Effect of isothermal annealing on degree of crystallinity and mechanical properties of poly(l‐lactide‐co‐glycolide)

We investigated the effect of isothermal annealing on the degree of crystallinity and mechanical properties of a random copolymer‐poly(l‐lactide‐co ‐glycolide) (PLLGA)‐with monomer molar ratios of 85/15 (PLLGA85/15) by performing polarizing optical microscopy, differential scanning calorimetry, and X‐ray diffraction, and studying the tensile properties. Isothermal annealing of PLLGA at 130 °C was conducted to improve the degree of crystallinity of the copolymer; the maximum degree of crystallinity (44.5%) was achieved after 60 min of annealing. The crystal size/perfection was observed to increase with annealing time. The highest tensile strength of 65.8 MPa was achieved after 80 min of annealing. However, the degree of crystallinity and tensile strength can only reach to certain extent. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Isothermal crystallization behavior and crystal structure of poly(ethylene terephthalate‐co‐1,4‐cyclohexylene dimethylene terephthalate) (P(ET/CT)) copolyesters

Poly(ethylene terephthalate) (PET) and a series of poly(ethylene terephthalate‐co‐1,4‐cyclohexylene dimethylene terephthalate) (P(ET/CT)) copolyesters with different molar ratios of ethylene glycol (EG) to 1,4‐cyclohexanedimethanol (1,4‐CHDM) were investigated by their isothermal crystallization behavior, transparency, crystal structure and morphology changes in different isothermal crystallization process using differential scanning calorimetry (DSC), digital photos, wide‐angle X‐ray diffraction (WAXD) and polarized optical microscopy (POM). The results revealed that P(ET/CT) copolyesters with ≤ 15 mol% 1,4‐CHDM and ≥ 50 mol% 1,4‐CHDM, such as P(ET/CT)(85/15), P(ET/CT)(50/50) and P(ET/CT)(30/70), were crystallizable, while that with 30 mol% 1,4‐CHDM, namely P(ET/CT)(70/30), was amorphous. The crystallization rate, crystallinity and transparency of these copolyesters underwent the same isothermal crystallization process, i.e. it first decreased and then increased remarkably with the increase of 1,4‐CHDM content. Accordingly, the crystal structure of these copolyesters changed from PET‐type lattice to PCT‐type lattice when the copolyester with around 30 mol% 1,4‐cyclohexylene dimethylene terephthalate (CT) unit. Interestingly, the small incorporation of 1,4‐CHDM into PET could lead to the formation of larger spherulite crystals than that of PET. But small and grainy crystallites appeared with further increase in the 1,4‐CHDM content.     

Phosphoric acid purification by suspension melt crystallization: Parametric study of the crystallization and sweating steps

In order to purify phosphoric acid, the suspension melt crystallization process was studied. The suspension crystallization experiments were carried out with 80, 84 and 88 wt% phosphoric acid melt at the cooling rates of 0.05, 0.1 and 0.2 K/min, respectively. Sweating experiments were executed for various crystals obtained in suspension crystallization step. The purification effects of the sweating parameters including sweating time, initial inclusion amount and initial impurity content were studied. The inclusion fraction increases with the increase in cooling rate. The inclusion fraction of the crystals which were formed with feed concentration of 84 wt% phosphoric acid melt is lowest among the three feed concentrations. Different impurities have different purification performances during sweating. High inclusion amount and low impurity concentration favor the purification of H₃PO₄·0.5H₂O crystals during sweating.     

Analysis of the feasibility of Avrami equation to amino acid endcapped PPDO crystallization by using Malek and Mitsuhashi method

Typically, the crystallization kinetics behaviour of biodegradable polyester could be described by the Avrami equation; however, for the appropriate application of this equation, some assumptions are required. Under conventional conditions of isothermal crystallization without nucleating agents, homogeneous nucleation is the key factor for the compatibility of this equation. On account of structural characteristics, the homogeneous nucleation of polymers is possibly affected by several factors. In this study, we investigated the isothermal crystallization of four PPDOs end‐capped with amino acids, followed by the Avrami equation analysis. After employing the method reported by Malek and Mitsuhashi, the L‐Gly‐ end‐capped PPDO and L‐Leu‐, L‐Phe‐end‐capped PPDO crystallized at higher temperatures were not suitable for analysis by the Avrami equation. In addition, this result was substantiated by reasoning the polymer chains’ mobility which could be affected by the terminated amino acids’ steric hindrance and crystallization temperatures, subsequently affecting homogeneous nucleation.     

Influence of a polyacrylate antiscalant on gypsum nucleation and growth

Calcium sulphate dihydrate (gypsum) crystallization was studied under conditions, of supersaturation and temperature, simulating a brackish water desalination unit using solar energy. The effect of an commercial sodium salt of poly(acrylic acid), based compound known as RPI, on homogeneous nucleation and growth of gypsum was also examined. Gypsum was precipitated by mixing aqueous CaCl₂ and Na₂SO₄ solutions. It was found that, with increasing temperature or supersaturation, the induction time decreases and the growth rate increases. By using classical nucleation theory, the interfacial tension and the nucleation rate values were estimated. It was shown that the interfacial tension is temperature dependent. The addition of increasing quantities of RPI, in the same conditions of temperature and supersaturation, prolongs the induction time, decreases the nucleation rate and increases the interfacial tension. The addition mode of RPI (in calcium or in sulphate solution) was found as an important parameter in controlling the inhibition process of gypsum crystallization. XRD and SEM analysis showed that RPI antiscalant strongly affected the texture and the morphology of the deposit gypsum. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Analysis of proton-conducting organic-inorganic hybrid materials based on sulphonated poly(ether ether ketone) and phosphotungstic acid via ASAXS and WAXS

In the present paper the distribution of phosphotungstic acid (PTA, H₃PW₁₂O₄₀) dispersed in sulfonated poly(ether ether ketone), SPEEK, was investigated by Anomalous Small Angle X-ray scattering (ASAXS) and Wide-Angle X-Ray Scattering WAXS techniques. The hydrolysis and condensation of 3-aminopropyltrimethoxysilane or zirconium tetrapropylate in this polymeric matrix were used to produce poly(3-aminopropyl silsesquioxane) or ZrO₂, as nanofillers. Contrary to previous results reported for membranes containing phosphomolybdic acid, the PTA could be completely dissolved in the SPEEK matrix. The applicability of the Guinier approximation for the SPEEK/PTA membrane confirmed that the PTA was dispersed as isolated nanoparticles. The incorporation of poly(3-aminopropyl silsesquioxane) in the SPEEK/PTA system caused the agglomeration of the heteropolyacid as 30 nm large particles. The ZrO₂ had little effect on the distribution of PTA in the SPEEK matrix. On the other hand, no crystallization of the heteropolyacid was observed in the membranes.     

Crystallization of potassium sulfate by cooling and salting‐out using 1‐propanol in a calorimetric reactor

A study was made on a isothermal process for the crystallization of potassium sulfate as an alternative to the cooling process. The process employs addition of 1‐propanol to aqueous salt solutions to achieve the “saltingout” of the K₂SO₄. This work was carried out using an automated Mettler Toledo model RC1 reactorcrystallizer with 800 ml capacity, and controlled isothermally at 25 °C to test the crystallization of K₂SO₄ by addition of the alcohol, and from 50 to 10 °C for the cooling crystallization. In both systems, the line of nucleation points was shown to be approximately parallel to the saturation curve, with an average width of 13°C or 3 % mass for crystallization by cooling, compared with 0.2 to 1 % by salting‐out. In experiments on crystallization by cooling, the K₂SO₄ crystals were 0.27 mm in mean size, showed 7 % agglomeration, and contained 8.5 % moisture. Crystals obtained by salting‐out had a mean size of 0.79 mm, 28 % agglomeration, and 9‐10 % moisture content. A crystal shape factor of approximately of 0.7 was obtained in both systems, apart from the agglomeration.     

Isothermal and non-isothermal crystallization kinetics in amorphous Ni45.6Ti49.3Al5.1 thin films

The crystallization kinetics in Ni_45.6Ti_49.3Al_5.1 film were studied by differential scanning calorimetry through isothermal and non-isothermal approaches. The activation energy for crystallization was determined to be 374 and 280 kJ/mol by the Kissinger and the Augis & Bennett method, respectively, in non-isothermal methods. In the isothermal annealing study, the Avrami exponents were in the range of 2.78-3.80 between 793 and 823 K, suggesting that the isothermal annealing was governed by three dimensional diffusion-controlled growth for Ni_45.6Ti_49.3Al_5.1 thin films, in which the activation energy of nucleation is higher than that of growth. In addition, the transformation rate curves of Ni_45.6Ti_49.3Al_5.1 film were also constructed by isothermal methods. The crystallization kinetics of amorphous Ni_45.6Ti_49.3Al_5.1 film can thus be appreciated and the transformation rate also can be employed to control the degree of crystallization.     

Crystallization of calcium sulfate dihydrate at different supersaturation ratios and different free sulfate concentrations

Free Sulfate is a major parameter affecting gypsum crystallization during phosphoric acid production. Gypsum crystal size, shape and filtration rate are significantly affected by the concentration of free sulfate. It is, therefore, important to evaluate the effectiveness of different sulfate levels from 1.5% to 3.5% on induction time and gypsum morphology. The crystallization of gypsum was carried out under simulated conditions of phosphoric acid production by the dihydrate process. Calcium hydrogen phosphate and sulfuric acid were mixed with dilute phosphoric acid at 80 °C, and the turbidity of the reaction mixture was measured at different time periods to calculate the induction time of gypsum crystals formation. With increasing free sulfate concentration, the induction time was significantly decreased. Chemical processing of Central and South Florida phosphate concentrates under different concentrations of free sulfate from 1.5% to 5.5% was carried out. The change on crystal size distribution and filtration rate were traced with free sulfate concentrations. The results show that, filtration rate of phosphogypsum was correlated to the mean diameter of crystals. In addition, induction time and co‐crystallized (lattice) P₂O₅ % in gypsum are decreased with increasing free sulfate content from 1.5% to 3.5%. Morphology of formed gypsum crystals at different sulfate contents and different supersaturation ratios are investigated. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

On the interpretation of metastable zone width in anti‐solvent crystallization

Considering nonlinear dependence of solute concentration on anti‐solvent–solvent composition a modified Nývlt‐like equation based on traditional power‐law relation between nucleation rate and developing supersaturation and a new equation based on the classical theory of three‐dimension nucleation are proposed to explain the dependence of anti‐solvent addition rate on metastable zone width defined as excessive anti‐solvent composition in anti‐solvent crystallization. The experimental data on the metastable zone width in anti‐solvent crystallization of benzoic acid are analyzed and discussed from the standpoint of these equations. It is found that the new approach based on the classical nucleation theory provides better insight into the processes involved in anti‐solvent crystallization. Analysis of the experimental results on anti‐solvent crystallization of benzoic acid [D. O'Grady, M. Barret, E. Casey, and B. Glennon, Trans. IChemE A 85 , 945, (2007)] revealed that: (1) the value of metastable zone width for a solvent–anti‐solvent system is determined by the solute–solvent and solute–anti‐solvent interactions, (2) the dependence of the metastable zone width on stirring is associated with the enthalpy of mixing, and (3) the new approach predicts a threshold anti‐solvent addition rate associated with the setting up of an equilibrium between solvent and anti‐solvent and a maximum anti‐solvent addition rate connected with the induction period t_ind for the onset of crystallization. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Static and dynamic crystallization in Mg–Cu–Y bulk metallic glass

The static and dynamic crystallization behavior of Mg₆₅Cu₂₅Y₁₀ bulk metallic glass was investigated by X-ray diffraction, differential scanning calorimetry and transmission electron microscopy. It was found that the kinetics of both anisothermal and isothermal crystallization were adequately represented by the Kissinger and KJMA relations, respectively. The apparent activation energy for crystallization was calculated to be 139 kJ/mol; this value is close to the self diffusion of Mg in both a crystalline and non-crystalline matrix. The Avrami exponent was found to vary from 2.2 to 2.5 with increasing annealing temperature which implies that, at high annealing temperatures, nucleation occurs at a constant rate accompanied by diffusion-controlled growth of spherical grains. Tensile straining in the supercooled liquid region indicated that crystallization is slightly accelerated compared with static crystallization; this phenomenon was found to adversely affect the ductility of the alloy.     

The crystallization of lysozyme in the system of ionic liquid [BMIm][BF4]‐water

Lysozyme crystallization was conducted in the ionic liquid (IL) 1‐butyl‐3‐methylimidizolium tetrafluoroborate ([BMIm][BF₄]) with different buffer/IL proportions. It was found that the addition of [BMIm][BF₄] could promote the crystallization process, during which more lager single crystals with controllable morphologies could be obtained due to the manageable crystal growth velocity. A probable explanation was proposed based on the influence of the ionic polarization and kinetics in the lysozyme crystallization. Moreover, the transform in coordination number and the relative growth rate of different crystal faces were discussed. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Phase transformation and crystallization kinetics of melt-spun Ni60Nb20Zr20 amorphous alloy

The glass transition and crystallization kinetics of melt-spun Ni₆₀Nb₂₀Zr₂₀ amorphous alloy ribbons have been studied under non-isothermal and isothermal conditions using differential scanning calorimetry (DSC). The dependence of glass transition and crystallization temperatures on heating rates was analyzed by Lasocka's relationship. The activation energies of crystallization, E_x, were determined to be 499.5 kJ/mol and 488.6 kJ/mol using the Kissinger and Ozawa equations, respectively. The Johnson–Mehl–Avrami equation has also been applied to the isothermal kinetics and the Avrami exponents are in the range of 1.92–2.47 indicating a diffusion-controlled three-dimensional growth mechanism. The activation energy obtained from the Arrhenius equation in the isothermal process was calculated to be E_x = 419.5 kJ/mol. The corresponding three dimensional (3D) time–temperature–transformation (TTT) diagram of crystallization for the alloy has been drawn which provides the information about transformation at a particular temperature. In addition, the intermetallic phases and morphology after thermal treatment have been identified by X-ray diffraction (XRD) and scanning electron microscope (SEM).     

Study on crystallization kinetics of Al65Cu20Ti15 amorphous alloy

The crystallization behavior and thermal stability of amorphous phases of Al₆₅Cu₂₀Ti₁₅ alloy obtained by mechanical alloying were investigated by using in-situ X-ray diffraction and differential scanning calorimetry (DSC) under non isothermal and isothermal conditions. The result of a Kissinger analysis shows that the activation energy for crystallization is 1131 kJ/mol. The higher stability against crystallization of Al₆₅Cu₂₀Ti₁₅ amorphous alloy is attributed to the stronger interaction of atoms in the Al-Cu-Ti system and formed of complicated compound like Al₅CuTi₂ and Al₄Cu₉ as primary phases. The isothermal crystallization was modeled by using the Johnson-Mehl-Avrami (JMA) equation. The Avarami exponents suggest that the isothermal crystallization is governed by a three-dimensional diffusion-controlled growth.     

Silicon nanowire growth on poly‐silicon‐on‐quartz substrates formed by aluminum‐induced crystallization

The vertical growth of Si nanowires on non‐monocrystalline substrates is of significant interest for photovoltaics and other energy harvesting applications. In this paper, we present results on using poly‐Si layers formed by aluminum‐induced crystallization (AIC) on fused quartz wafers as an alternative substrate for the vapor‐liquid‐solid (VLS) growth of vertical Si nanowires. Oxidation of the Al surface to Al₂O₃ before the a‐Si deposition was shown to be a key requirement in the formation of the poly‐Si template since it promotes the crystallization of the a‐Si into Si(111) which is required for vertical silicon nanowire growth. The effect of Al deposition technique (DC sputtering versus thermal evaporation) on a‐Si crystallization and Si nanowire growth was investigated. The use of Al thermal evaporation yielded AIC poly‐Si layers with the highest fraction of 〈111〉 grains as measured by orientation imaging microscopy (OIM) which enabled the growth of vertical Si nanowires. Cross‐sectional transmission electron microscopy analysis confirmed that the 〈111〉 Si nanowires grew epitaxially off of {111}poly‐Si grains in the AIC layer. This study demonstrates the potential of using AIC poly‐Si as a template layer for the vertical growth of silicon nanowires on amorphous substrates.     

A modeling approach for the non‐isothermal antisolvent crystallization of a solute with weak temperature dependent solubility

Antisolvent crystallization is a crystallization technique, which is normally operated isothermally. However, non‐isothermal operation is on occasions performed for solutes that have temperature dependent solubility. This paper shows that it is beneficial to operate antisolvent crystallizers non‐isothermally even for solutes whose solubility is weakly dependent on temperature. In this context, it demonstrates the joint control of particle mean size and size distribution coefficient of variation. A non‐isothermal crystallization model‐based framework is developed for the sodium chloride‐ethanol‐water system and validated for both isothermal and non‐isothermal operations. This framework was used to systematically determine both the optimal antisolvent feed rate and temperature profiles that minimize the coefficient of variation while producing a specified mean crystal size. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Ni1‐x Crx alloy for self controlled magnetic hyperthermia

It is known now that an increase in tumor temperature decreases the tumor resistance to chemo‐ and radiation therapies. Hyperthermia treatment of the tumor cells where damage to the healthy cells can be avoided is viable by using magnetic nanoparticles with controlled Curie temperatures. Nickel‐Chromium (Ni_1‐x Cr_x) particles with varying compositions have been investigated as thermoseeds for use in localized self controlled hyperthermia treatment of cancer. A series of Ni_1‐x Cr_x alloys, have been prepared to find the specific composition which has Curie temperature around 316‐317 K. The samples were cast by arc melting technique, and were annealed at 850 ^oC for 5 hours in sealed quartz tubes. Magnetic properties of the samples were investigated, including Curie temperature, saturation magnetization and hysterisis using Superconducting Quantum Interference Device (SQUID) and Vibrating Sample Magnetometer (VSM). The Curie temperatures of the alloys were found to decrease almost linearly from 401 K to 289 K as the Cr concentration was increased from x = 4.54 wt% to x = 5.90 wt%. The results showed that Ni_1‐x Cr_x alloys might be good candidates for self regulating magnetic hyperthermia applications. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The influence of low-concentration γ-Fe2O3 magnetic nanoparticles in a poly(ether–b–ester) copolymer matrix on interfacial polarization and glass transition

A new poly(ester–b–ether) copolymer filled with magnetic γ-Fe₂O₃ nanoparticles of low concentration (0.1 and 0.3 wt%) has been synthesized, with poly(ethylene terephthalate) (PET) as a rigid segment and poly(tetraoxymethylene) (PTMO) as the flexible segment at the ratio of 50 wt%. The filler was used in the form of suspension or solid powder. The dielectric spectroscopy results for these materials, in a wide range of frequencies and temperatures, are presented and compared with similar results for a pure polymeric matrix. Two relaxation processes, α and β, have been observed clearly in all samples at about 253 K and 163 K, respectively. The α process was shifted towards the lower temperatures as the nanoparticle content increased, while the β-relaxation was unaffected by the doping. The interfacial polarization effect was observed in the samples with nanoparticles, less pronounced for the larger nanoparticles.     

Crystallization kinetics in Cu46Zr45Al7Y2 bulk metallic glass by differential scanning calorimetry (DSC)

Crystallization transformation kinetics in isothermal and non-isothermal (continuous heating) modes were investigated in Cu₄₆Zr₄₅Al₇Y₂ bulk metallic glass by differential scanning calorimetry (DSC). In isochronal heating process, activation energy for crystallization at different crystallized volume fraction is analyzed by Kissinger method. Average value for crystallization in Cu₄₆Zr₄₅Al₇Y₂ bulk metallic glass is 361 kJ/mol in isochronal process. Isothermal transformation kinetics was described by the Johnson-Mehl-Avrami (JMA) model. Avrami exponent n ranges from 2.4 to 2.8. The average value, around 2.5, indicates that crystallization mechanism is mainly three-dimensional diffusion-controlled. Activation energy is 484 kJ/mol in isothermal transformation for Cu₄₆Zr₄₅Al₇Y₂ bulk metallic glass. These different results were discussed using kinetic models. In addition, average activation energy of Cu₄₆Zr₄₅Al₇Y₂ bulk metallic glass calculated using Arrhenius equation is larger than the value calculated by the Kissinger method in non-isothermal conditions. The reason lies in the nucleation determinant in the non-isothermal mode, since crystallization begins at low temperature. Moreover, both nucleation and growth are involved with the same significance during isothermal crystallization. Therefore, the energy barrier in isothermal annealing mode is higher than that of isochronal conditions.     

Crystallization kinetic of glass particles prepared from a mixture of coal ash and soda-lime cullet glass

The crystallization capability of a parent glass made from a mixture of coal ash (40 wt%) and soda-lime glass was investigated using differential thermal analysis (DTA), X-ray diffraction (XRD) and scanning electron microscopy. Different glass particle size distributions were considered in the range 20-500 μm. Two crystallization exotherms in DTA were attributed to the formation of both pyroxenes (diopside Ca(Mg,Al)(Si,Al)₂O₆ and augite Ca(Mg,Fe)Si₂O₆) and plagioclase (Na,Ca)(Si,Al)₄O₈. These phases were confirmed by XRD analyses. Analysis of non-isothermal DTA data yielded values of 545 kJ/mol and 1.8 for the activation energy of crystallization and the Avrami exponent, respectively. This value for the Avrami exponent was consistent with a decreasing nucleation rate and the observed dendritic morphology. The data on crystallization kinetics obtained in this study are relevant for the production of glass-ceramic materials by a sintering/crystallization method from powder compacts made of this parent glass.