Induction Period For Nucleation Research Articles

Rheology and Crystallization of Long-Chain Branched Poly(l-lactide)s with Controlled Branch Length

A series of long-chain branched poly(l-lactide)s (LCB-PLAs) with controlled branch length were prepared by a simple and efficient method through a combination of ring-opening polymerization (ROP) of l-lactide and a coupling reaction between the terminal OH groups of the PLA prepolymers and the NCO groups of HDI. The influences of reaction conditions on the synthesis of the LCB-PLAs were investigated, and the structures of the resultant LCB-PLAs were characterized by 1H NMR spectroscopy and SEC-MALLS. By adjusting the degree of polymerization and the composition of the prepolymers, LCB-PLAs with different branch densities and molecular weights between branch points were obtained. The effect of macromolecular chain branching on the rheology and crystallization of PLA was also investigated. The LCB structure contributed to the enhancement of the zero-shear viscosity, complex viscosity, storage modulus, melt strength, and strain hardening under elongational flow. Thermal behavior indicated that the branch structure resulted in a short nucleation induction period and more rapid crystallization, which can be a guarantee of high-strength foams.

Comment on He, J.H. et al., Quasistatic model for two-strand yarn spinning [Mech. Res. Commun. (2005), doi:10.1016/j.mechrescom.2004.05.010

The crystallization behaviors of polymers can be changed by grafting them onto the surface of fillers. In this work, the effect of molecular weight of polymers grafted in two-dimensional filler on the isothermal crystallization behaviors was studied by the dynamic Monte Carlo simulation. The result shows that the grafted polymer system can shorten the nucleation induction period compared with ungrafted polymer systems, but its ultimate crystallinity is reduced. With increasing molecular weight, the nucleation induction period becomes shorter and the final crystallinity becomes higher. The polymer system with low molecular weight shows the intermolecular nucleation, while it exhibits the intramolecular nucleation for the one with high molecular weight. In addition, at lower molecular weights, the crystalline bonds tend to perpendicular to the filler. As the molecular weight increases, the crystalline bonds progressively inclined to deviate from the Z direction and return to the direction parallel to the Z axis ultimately. These simulation results can illustrate the micro-mechanism of the effect of molecular weight on crystallization behaviors of polymer composites from a microscopic perspective, which can provide important theoretical basis for improving the properties of polymer composites.

In Situ Time-Resolved XAFS Study on the Formation Mechanism of Cu Nanoparticles Using Poly(N-vinyl-2-pyrrolidone) as a Capping Agent

The formation mechanism of copper nanoparticles (NPs) using poly(N-vinyl-2-pyrrolidone) (PVP) as a capping agent was investigated by measurements of X-ray diffraction (XRD), transmission electron microscopy (TEM), in situ time-resolved X-ray adsorption fine structure (XAFS) analysis, in situ UV-vis spectroscopy, and an indicator method. XAFS analyses, in combination with TEM observations and the indicator method, revealed that the stable intermediates such as Cu(OH)(2) and Cu(+)-PVP intermediate were formed during an induction period of nucleation of Cu NPs, which play a critical role in the Cu NP formation. Our results suggest that the PVP capping agent is important not only to protect NPs from overgrowth and aggregation but also to control the reaction kinetics of NP formation.

Formation and Stability of Ferroelectric BaTi2O5

BaTi2O5 (BT2) is thermodynamically stable over a very narrow temperature range between 1220° and 1230°C: a modification to the BaO–TiO2 phase diagram is proposed. This thermodynamic stability was shown by constructing a time–temperature transformation diagram for the decomposition of BT2. Once formed, BT2 appears to be stable indefinitely at 1220°–1230°C; at higher temperatures, the decomposition rate increases with temperature; at lower temperatures, the decomposition rate increases with decreasing temperature and passes through a maximum at ∼1200°C; below ∼1150°C, BT2 has long‐lived kinetic stability. Kinetic considerations show a nucleation and growth mechanism for decomposition, with a nucleation induction period that is very temperature dependent. BT2 can be prepared by various routes, including solid‐state reaction of oxides below ∼1100°C; because it is metastable at all temperatures other than 1220°–1230°C, its formation is an example of Ostwald's rule of successive reactions. Discrepancies in the literature concerning the reported stability range of BT2 can be explained by the complex dependence on temperature and time of both its formation and decomposition, for both of which, the nucleation stage is rate limiting.

Influence of Co-Doped Bimetallic Impurities on the Metastable Zone Width and Induction Period for Nucleation of KDP from Aqueous Solutions

Influence of Co-Doped Bimetallic Impurities on the Metastable Zone Width and Induction Period for Nucleation of KDP from Aqueous Solutions

Influence of airflow rate and substrate nature on heterogeneous struvite precipitation

In wastewater treatment plants a hard scale consisting of struvite crystals can be formed, in pipes and recirculation pumps, during anaerobic digestion of wastewater. This study was conducted to evaluate the effect of airflow rate and substrate nature on nucleation type, induction period and supersaturation coefficient during struvite precipitation. A crystallization reactor similar to that designed for calcium carbonate precipitation was used. The pH of synthetic wastewater solution was increased by air bubbling. Experimental results indicated that the airflow increased heterogeneous precipitation of struvite. The susceptibility to scale formation was more important on polyamide and polyvinyl chloride than on stainless steel. In all cases, X‐ray diffraction and infrared spectroscopy showed that the precipitated solid phase was solely struvite. No difference in crystal morphology was observed. However, at similar experimental conditions, the particle size of struvite was higher for stainless‐steel material than that for plastic materials.

Molecular Dynamics Study on the Crystallization of a Cluster of Polymer Chains Depending on the Initial Entanglement Structure

Using a large polymer chain cluster, we have examined entanglement densities of the polymer chains in the bulk and in the surface by molecular dynamics simulation. The entanglement density was measured by the interchain contact fraction (ICF), which was developed as a structural parameter of the entanglement. It was found that the surface location has a lower ICF than the bulk core does. Different entanglement structures then underwent crystallization processes. It resulted in a multidomain system, in which the directors of the order domains are in isotropic distributed. For calculating the crystallinity of the systems, we developed a method, the site order parameter (SOP), which endows local order to every particle in the system; when determining the order for crystalline phase, one can count the crystallinity. SOP enables us to transform a molecular chain picture into an order image, where white stands for the crystalline phase and black for the amorphous. Snapshots of the simulations show evolution of the nucleation and the crystal growth through SOP images. The dynamic effect of the entanglement was observed in the crystallization behaviors. It shows that the entanglement has a higher density in the bulk and a lower density in the surface. The result shows that a shorter induction period of nucleation and a higher crystal growth rate was found in the surface location, since possessing lower ICF.

Effect of amino acid additives on crystal growth parameters and properties of ammonium dihydrogen orthophosphate crystals

Ammonium dihydrogen orthophosphate (ADP) crystals were grown with amino acid materials l-arginine monohydrochloride and l-alanine as new additives in different molar concentrations. The metastable zonewidth of ADP solution with and without these additives was determined and compared. Induction periods for nucleation as a function of supersaturation in different concentrations of additives were measured. Nucleation studies show that metastable zonewidth and induction period of ADP are enhanced by the addition of these amino acids. With increasing additive concentration, the growth rate was found to increase. Amino acid added ADP crystals were grown from the aqueous solution with a simple apparatus that can be applied in certain forced convection configurations to maintain a higher homogeneity of the solution. Moreover, the addition of these amino acids improves the quality of the crystal and yields highly transparent crystals with well-defined features. The high-resolution X-ray diffraction (XRD) analysis reveals that the crystalline perfection of these amino acid doped ADP crystals is extremely good without having any internal structural grain boundaries and mosaic nature. The effect of additives on the growth, nucleation kinetics, structural, optical and mechanical properties has been investigated.

Influence of dispersed organophilic montmorillonite at nanometer‐scale on crystallization of poly(L‐lactide)

AbstractWe investigated the effects of surface‐treated organophilic clay on the crystallization of poly(L‐lactide) (PLLA) in their hybrids. The natural nano‐clay in PLLA/clay hybrids acts as a heterogeneous nucleating agent to facilitate crystallization. On the contrary, extensive distributions of induction periods for nucleation are observed in the individual spherulites of neat PLLA and PLLA/organophilic clay hybrids. Therefore, it is suggested that nucleation type of neat PLLA and PLLA/organophilic clay hybrids implies nearly growth geometry as a homogeneous one. Further, under the presence of nano‐clay in their composites, PLLA matrix form the orthorhombic lattice structure corresponded to the α‐form crystal. Since this experimental fact implies little effect of the clay particles on polymorphism of PLLA crystal, the nucleating effect of the organophilic clay seems weaker than the natural clay itself. However, an increase in clay content enhances the growth rates of spherulite for hybrids. Consequently, most of hybrids exhibit an increase in overall crystallization rates at any crystallization temperature in spite of relatively lower nucleation rate of PLLA crystallites itself. In addition, the Avrami exponents (n) obtained by relatively low crystallization temperature ranged from 4 to 6, implying that the growth geometry was dominated sheaf‐like structure in early stage of isothermal crystallization. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers

Kinetics of growth of carbon fibers on an iron catalyst in methane pyrolysis: A measurement procedure with the use of an optical microscope

A procedure was developed for measuring the kinetic parameters of growth of carbon fibers in the catalytic pyrolysis of hydrocarbons. In this procedure, the dependence of the averaged length of grown fibers on growth time is constructed based on the measurements of fiber lengths with an optical microscope. The procedure proposed allowed us to reliably determine the Arrhenius parameters of the rate of fiber growth and the induction period of fiber nucleation in the measurement of carbon fibers 100 nm or more in diameter. The kinetics of growth of carbon fibers from methane-hydrogen mixtures on an iron catalyst was measured over a temperature range of 950–1050°C. It was found that the rate of fiber growth as a function of methane activity in the gas phase exhibited a maximum in the activity range 200–300; the value of this maximum depends on the contribution of the products of gas-phase methane pyrolysis to the growth of fibers. It was also found that the rate of fiber growth dramatically increased as the critical concentration of pyrolysis products in the mixture was reached. This increase was interpreted as a change from one growth mechanism (growth from methane) to another growth mechanism (growth from acetylene). The experimental data explained the high sensitivity of the process of carbon fiber growth from methane to temperature and the residence time of the gas in the reactor.

Effects of temperature on the scaling of calcium sulphate in pipes

Calcium sulphate scaling is a serious problem encountered in many industrial and domestic applications. Supersaturation has been proven to be the major driving force of scale formation, but the solubility of calcium sulphate changes with temperature. The main purpose of this work is to investigate the effects of temperature on the formation of calcium sulphate scales in pipes, using a pipe flow system. Various levels of supersaturation of the calcium sulphate solution have been employed at different temperatures. Results indicated that higher temperature produced a large increase of scale amounts and a significant decrease of induction periods. Many forms of hydrated calcium sulphate were created at high temperature. The relationship between deposited scale mass and temperature was deduced from experimental data. From the relationship between induction period and temperature activation energies of the surface nucleation were estimated to be in the range of 42 to 48 kJ mol − 1 .

The effect of initial precipitates on the induction period of l-ornithine- l-aspartate during semi-batch drowning out crystallization

Crystallization of l-ornithine- l-aspartate (LOLA) by drowning out was carried out using methanol as an antisolvent in a semi-batch crystallizer. Induction period for nucleation was obtained by a conductivity measurement to characterize the formation of LOLA crystals at different levels of supersaturation and various operating conditions (feed concentration, temperature and the presence of seeds with varying seed mass). It was found that generation and redissolution of initial precipitates occurred simultaneously in early feeding stage of aqueous LOLA solution into the antisolvent. At low feed concentration (⩽0.5 mol LOLA/kg H 2O), the initial precipitates dissolved again and then supersaturation was generated. However, an enormous shower of nuclei during feeding was observed at high feed concentration and then supersaturation was generated with the presence of precipitates. These undissolved precipitates played as seeds which reduced the induction period in comparison with the low feed concentration case. The experimental results indicated that the LOLA crystals were formed by catastrophic secondary nucleation at high feed concentration. To confirm this nucleation phenomenon, the induction periods at various operating conditions were measured and compared with a cluster coagulation model which was a suitable nucleation model to interpret the catastrophic nucleation. The results showed a good agreement between the experimental data and the present model.

A Comparative Study of Apatite Deposition on Polyamide Films Containing Different Functional Groups under a Biomimetic Condition

This work compares the induction period for the nucleation of apatite, rate of crystal growth, and adhesive strength of apatite formed on polyamide films containing either carboxyl or sulfonic groups incorporated with calcium chloride by immersing them in 1.5SBF, whose ion concentrations are 1.5 times those of a simulated body fluid (SBF). The induction period for apatite nucleation on polyamide containing-SO 3 H was shorter than for polyamide containing-COOH, while the rate of crystal growth did not depend on the kind of functional group, but depended on the degree of supersaturation of the surrounding solution. The more ready association of sulfonic groups with calcium ions may lead to a shorter induction period for apatite nuclei than that with carboxyl groups. Adhesive strength of apatite layer with polyamide film containing - SO 3 H was significantly lower than that with - COOH depending not only on the chemical interactions but also on the mechanical properties of the polyamide film.

Influence of Surface Carbon Deposition on the Bulk Crystallization of Isotactic Polypropylene

AbstractSummary: The effect of a surface‐vacuum‐evaporated ultrathin carbon layer on the bulk crystallization behavior of isotactic polypropylene (iPP) films was investigated by means of polarized optical microscopy, differential scanning calorimetry (DSC), and wide‐angle X‐ray diffraction. Results obtained from both isothermal crystallization and constant cooling experiments indicate that surface carbon deposition on an iPP film can remarkably increase the crystallization ability of it during the melt recrystallization process. Optical microscopy observations show that the melt recrystallization of iPP with a carbon coating starts about 10 °C earlier than the pure iPP during a re‐cooling process of 1 °C · min−1. Under isothermal crystallization conditions, a much shorter induction period for nucleation is required for the carbon‐coated iPP than the non‐carbon‐coated one. The wide‐angle X‐ray diffraction and DSC analyses further demonstrate a crystallinity increment of the carbon‐coated iPP with respect to its non‐carbon‐coated counterpart. This clearly indicates that the strong influence of the interfacial interaction between the vacuum‐evaporated carbon and the iPP film on its bulk crystallization can be extended deep into the bulk material.An OM micrograph of a hot‐pressed partially carbon‐coated (top half) iPP film.magnified imageAn OM micrograph of a hot‐pressed partially carbon‐coated (top half) iPP film.

The Retarding Effect of Citric Acid on Calcium Sulfate Nucleation Kinetics

Measurements of the induction period for gypsum nucleation, when citric acid is added in solution as an additive to retard calcium sulfate nucleation, are reported in the present paper. The supersa...

Kinetics of the precipitation of calcium sulfate dihydrate in a desalination unit

The kinetics of calcium sulfate dihydrate spontaneous precipitation in supersaturation solutions for temperatures ranging between 20 and 70°C was investigated The quartz microbalance technique (QMC) was used. From measurements of the induction period for gypsum nucleation, interfacial tension was found to be 48.5 and 46.2 mJ.m 2 at 20 and 70°C, respectively. The dependence of the induction period on temperature determined the activation energy, which ranged from 71 to 51 kJ.mol −1 for the supersaturation range between 2.5 and 11. From the dependence of the induction period on temperature and supersaturation, it was possible to distinguish between the homogeneous and heterogeneous nucleation mechanisms. It was also observed that the induction period depends highly on the lattice cation/anion molar ratio. This dependence is more important for the lower supersaturations. In all cases, the DRX technique showed that the formed solid phase was solely the calcium sulfate dihydrate. Finally, this work demonstrates that the QMC is a very attractive instrument to identify precipitation mechanisms and to measure the amount of the deposited crystals.

Nucleation Rates of Zeolite A Crystals from Clear Aluminosilicate Solutions.

In order to understand the crystallization conditions of zeolite A, induction periods for nucleation from clear aluminosilicate solutions were measured and correlated with operating variables. The induction periods were found to decrease with supersaturation of reactant solutions and were represented by two lines when they were plotted against the supersaturation on a semilog ordinate system at given temperature and given NaOH concentration. At higher supersaturations, amorphous phases precipitated, while zeolite A crystals were obtained at lower supersaturations.For precipitation of zeolite A, the induction periods decreased with the NaOH concentrations, indicating that the crystallization rate of zeolite A was accelerated by the presence of NaOH. The boundary between the precipitation of zeolite A and that of amorphous phases was found to depend on the NaOH concentration and the ratio of [Al3+] to [NaOH] as well as saturation ratios.Finally, an attempt was made to determine the activation energy for nucleation and the surface energy of zeolite A on the basis of measured induction periods.

Kinetics analysis of colloidal crystallization of silica spheres modified with polymers on their surfaces in acetonitrile

The nucleation and growth rates in the colloidal crystallization of silica spheres (136 nm in diameter) modified with polymers on their surface were measured by time-resolved reflection spectroscopy. The polymers were poly(maleic anhydride-co-styrene) [P(MA-ST)] and poly(methyl methacrylate) (PMMA). The induction period for nucleation decreased sharply when the sphere concentration increased. The crystal growth process consisted of a fast growing step leading to metastable crystals (rate v1) and a slow growth rate accompanied by the formation of stable crystals. The crystal size of the P(MA-ST)/SiO2 particles decreased from 0.4 to 0.06 mm, whereas v1 increased from 13 to 37 μm/s, when the particle concentration increased. The slow step was also observed for almost all the samples but was not analyzed since the rate was too small. For PMMA/SiO2 dispersions, the crystal size (0.17–0.3 mm) and v1 (43–166 μm/s) did not show any relation to the particle concentration but showed a linear relationship with the molecular weight of PMMA. These results suggest the important role of the excluded-volume effects of the polymer layers around the silica surface. The contribution of the repulsion due to the electrical double layers is still effective in the colloidal crystallization in acetonitrile.

Effect of Supersaturation on Crystal Size and Number of Crystals Produced in Antisolvent Crystallization.

The antisolvent crystallization of sodium chloride was carried out in batches to observe crystallization phenomena using ethanol as an antisolvent. To decrease high local supersaturation generated by the addition of antisolvent, the authors have proposed a new idea by choosing high ethanol concentration operational conditions. Obtained crystals seemed to be unagglomerated and monodispersed in the optimum range of ethanol concentration. From the observation of nucleation phenomena, it was considered that nucleation induced by the antisolvent addition occurred due to the local supersaturation generated at the boundary of the starting and feed solutions. On the other hand, crystal growth is considered to proceed by the supersaturation generated by sufficient mixing between the starting and feed solutions. Average crystal size increased with the increase of σg. The number of produced crystals increased with the increase of the starting solution concentration C. The induction period for nucleation tended to decrease with increase of C and σg. Since the number of produced crystals depended on C, nucleation was considered to be influenced by local mixing between the starting and feed solutions.

Analysis of nucleation of zeolite A from clear solutions

It is recently reported that the preparation of zeolite membranes from clear solutions with low concentrations of Si 4+ and Al 3+ could prevent secondary nucleation and hence results in tight structures. Therefore, it is essential to study the phenomena of nucleation and crystal growth of zeolites from low concentration and clear solutions. In this study, the dependence of induction periods for nucleation of zeolite A on solution compositions and temperatures was analyzed. The induction periods of nucleation were found to decrease with the saturation ratios. The dependence of induction periods on saturation ratios was represented by two straight lines and the lines intersected at critical points that corresponded to the points of changing the types of precipitate structures. In the range of the saturation ratio above the critical point, precursors precipitated, while zeolite A precipitated in the rest of the range. This suggests that the induction periods are related to the type of precipitate structures.