Influence of Solvents on Solution‐Mediated Polymorphic Transformation of the Polymorphs of L‐Histidine

Solution‐mediated polymorphic transformation (SMPT) of the metastable polymorph B of L‐histidine (L‐his) into the stable polymorph A in water‐antisolvent solutions was performed. Methanol, acetone, and acetonitrile served as antisolvents. The SMPT was studied via nucleated batch antisolvent crystallization process by determining the change of the fraction of the stable polymorph A of L‐his in the crystal phase with time during the crystallization process. The fraction of the stable polymorph A of L‐his was assessed offline by Raman spectroscopy. The transformation time depended on the fractions of form A obtained at the initial stage of crystallization. The transformation rate of the metastable polymorph B into the stable polymorph A at lower antisolvent volume fraction was faster than at higher antisolvent volume fraction. The transformation time of polymorph B into polymorph A in water‐acetonitrile solution was the shortest compared to the other solutions.     

Crystallization of selective polymorph using relationship between supersaturation and solubility

For polymorph screening, the plot of ΔC_met/C_c against C^*/C_c in nucleation kinetics was investigated. The polymorph screening for forms I and II and amorphous form of clopidogrel hydrogen sulfate was carried out according to the nucleation kinetics expressed by this plot. The stability order of polymorphs for famotidin was also predicted successfully by this model. This model was used in the expectation of supersaturation level for polymorphic formation. Two types of polymorphic crystallization: transformation from metastable form to stable form and nucleation and growth of polymorphic form without transformation can be explained. Amorphous form was also expected by this model. Even though polymorphs depend on lots of crystallization parameters such as solvent, temperature, concentration, cooling rate, and so forth, plot of ΔC_met/C_c and the C^*/C_c in various nucleation kinetics gives a guide line for screening of polymorph. © 2015 American Institute of Chemical Engineers AIChE J, 61: 1372–1379, 2015     

Crystallization kinetics of palm stearin in blends with sesame seed oil

This study investigates the crystallization kinetics of palm stearin (PS), a palm oil fraction, in blends with sesame seed oil. The results indicate that the crystallization behavior of PS in sesame oil is mainly associated with the crystallization of tripalmitin. Therefore, crystallization of blends of 26, 42, 60, and 80% (wt/vol) PS in sesame oil was described by equations developed for simpler systems (e.g., Fisher and Turnbull equation). The isothermal crystallization, melting profile, and fitting of the kinetics of nucleation to the Fisher and Turnbull equation showed that the 26, 42, and 60% PS/sesame oil blends crystallized mainly in the β₁′ polymorph state. In contrast, the 80% blend crystallized in two different polymorph states (i.e., β₁′ at T⪯307.6 K and β₁ at T≽308.2 K). The data indicated that, in spite of the higher concentration of PS in the 80% PS/sesame oil system, crystallization in the β₁ state required more free energy for nucleation (δG _c ) than β₁′ crystallization in the 26, 42, and 60% PS/sesame oil. At the low cooling rate used (1 K/min) it was observed that, for a particular PS blend, the higher the effective supercooling the higher the viscosity of the oil phase and the smaller the induction time of crystallization (T_i). Additionally, the β₁′ crystals from PS, developed at the highest effective supercooling investigated, were smaller than the β₁ crystals obtained at lower effective supercooling.     

Crystallization kinetics of fully hydrogenated palm oil in sunflower oil mixtures

The crystallization kinetics of mixtures of fully hydrogenated palm oil (HP) in sunflower oil (SF) was studied. The thermal properties and phase behavior of this model system were characterized by means of differential scanning calorimetry and X-ray diffraction. From the melting enthalpy and clear point of HP, it was possible to calculate the supersaturation at a given temperature for every composition of the model system. Supersaturation of the model system for the β′ but not for the α polymorph yielded the β′ polymorph, while supersaturation for the α polymorph yielded a mixture of mainly β and some β′ polymorphs. The crystallization kinetics of HP/SF mixtures were determined by pulsed wide-line proton nuclear magnetic resonance for various initial supersaturations in the β′ polymorph. The determined curves were modeled by a modified classical nucleation model and an empirical crystal growth function, which are both functions of supersaturation. Heterogeneous nucleation rates in the β′ polymorph yielded a surface Gibbs energy for heterogeneous nucleus formation of 3.8 mJ·m⁻². About 80% of the triglyceride was assumed to be in a suitable conformation for incorporation in a nucleus. Induction times for isothermal crystallization in the β′ polymorph yielded a surface free energy for heterogeneous nucleus formation of 3.4 to 3.9 mJ·⁻².     

溶解度与多晶型浅谈

许多热力学和动力学因素都对晶体的晶型控制有影响,文章中主要介绍溶解度与晶体多晶型之间的内在联系.首先,介绍了晶体学的一些基本理论,其次,介绍溶解度曲线在判定多晶型稳定性中的应用；溶解度曲线对多晶型成核速率的影响,最后,介绍溶解度曲线在溶剂介导晶型转化机制中的贡献.     

Regulating the β′→β polymorphic transition in food fats

Hydrogenated cottonseed oil (HCSO) is commonly used as a β′-stable fat in margarines and shortenings. In the present study, the crystallization behavior of HCSO is altered via dilution, agitation, tempering regime, and the addition of an emulsifier [polyglycerol polyricinoleate (PgPr)]. Key properties assessed include crystal morphology (with polarized light microscopy), polymorphic behavior (with X-ray diffraction), and crystallization kinetics (with DSC). It is demonstrated that on considerable dilution with canola oil (4% w/w), HCSO can be crystallized in the β′ or β polymorph with associated changes in crystal morphology, depending on tempering regime. Crystallization from the melt to 25°C results in the β′-form, as there is insufficient supercooling to form the β polymorph but enough to form the metastable β′. With cooling from the melt to 5°C, there is adequate supercooling for the δ polymorph to form, with the presence of the canola oil facilitating the transformation toward this stable phase. Static vs. crystallization under agitation does not lead to visible changes in either polymorphic behavior or crystal morphology. However, there is extensive secondary nucleation and growth as a result of crystals breaking off accreting agglomerates. The presence of PgPr, added as a crystal modifier, does not affect the final crystal polymorph or morphology, except under one set of conditions—crystallization from the melt to 5°C with agitation, whereby it considerably alters crystallization behavior.     

Kinetics of melt crystallization and transformation of tripalmitin polymorphs

The rates of melt crystallization and phase transformation of three polymorphs of tripalmitin were examined by optical microscopy, X-ray diffractometry and DSC with and without surfactant additives (sorbitan mono- and tristearates). The following results were obtained: (a) Crystallization rate increased in order ofα, β′ andβ; (b) transformation rate was slower than crystallization rate for each polymorph at the same temperature examined; (c) when the most stableβ form was recrystallized from the melt just after the melting ofα, its recrystallization rate was much higher than that by simple melt-cooling; (d) surfactant additives retarded both the crystallization and transformation of all the polymorphs, yetβ′ was influenced the most. A mechanistic interpretation based on the molecular structures both of the melt and of each polymorph is presented.     

Perils of Polymorphism: Size Matters

Since the discovery of polymorphism in 1832, the ability of molecular crystals to adopt different crystal forms has intrigued and frustrated the scientific community. Control of this property has been limited by poor understanding of nucleation pathways and the absence of methods that can intercept crystallization at its earliest stages. Nonetheless, polymorphism, a favorite subject of Jack Dunitz, who is honored in this special issue, has opened a window into the forces that govern molecular organization in the solid state, while enabling discovery of new crystal forms with improved properties. This review illustrates the growth of crystals in nanoscale reactors with sizes comparable with those for nucleation, where thermodynamics and kinetics intersect, changing polymorph stabilities, providing insights into nucleation, enabling discovery of new polymorphs, and even suggesting a route to disappearing polymorphs, brought to our attention by Dunitz and Bernstein twenty years ago.     

Polymorphic and microstructural behaviors of palm oil/lecithin blends crystallized under shear

There has been much work on polymorphism and crystal habit of quiescently crystallized palm oil. However, researchers have found it difficult to probe the process of sheared crystallization. The effect of surface-active molecules as nucleation agents or habit modifiers was demonstrated in quiescent systems. The aim of this work is to explore the effects of shear and specific lecithins (soy and sunflower) on palm oil crystallization by monitoring crystallization under shear using a synchrotron radiation source, as well as microscopy and DSC. It was found that increasing shear led to increasing β′ stabilization in all situations. Soybean lecithin had little effect on behavior. Sunflower lecithin led to even greater β′ stabilization. The different lecithins interact with the crystallizing fat changing rates of nucleation and crystal growth. Thus, the structure of the overall system can be dramatically altered. Microscopy revealed very different structures even if the polymorphism of the different systems was similar. Consequently, specific interactions can be manipulated in order to control the system. In particular, control of lecithin composition affects the stability of the different polymorphs. Palm oil crystallization under realistic processing conditions has been characterized. Under these conditions, increasing shear rates give higher β′ stability. Specific lecithins have different effects. In particular, soybean lecithin is β′ stabilizing, whereas sunflower lecithin has limited effects. Thus the overall structure of lecithin is important in determining the efficacy. This can be applied to control the structure and properties of different systems such as shortenings or spreads where crystalline interactions create the macro-structure that determines product properties.     

Characterizing cocoa butter seed crystals by the oil-in-water emulsion crystallization method

Unambiguous quantitative evidence for the catalytic action of seed crystals in cocoa butter is presented. We used an ultrasound velocity technique to determine the isothermal growth of solid fat content in cocoa butter oil-in-water emulsions, in which the probability of finding a seed crystal in any one droplet was around 0.37 at 14.2°C. The upper limit for the size of seed crystals in West African cocoa butter was around 0.09 μm, the Gibbs free energy for nucleation was 0.11 mj m⁻², and the concentration of seed crystals was in the range of 10¹⁶ to 10¹⁷ m⁻³. X-ray diffraction measurements showed that emulsified cocoa butter crystallizes in the α polymorph and does not appear to transform to the β′ form within the first 25 min of crystallization. Primary nucleation events in cocoa butter emulsions are accounted for by seed crystals. Collision-mediated nucleation, a secondary nucleation mechanism, in which solid droplets (containing seed crystals) catalyze nucleation in liquid droplets, is shown to account for subsequent crystallization. This secondary nucleation mechanism is enhanced by stirring.     

Kinetics of growth of spinel crystals in a borosilicate glass

Three aspects of the kinetics of spinel crystallization in a high-level waste (HLW) glass were studied: (1) the effect of nucleation agents on the number density (n_s) of spinel crystals, (2) crystallization kinetics in a crushed glass, and (3) crystallization kinetics in a glass preheated at T>T_L (liquidus temperature). In glass lacking in nucleation agents, n_s was a strong function of temperature. In glasses with noble metals (Rh, Ru, Pd, and Pt), n_s increased by up to four orders of magnitude and was nearly independent of temperature. The kinetics of spinel crystallization in crushed glass lacking nucleation agents was dominated by surface crystallization and was described by the Kolmogorov-Johnson-Mehl-Avrami (KJMA) equation with the Avrami exponent n≅0.5. For application to HLW glass melter processing, it was necessary to preheat glass at T>T_L to eliminate the impact of temperature history and surface crystallization on crystal nucleation and growth. In the temperature range of glass processing, crystals descend under gravity when they reach a critical size. Below this critical size, crystallization kinetics is described by the KJMA equation and above the critical size by the Hixson-Crowell equation. At low temperatures, at which glass viscosity is high and diffusion is slow, the KJMA equation represents crystal growth from nucleation to equilibrium. As n_s increases, the temperature interval of the transition from the KJMA to Hixson-Crowell regime shifts to a higher temperature.     

Kinetics of isothermal and nonisothermal crystallization of nylon 1313

The crystallization process of a new polyamide, nylon 1313, from the melt has been thoroughly investigated under isothermal and nonisothermal conditions. During isothermal crystallization, relative crystallinity develops in accordance with the Avrami equation with the exponent n ≈ 2 based on DSC analysis. Under nonisothermal conditions, several different analysis methods were used to elucidate the crystallization process. The Avrami exponent n is greater in the isothermal crystallization process, indicating that the mode of nucleation and the growth of the nonisothermal crystallization for nylon 1313 are more complicated, and that the nucleation mode might include both homogeneous and heterogeneous nucleation simultaneously. The calculated activation energy is 214.25 kJ/mol for isothermal crystallization by Arrhenius form and 135.1 kJ/mol for nonisothermal crystallization by Kissinger method, respectively. In addition, the crystallization ability of nylon 1313 was assessed by using the kinetic crystallizability parameters G. Based on this parameter, the crystallizability of many different polymers was compared theoretically. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 1415–1422, 2007     

Nucleation effect of inclusion complexes with different polyolefin as guest molecules on the crystallization of polypropylene

The γ‐cyclodextrin (γ‐CD) inclusion complexes (ICs) with four kinds of polyolefin (PO) as guest molecules were prepared. The crystallization behavior of isotactic polypropylene (iPP) blended with the γ‐CD and γ‐CD–PO ICs was investigated by differential scanning calorimetry, polarized optical microscopy, and light scattering. The iPP blended with the ICs was found to exhibit higher crystallization temperature (T_C), smaller spherulites, and faster crystallization rate than those of neat iPP, indicating that the ICs play a role of nucleating agent on the crystallization of iPP and induce accelerated crystallization. The IC with PO having higher T_C as guest molecules showed higher nucleation effect than the IC with PO having lower T_C as guest molecules. The results suggest that the nucleation effect of these ICs was affected by the kinds of the guest molecules. The higher T_C guest molecules could result in higher nucleation effect. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Effect of silicon dioxide on crystallization and melting behavior of polypropylene

The crystallization and melting behavior of isotactic polypropylene (iPP) and polypropylene copolymer (co‐PP) containing silicon dioxide (SiO₂) were investigated by differential scanning calorimeter (DSC). SiO₂ had a heterogenous nucleating effect on iPP, leading to a moderate increase in the crystallization temperature and a decrease in the half crystallization time. However, SiO₂ decreased the crystallization temperature and prolonged the half crystallization time of co‐PP. A modified Avrami theory was successfully used to well describe the early stages of nonisothermal crystallization of iPP, co‐PP, and their composites. SiO₂ exhibited high nucleation activity for iPP, but showed little nucleation activity for co‐PP and even restrained nucleation. The iPP/SiO₂ composite had higher activation energy of crystal growth than iPP, indicating the difficulty of crystal growth of the composite. The co‐PP/SiO₂ composite had lower activation energy than co‐PP, indicating the ease of crystal growth of the composite. Crystallization rates of iPP, co‐PP, and their composites depended on the nucleation. Because of its high rate of nucleation, the iPP/SiO₂ composite had higher crystallization rate than iPP. Because of its low rate of nucleation, the co‐PP/SiO₂ composite had lower crystallization rate than co‐PP. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 1889–1898, 2006     

Non‐isothermal crystallization and multiple melting behavior of syndiotactic polystyrene—pre‐melting temperature effects

This work investigated how pre‐melting temperature (T_max) and cooling rate (C) affected the non‐isothermal melt crystallization, melting behavior and crystal structure of syndiotactic polystyrene (sPS) by using differential scanning calorimetry (DSC) and wide angle X‐ray diffraction (WAXD) techniques. Experimental results indicated that raising T_max or C decreased the crystallization peak temperature (T_p) and crystallization initiating temperature (T_i). The crystallization kinetics was analyzed through the Ozawa equation. Although the Ozawa exponent n and cooling function K(T) were determined for T_max = 340°C and T_max = 315°C specimens, for T_max = 290°C specimens, the Ozawa equation was not applicable. Activation energies for the non‐isothermal crystallization processes of different T_max specimens were estimated to be approximately 418 kJ/mol. As T_max was raised the nucleation rate of sPS became slower. The multiple melting peaks were associated with different polymorphs as well as recrystallized crystals that formed during heating scans. The percentage content of α polymorph formed in the crystals under various crystallization conditions was estimated through WAXD experiments.     

Interaction of self‐nucleation and the addition of a nucleating agent on the crystallization behavior of isotactic polypropylene

The relationship between heterogeneous or homogeneous nucleation and self‐nucleation of polypropylene (PP) and PP nucleated by an organic phosphate salt (PPA) was studied by DSC. For pure PP, it homogeneously nucleated during cooling after melting at the selected temperature (T_s) of 170–200°C for 3 min, but at the T_s of 160–168°C self‐nucleation occurred; PPA only nucleated heterogeneously at the T_s of 168–200°C, and there existed self nucleation at the T_s of 160–168°C. The double melting peaks of PP and PPA at the T_s of 162°C were observed. Once the self‐nucleation occurred, the change of the crystallization temperature and heat of fusion of PP is more significant than that of PPA with the change of the T_s, depending upon the crystallization conditions. Results were explained by homogeneous nucleation, heterogeneous nucleation, self‐nucleation, and annealing crystallization. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 78–84, 2001     

Crystallization kinetics of PPS composites and quantification of fiber nucleation density using a computer simulation

Differential scanning calorimetry and hot-stage optical microscopy were used to study the isothermal crystallization kinetics of unreinforced poly(phenylene sulfide) (PPS) and PPS reinforced with aramid, carbon, and glass fibers. The influence that fibers have on the crystallization kinetics of PPS was found to depend on the characteristics of the fiber as well as the type of PPS used. For one kind of PPS, fibers enhanced the crystallization rate, while for another type of PPS, reinforcing fibers had a moderate depressing effect on the polymer crystallization rate. To clarify these effects, we used a new method of quantifying the nucleation process in fiber-reinforced composites that employs a 3-D computer simulation of spherulitic crystallization. Using this method, the nucleation density in the bulk polymer, N_b, and the nucleation density on fiber surfaces, N_f, were calculated for PPS composites as a function of crystallization temperature. The calculated values of N_b and N_f were used to explain differences in the effectiveness of the fibers as well as differences in the nucleating characteristics of the two polymers. © 1995 John Wiley & Sons, Inc.     

Determination of Crystal Growth Rates in Glasses Over a Temperature Range Using a Single DSC Run

In this work, we propose and test a simple and accurate technique capable of determining the crystal growth rate, U(T), over a fairly wide temperature range by means of a single differential scanning calorimetry run. This method is based on using 50–200 μm‐thick samples with parallel rough surfaces so that crystal growth is effectively unidirectional and the crystallization fronts have a constant area during the entire crystallization process. Growth rates are calculated from the expression U(T) = L·q·DSC(T)/A_peak, where DSC(T) is the value of the differential scanning calorimetry (DSC) crystallization curve at each temperature T, A_peak is the overall peak area, L is half the sample thickness, and q is the heating rate. This method has been tested for different values of L and q for three glasses undergoing predominantly surface nucleation, that possess distinctly different crystallization behaviors: stoichiometric lithium disilicate and diopside (CaO·MgO·2SiO₂) and a nonstoichiometric lithium‐calcium metasilicate. Growth rates spanning temperature intervals of more than 100 K, including temperature ranges where literature data are scarce due to experimental difficulties, were determined using a single DSC run. The resulting U(T) data were compared with literature data obtained using optical microscopy. The growth rates determined using the proposed method show excellent agreement with the published data for both stoichiometric glasses and only small errors for the nonstoichiometric glass.     

Effect of nucleation agents on the crystallization of poly(3‐hydroxybutyrate‐co‐4‐hydroxybutyrate) (P3/4HB)

Boron nitride (BN), talc, hydroxyapatite (HA), and zinc stearate (ZnSt) were investigated as nucleation agents (NA) for nonfossil‐based poly(3‐hydroxybutyrate‐co‐4‐hydroxybutyrate) (P3/4HB) plastics. Nonisothermal crystallization behaviors of the P3/4HB/NA blends were examined by DSC. It revealed that BN is the most efficient nucleation agent to promote the crystallization rate, however, but not the crystallization degree. The lasting crystallization of P3/4HB was also removed. The nucleation effect was strengthened with increase of BN content up to 1% and then slackened deeply when further BN was added. Isothermal crystallization analysis revealed that the addition of nucleation agent BN does not alter the crystal growth mode of P3/4HB, with maintaining the Avrami parameter n value around 2.40. Talc did enhance the crystallization of P3/4HB with however milder crystal growth rate. HA and ZnSt did not promote, but depressed the crystallization of P3/4HB plastics. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Isothermal and nonisothermal crystallization kinetics of isotactic polypropylene nucleated with substituted aromatic heterocyclic phosphate salts

The crystallization kinetics of isotactic polypropylene (iPP) and nucleated iPP with substituted aromatic heterocyclic phosphate salts were investigated by means of a differential scanning calorimeter under isothermal and nonisothermal conditions. During isothermal crystallization, Avrami equation was used to describe the crystallization kinetics. Moreover, kinetics parameters such as the Avrami exponent n, crystallization rate constant Z_t, and crystallization half‐time t_1/2 were compared. The results showed that a remarkable decrease in t_1/2 as well as a significant increase in overall crystallization rate was observed in the presence of monovalent salts of substituted aromatic heterocyclic phosphate, while bivalent and trivalent salts have little effect on crystallization rate of iPP. The addition of monovalent metal salts could decrease the interfacial free energy per unit area perpendicular to PP chains σ_e value of iPP so that the nucleation rate of iPP was increased. During nonisothermal crystallization, Caze method was used to analyze the crystallization kinetics. It also showed that monovalent metal salts had better nucleation effects than bivalent and trivalent metal salts. From the obtained Avrami exponents of iPP and nucleated iPP it could be concluded that the addition of different nucleating agents changed the crystal growth pattern of iPP. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 3307–3316, 2006