excess Tween 20 concentration remaining in the aqueous phase was measured by surface tensiometry. Emulsion droplets after homogenization had a mean particle diameter of 134.1+/-2.0 nm and a polydispersity index of 0.08+/-0.01. After cooling to 5 degrees C at low Tween 20 concentrations, SLN dispersions rapidly gelled due to aggregation of particles driven by hydrophobic attraction between insufficiently covered lipid crystal surfaces. Upon addition of 1-5% w/w Tween 20, SLN dispersions became increasingly stable. At low added Tween 20 concentration ( 1% w/w). The Tween 20 concentration in the aqueous phase decreased after tripalmitin crystallization suggesting additional surfactant adsorption onto solid surfaces. At higher Tween 20 concentrations, SLN had increasingly complex crystal structures as evidenced by the appearance of additional thermal transition peaks in the DSC. The results suggest that surfactant coverage at the interface may influence crystal structure and stability of solid lipid nanoparticles via surface-mediated crystal growth.

High gas barrier from confined crystallization of polyethylene oxide in nanolayer assemblies

Abstract: The design and fabrication of ultrathin polymer layers are of increasing importance because of the rapid development of nanoscience and nanotechnology. Confined, two-dimensional crystallization of polymers presents challenges and opportunities due to the long-chain, covalently bonded nature of the macromolecule. Using an innovative layer-multiplying coextrusion process to obtain assemblies with thousands of polymer nanolayers, we discovered a morphology that emerges as confined polyethylene oxide (PEO) layers are made progressively thinner. When the thickness is confined to 20 nanometers, the PEO crystallizes as single, high-aspect-ratio lamellae that resemble single crystals. Unexpectedly, the crystallization habit imparts two orders of magnitude reduction in the gas permeability.

Effect of LiClO4 on the Structure and Mobility of PEO-Based Solid Polymer Electrolytes

Abstract: The relationship between structure, PEO mobility, and ionic conductivity is investigated for the solid polymer electrolyte, PEO/LiClO4. Amorphous and semicrystalline samples with ether-oxygen-to-lithium ratios ranging from 4:1 to 100:1 are measured. Previous X-ray diffraction results show that three crystalline phases can form in this system depending on the LiClO4 concentration: (PEO)3:LiClO4, pure PEO, and (PEO)6:LiClO4. We use SANS to determine that the (PEO)3:LiClO4 phase forms cylinders with a radius of 125 A and a length of 700 A. We also measure the amount and size of pure PEO lamellae by exploiting the neutron scattering length density contrast that arises because of crystallization. The samples are thermally treated such that the (PEO)6:LiClO4 phase does not form. QENS is used to measure PEO mobility directly in amorphous and semicrystalline samples, and it reveals two processes. The first process at short times is attributed to the segmental mobility of PEO, and the second process at longer time...

The multiple roles of additives in CaCO3 crystallization : a quantitative case study

Abstract: In the classical picture of crystallization, nucleation is considered to take place in a solution ofions or molecules exceeding a critical supersaturation, leading to the nucleation of the new phaseYJ The growth of nucleated particles and crystals is then considered to take place via addition of single ions or molecules. The role of additives, which modify crystal growth, is restricted in such a view: they can either bind ions or interact with the crystal. Structural complexity found in biominerals,f2-4J and also many synthetic crystallization patterns found in the presence of additives,fSi raised serious doubts on such a simplified view, and meanwhile, a series of novel intermediates and mechanisms have been postulated to occur.[S-7J The role of additives is, however, still hard to attribute, and empirical control of morphology is the rule, not the exception. In our opinion, this is caused by the multiple roles of additives in such processes, which in addition depends on concentrations and other experimental conditions. A possible quantification, at least a classification of all the different interactions, is eagerly needed, but simple tools to do so are not currently known. This paper aims at opening the pathway to such system­ atization, here exemplified for calcium carbonate as a model system of complex crystallization. The choice is based on relevance: calcium carbonate is not only of great industrial importance, the major source of water hardness, and the most abundant biomineral, but also one of the most frequently studied minerals, with great scientific relevance in biomineralizatiol1 and geosciences. Scale formation (incrustation) is also a substantial issue in daily life, industry, and technology, rendering the addition of scale inhibitors to laundry detergents, household cleaners, and also in many industrial applications unavoidable. Only little is known about the early stages of CaCO 3 crystal­ lization, though this mineral has been studied for more than a century now. Identified precursor phases are amorphous calcium carbonate (ACC) in bio J8J and biomimetic mineralization i9 ! and liquid precursors (PILP), which have been found in some cases.l 6 ] Directly after ion contact forming precursor species -that is, before nucleation of the novel phase occurs- were postulated,Po,l1 J and could be revealed.f 12 ,uJ Up to then, Ace was the only known species, which occurs after nucleationP4! It will be shown that a simple titration of Ca 2 + ions under otherwise carefully moderated conditions is enough to "fingerprinf' most of the possible interactions and influences of additives and crystallization conditions. Using different features of the titration curve, it is

Seed-Induced Crystallization of Nanosized Na-ZSM-5 Crystals

Abstract: ZSM-5 nanocrystals were synthesized from organic-template-free gel systems containing 0.1, 1.0, and 3.0 wt % of 80 nm silicalite-1 seeds. The syntheses were performed at 100, 120, 150, and 170 °C for periods of time ranging between 3 and 190 h. Physicochemical characteristics derived from XRD, NMR, TG/DTA, SEM/TEM, DLS, N2 adsorption and chemical analyses of Na-ZSM-5 nanocrystals were compared with those of the nanosized tetrapropylammonium (TPA)-promoted counterpart. Crystalline yield and colloidal stability of ZSM-5 nanosized materials obtained under different conditions (seed content, crystallization temperature, and time) were also studied. Another issue of interest in the framework of this study was the effect of seed pretreatment (drying, calcination) on the characteristics of crystalline product. Thus, factors controlling seeded growth of nanosized zeolite particles were established and the mechanism of formation, including parameters governing the formation of single or aggregated crystals, revealed.

Experimental evidence for two-step nucleation in colloidal crystallization.

Abstract: We investigated the freezing of colloidal spheres in two dimensions with single-particle resolution. Using micron-size, charge-stabilized polystyrene spheres with a temperature-dependent depletion attraction induced by surfactant micelles, we supercooled an initially amorphous (gaslike) system. Particle motions were monitored as crystallization proceeded. At low concentrations, freezing occurred in a single step in a manner consistent with classical nucleation theory. In other samples two-step nucleation was found, in which amorphous clusters grew to $\ensuremath{\approx}30$ particles, then rapidly crystallized. Measured free energies show the role of metastable gas-liquid coexistence, which also enhanced the rate of nucleation following deeper quenches.

Hard spheres: crystallization and glass formation

Abstract: Motivated by old experiments on colloidal suspensions, we report molecular dynamics simulations of assemblies of hard spheres, addressing crystallization and glass formation. The simulations cover wide ranges of polydispersity s (standard deviation of the particle size distribution divided by its mean) and particle concentration. No crystallization is observed for s>0.07. For 0.02

Cloning polymer single crystals through self-seeding.

Abstract: In general, when a crystal is molten, all molecules forget about their mutual correlations and long-range order is lost. Thus, a regrown crystal does not inherit any features from an initially present crystal. Such is true for materials exhibiting a well-defined melting point. However, polymer crystallites have a wide range of melting temperatures, enabling paradoxical phenomena such as the coexistence of melting and crystallization. Here, we report a self-seeding technique that enables the generation of arrays of orientation-correlated polymer crystals of uniform size and shape ('clones') with their orientation inherited from an initial single crystal. Moreover, the number density and locations of these cloned crystals can to some extent be predetermined through the thermal history of the starting crystal. We attribute this unique behaviour of polymers to the coexistence of variable fold lengths in metastable crystalline lamellae, typical for ordering of complex chain-like molecules.

Phase behavior of poly(vinylpyrrolidone) containing amorphous solid dispersions in the presence of moisture.

Abstract: The objective of this study was to investigate the phase behavior of amorphous solid dispersions composed of a hydrophobic drug and a hydrophilic polymer following exposure to elevated relative humidity. Infrared (IR) spectroscopy, differential scanning calorimetry (DSC) and moisture sorption analysis were performed on five model systems (nifedipine-poly(vinylpyrrolidone) (PVP), indomethacin-PVP, ketoprofen-PVP, droperidol-PVP, and pimozide-PVP) immediately after production of the amorphous solid dispersions and following storage at room temperature and elevated relative humidity. Complete miscibility between the drug and the polymer immediately after solid dispersion formation was confirmed by the presence of specific drug-polymer interactions and a single glass transition (T(g)) event. Following storage at elevated relative humidity (75-94% RH), nifedipine-PVP, droperidol-PVP, and pimozide-PVP dispersions formed drug-rich and polymer-rich amorphous phases prior to crystallization of the drug, while indomethacin-PVP and ketoprofen-PVP dispersions did not. Drug crystallization in systems exhibiting amorphous-amorphous phase separation initiated earlier ( or=46 days at 94% RH). Evidence of moisture-induced amorphous-amorphous phase separation was observed following storage at as low as 54% RH for the pimozide-PVP system. It was concluded that, when an amorphous molecular level solid dispersion containing a hydrophobic drug and hydrophilic polymer is subjected to moisture, drug crystallization can occur via one of two routes: crystallization from the plasticized one-phase solid dispersion, or crystallization from a plasticized drug-rich amorphous phase in a two-phase solid dispersion. In the former case, the polymer is still present in the same phase as the drug, and can inhibit crystallization to a greater extent than the latter scenario, where the polymer concentration in the drug phase is reduced as a result of the amorphous-amorphous phase separation. The strength of drug-polymer interactions appears to be important in influencing the phase behavior.

Structure versus properties relationship of poly(lactic acid). I. Effect of crystallinity on barrier properties

Abstract: Poly(lactic acid) is at present the most promising and commercially available bio-based and biocompostable (bio)plastic. The properties of PLA, however, are rather poor, notably its low glass transition temperature Tg of ∼55 °C, its intrinsic low crystallization rate and the limited barrier properties with respect to water, oxygen and carbon dioxide in comparison with to oil-based counterparts, notably polyesters (PET). In this manuscript we address the question of what could ultimately be achieved in terms of barrier properties when PLA is crystallized fully. In fact, this question is rather academic in view of the long crystallization/annealing times needed to obtain maximum crystallinity but serves the purpose of better understanding current limits and notably a better understanding of the role of crystallinity on the barrier properties. A correlation is made between crystallization, various morphological parameters and the barrier properties of PLA. Crystallization of PLA causes a decrease of the oxygen permeability, but not in linear proportion with the decrease in amorphous volume. We explain this in terms of influence of the space filling and the inner crystalline structure on gas transport properties. Furthermore we suggest that the presence of a rigid amorphous fraction with lower density can have strong impact on PLA gas permeability, in particular on the gas solubility coefficient of PLA. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2247–2258, 2009

Intrinsic dipole-field-driven mesoscale crystallization of core-shell ZnO mesocrystal microspheres.

Abstract: Novel uniform-sized, core−shell ZnO mesocrystal microspheres have been synthesized on a large scale using a facile one-pot hydrothermal method in the presence of the water-soluble polymer poly(sodium 4-styrenesulfonate). The mesocrystal forms via a nonclassical crystallization process. The intrinsic dipole field introduced by the nanoplatelets as a result of selective adsorption of the polyelectrolyte on some polar surfaces of the nanoparticles acts as the driving force. In addition, it plays an important role throughout the mesoscale assembly process from the creation of the bimesocrystalline core to the apple-like structure and finally the microsphere. Our calculation based on a dipole model confirms the dipole-field-driven mechanism forming the apple-like structure.

Unusual Tuning of Mechanical Properties of Isotactic Polypropylene Using Counteraction of Shear Flow and β-Nucleating Agent on β-form Nucleation

Abstract: Isotactic polypropylene (iPP) has good comprehensive properties, viz. easy processing, high heat resistance, and good stiffness, etc., and in turn is widely used as a commodity plastic. However, under conventional processing conditions, iPP crystallizes into sizable spherulites with few tie molecules between spherulites. With such a crystalline texture, iPP exhibits very low impact toughness, especially at lower temperature, which restricts its more extensive application. Therefore, toughening of iPP has always been an open research. Up to now, four routes have been taken to improve impact strength of iPP, including copolymerizing with other olefin monomers, blending with rubber or thermoplastic elastomer, compounding with organic or inorganic fillers (e.g., nanoparticles), and adding β-nucleating agent. Without doubt, the modified iPP, more or less, increases its toughness. The enhancement of toughness is, unfortunately, at sacrifice of other properties, e.g., strength, heat resistance, etc. There are extremely few successful examples for simultaneously efficiently reinforcing and toughening iPP. It has been well established that for iPP oriented crystals (i.e., shish-kebabs) can bring out notable reinforcement on iPP, while β-form crystals of iPP can greatly increase its toughness. Flow (shear, elongational, or mixed) would induce formation of shish-kebabs whose content is governed to shear rate, shear duration, and molecular species and weight, etc. On the other hand, β-form crystals can be high production generated by addition of β-nucleating agent under quiescent conditions. This type of iPP crystals causes high toughness due to the β-R polymorphous transition and the loose structure of β-form crystals compared with R-crystals in favor of absorbing impact energy. Naturally, an idea arises that combination of flow-induced molecular orientation crystallization and β-nucleant-induced β-form crystals produces efficient reinforcement and toughening on iPP. However, as a matter of fact, there is not any example of it in the open literature. Themajor reason is that the coexistence of a β-nucleating agent and a shear flow at above a certain but low level of shear rate depresses β-form nucleation. Huo et al. performed a delicate study on iPP crystallization with β-nucleating agent and observed that as the shear rate rises, the content of β crystals constantly reduces, more greatly for higher nucleating agent content, showing counteraction between shear and β-nucleating agent for β-form formation. Apparently, it is a great practical challenge to prepare iPP parts withhighmolecular orientation and highβ-crystal proportion for the purpose of both strength and toughness enhancement. In the present work, we attempt to fabricate iPP with considerably increased strength and toughness, for the first time, utilizing the counteraction of shear-induced orientation crystallization and β-nucleating agent on β-form nucleation. The crystallization process of iPP is manipulated by two stages: one is flow-induced crystallization for high molecular orientation, and the other is β-nucleating-agent-induced nucleation for highβ-crystal content. More specifically, the oscillation shear supplied by an oscillation shear injectionmolding (OSIM) (its detailed definition is included in the Experimental Section) is imposed on β-nucleated iPP melt in the mold during injection molding. Initially, due to occurrence of shear flow and β-nucleating agent, β-form formation is restrained; thereby, the oriented R-crystals first form in the skin and intermediate layers of the sample.When the gate of the mold freezes, the shear ceases, and hereby, β-nucleating agent revives. The remainder of iPP melt in the core layer undergoes β-nucleation-induced crystallization. The results demonstrate that a large amount of shish-kebab structure appears in the surface and intermediate layers of the sample; at the same time, numerous β-form crystals form in the core layer, thus leading to prominent reinforcement and toughening for iPP.

Solubility of Small-Molecule Crystals in Polymers: d-Mannitol in PVP, Indomethacin in PVP/VA, and Nifedipine in PVP/VA

Abstract: Amorphous pharmaceuticals, a viable approach to enhancing bioavailability, must be stable against crystallization. An amorphous drug can be stabilized by dispersing it in a polymer matrix. To implement this approach, it is desirable to know the drug’s solubility in the chosen polymer, which defines the maximal drug loading without risk of crystallization. Measuring the solubility of a crystalline drug in a polymer is difficult because the high viscosity of polymers makes achieving solubility equilibrium difficult. Differential Scanning Calorimetry (DSC) was used to detect dissolution endpoints of solute/polymer mixtures prepared by cryomilling. This method was validated against other solubility-indicating methods. The solubilities of several small-molecule crystals in polymers were measured for the first time near the glass transition temperature, including d-mannitol (β polymorph) in PVP, indomethacin (γ polymorph) in PVP/VA, and nifedipine (α polymorph) in PVP/VA. A DSC method was developed for measuring the solubility of crystalline drugs in polymers. Cryomilling the components prior to DSC analysis improved the uniformity of the mixtures and facilitated the determination of dissolution endpoints. This method has the potential of providing useful data for designing physically stable formulations of amorphous drugs.

Layered metal phosphonate reinforced poly(L-lactide) composites with a highly enhanced crystallization rate.

Abstract: Layered metal phosphonate, zinc phenylphosphonate (PPZn), reinforced poly(l-lactide) (PLLA) composites were fabricated by a melt-mixing technique. The nonisothermal and isothermal crystallization, melting behavior, spherulite morphology, crystalline structure, and static and dynamic mechanical properties of the PLLA/PPZn composites were investigated. PPZn shows excellent nucleating effects on PLLA crystallization. With incorporation of 0.02% PPZn, PLLA can finish crystallization under cooling at 10 °C/min. The crystallization rate of PLLA further increases with increasing PPZn concentration. Upon the addition of 15% PPZn, the crystallization half-times of a PLLA/PPZn composite decrease from 28.0 to 0.33 min at 130 °C, and from 60.2 to 1.4 min at 140 °C, compared to the neat PLLA. With the presence of PPZn, the nuclei number of PLLA increases and the spherulite size reduces significantly. Through analysis of the crystal structures of PLLA and PPZn, it was proposed that the nucleation mechanism of the PLLA/...

A Study of the Crystallization, Melting, and Foaming Behaviors of Polylactic Acid in Compressed CO2

Abstract: The crystallization and melting behaviors of linear polylactic acid (PLA) treated by compressed CO2 was investigated. The isothermal crystallization test indicated that while PLA exhibited very low crystallization kinetics under atmospheric pressure, CO2 exposure significantly increased PLA’s crystallization rate; a high crystallinity of 16.5% was achieved after CO2 treatment for only 1 min at 100 °C and 6.89 MPa. One melting peak could be found in the DSC curve, and this exhibited a slight dependency on treatment times, temperatures, and pressures. PLA samples tended to foam during the gas release process, and a foaming window as a function of time and temperature was established. Based on the foaming window, crystallinity, and cell morphology, it was found that foaming clearly reduced the needed time for PLA’s crystallization equilibrium.

Al-Rich Zeolite Beta by Seeding in the Absence of Organic Template

Abstract: Low- and no-template precursor suspensions are studied to clarify the effects of Si/Al ratio, seed content, heating temperature, and crystallization time on the syntheses of Al-rich zeolite Beta. Syntheses using 2.5 wt % seeds resulted in well-shaped octahedral Al-rich Beta crystals with sizes between 0.4 and 1 μm after either 6 days hydrothermal treatment at 125 °C or 12 days at 100 °C. The crystalline Beta product has Si/Al ratios as low as 3.9 (16.4 Al atoms per unit cell), which are comparable to the natural counterpart of zeolite Beta, the mineral Tschernichite, and give the material high hydrophilic properties. The tetrahedral coordination of aluminum in the Al-rich Beta and their stability upon calcinations is demonstrated by NMR spectroscopy and TG analysis. The seed method opens new perspective in the development of economically and environmentally friendly synthesis approach for Al-rich zeolite Beta, where the amount of organic template is reduced significantly.

Observation of the Role of Subcritical Nuclei in Crystallization of a Glassy Solid

Abstract: Phase transformation generally begins with nucleation, in which a small aggregate of atoms organizes into a different structural symmetry. The thermodynamic driving forces and kinetic rates have been predicted by classical nucleation theory, but observation of nanometer-scale nuclei has not been possible, except on exposed surfaces. We used a statistical technique called fluctuation transmission electron microscopy to detect nuclei embedded in a glassy solid, and we used a laser pump-probe technique to determine the role of these nuclei in crystallization. This study provides a convincing proof of the time- and temperature-dependent development of nuclei, information that will play a critical role in the development of advanced materials for phase-change memories.

Effect of Polylactide Stereocomplex on the Crystallization Behavior of Poly(l-lactic acid)

Abstract: Effects of the addition of PDLA on the crystallization behavior of PLLA was investigated by means of differential scanning calorimetry, wide-angle X-ray diffraction, melt rheology, and polarized optical microscopy. Nonisothermal and isothermal crystallization behavior of PLLA including low (l-PDLA) and high molecular weight PDLA (h-PDLA) were studied. PLLA/PDLA asymmetric blends form stereocomplex (SC) crystal and stay unmelted at 200 °C in the PLLA melt. Nonisothermal crystallization measurement from 200 °C showed monotonous rise in the crystallization temperature for PLLA/h-PDLA blend, while peculiar concentration dependence was observed for PLLA/l-PDLA blends. The acceleration effect was more pronounced in PLLA/h-PDLA, although the crystallinity of SC was lower than PLLA/l-PDLA blends, which implies the importance of higher order structure of SC for the crystallization of PLLA. From isothermal crystallization kinetics measurements, the acceleration effect in PLLA/h- and l-PDLA blends was found to enhan...

Nucleation kinetics vs chemical kinetics in the initial formation of semiconductor nanocrystals.

Abstract: The initial formation of semiconductor nanocrystals/nanoclusters, that is, nucleation in the classic literature, was examined both theoretically and experimentally. An experimental method based on determining the initial reaction rate for the formation of nanocrystals/nanoclusters with fixed size and size distribution was developed using InP and CdS nanocrystals/nanoclusters systems, especially the InP one. This experimental strategy relies on the size-dependent absorption spectra of these semiconductor nanoparticles as quantitative probes. The experimental results along with theoretical analysis indicate that the classic nucleation model was unlikely relevant for such crystallization systems, whose bulk crystal solubility in a solution is extremely low. Instead, the formation process was found to match a reaction-controlled kinetics model. The results further imply that understanding of crystallization and development of controlled synthesis of high quality colloidal nanocrystals are both closely related to identifying the molecular mechanism and chemical kinetics.