Complex Crystallization Behavior Research Articles

Non‐isothermal crystallization of polypropylene with sorbitol‐type nucleating agents at cooling rates used in processing

AbstractSorbitol‐type nucleating agents used as clarifiers for polypropylene (PP) show a complex interplay of phase and crystallization behavior. To study the crystallization behavior, cooling measurements were performed by fast scanning calorimetry using rates between 10 and 4000 K s−1. These rates correspond to cooling rates used in processing. These measurements were combined with conventional differential scanning calorimetry (DSC) measurements at 10 K min−1. The optical properties were characterized by haze measurements. For this investigation the commercially available clarifiers 1,3:2,4‐bis(3,4‐dimethylbenzylidene)sorbitol and 1,2,3‐tridesoxy‐4,6:5,7‐bis‐O‐[(4‐propylphenyl)methylene]nonitol were added to PP in various amounts up to 0.6 wt%. At relatively slow cooling rates only a single crystallization process was observed. In contrast, fast cooling leads to a complex crystallization behavior with up to four different crystallization processes. It was found that the temperature of the main crystallization process during fast cooling correlates with the optical properties from haze measurements. Finally, the cooling rate dependence of the different crystallization processes is discussed in terms of nucleating efficiency of the clarifiers. We conclude that the results of conventional DSC measurements cannot be extrapolated in a simple way to describe the nucleation activity of nucleating agents at cooling conditions relevant during processing. © 2018 Society of Chemical Industry

Polymorphism of Indomethacin in Semicrystalline Dispersions: Formation, Transformation, and Segregation.

The crystallization of metastable crystal polymorphs in polymer matrices has been extensively reported in literature as a possible approach to enhance the solubility of poorly water-soluble drug compounds, yet no clarification of the mechanism of the polymorph formation has been proposed. The current work aims to elucidate the polymorphism behavior of the model compound indomethacin as well as the mechanism of polymorph selection of drugs in semicrystalline systems. Indomethacin crystallized as either the α- or τ-form, a new metastable form, or a mixture of the two polymorphs in dispersions containing different drug loadings in polyethylene glycol, poloxamer, or Gelucire as the result of the variation in the mobility of drug molecules. As a general rule, low molecular mobility of the amorphous drug favors the crystallization into thermodynamically stable forms whereas metastable crystalline polymorphs are preferred when the molecular mobility of the drug is sufficiently high. This rule provides insight into the polymorph selection of numerous active pharmaceutical ingredients in semicrystalline dispersions and can be used as a guide for polymorphic screening from melt crystallization by tuning the mobility of drug molecules. In addition, the drug crystallized faster while the polymer crystallized slower as the drug-loading increased with the maxima of drug crystallization rate in 70% indomethacin dispersion. Increasing the drug content in solid dispersions reduced the τ to α polymorphic transition rate, except for when the more stable form was initially dominant. The segregation of τ and α polymorphs as well as the polymorphic transformation during storage led to the inherent inhomogeneity of the semicrystalline dispersions. This study highlights and expands our understanding about the complex crystallization behavior of semicrystalline systems and is crucial for preparation of solid dispersions with reproducible and consistent physicochemical properties and pharmaceutical performance.

Application of a multi-phase multi-morphology crystallization model to isotactic polypropylenes with different molecular weight distributions

Flow-induced crystallization at elevated pressure of a set of metallocene isotactic polypropylenes (iPP) possessing different molecular weight distributions is studied using extended dilatometry experiments in an apparatus able to apply elevated pressure (up to 1200 bar) and strong shear flow (shear rates up to 180 s−1). The effect of flow on the crystallization temperature was quantified and the samples were analyzed using X-ray diffraction to measure the relative amounts of different crystal phases.The experimental results were used to test a flow induced crystallization model framework recently developed in our group which can describe the complex crystallization behavior of iPP and includes formation of multiple morphologies (spherulites, shish-kebab structure with lamellar branching) and crystal phases (α,β and γ). Almost all model parameters were left unchanged, except for the temperature description of the quiescent crystal growth rate and nucleation density (experimentally measured using optical microscopy) and an extra parameter which takes into account the fraction of high molecular weight for the creation of flow-induced nuclei.The model describes rather good the experimentally observed trends for what regards crystallization temperature and amount of α and γ-phase but substantial discrepancies were found in the amount of β-phase formed. This could be related to the presence of 2,1 insertion errors in metallocene samples which could inhibit the formation of this crystal phase.

Full Characterization of Multiphase, Multimorphological Kinetics in Flow-Induced Crystallization of IPP at Elevated Pressure.

Understanding the complex crystallization behavior of isotactic polypropylene (iPP) in conditions comparable to those found in polymer processing, where the polymer melt experiences a combination of high shear rates and elevated pressures, is key for modeling and therefore predicting the final structure and properties of iPP products. Coupling a unique experimental setup, capable to apply wall shear rates similar to those experienced during processing and carefully control the pressure before and after flow is imposed, with in situ X-ray scattering and diffraction techniques (SAXS and WAXD) at fast acquisition rates (up to 30 Hz), a well-defined series of short-term flow experiments are carried out using 16 different combinations of wall shear rates (ranging from 110 to 440 s–1) and pressures (100–400 bar). A complete overview on the kinetics of structure development during and after flow is presented. Information about shish formation and growth of α-phase parents lamellae from the shish backbones is extracted from SAXS; the overall apparent crystallinity evolution, amounts of different phases (α, β, and γ), and morphologies developing in the shear layer (parent and daughter lamellae both in α and γ phase) are fully quantified from the analysis of WAXD data. Both flow rate and pressure were found to have a significant influence on the nucleation and the growth process of oriented and isotropic structures. Flow affects shish formation and the growth of α-parents; pressure acts on relaxation times, enhancing the effect of flow, and (mainly) on the growth rate of γ-phase. The remarkably high amount of γ-lamellae found in the oriented layer strongly indicates the nucleation of γ directly from the shish backbone. All the observations were conceptually in agreement with the flow-induced crystallization model framework developed in our group and represent a unique and valuable data set that will be used to further validate and implement our numerical modeling, filling the gap for quantitatively modeling crystallization during complicated processing operations like injection molding.

How nucleation-growth kinetics is influenced by initial degree of material crystallinity

Differential scanning calorimetry was used to investigate the effect of initial partial crystallinity on the non-isothermal nucleation-growth Johnson-Mehl-Avrami kinetics – selected Se-Te chalcogenide glasses were used as model systems. Different reproducible degrees of initial crystallinity were achieved by non-isothermal heating to a selected temperature. In case of the materials that primarily crystallize from mechanically induced defects, no change of the activation energy or kinetic mechanism was observed with increasing degree of initial crystallinity. On the other hand, material that showed complex crystallization behavior consisting from two competing overlapping crystal growth processes originating from volume-located nuclei and mechanical defects, respectively, exhibited marked shift in the dominance between the two kinetic mechanisms. In particular, with the increasing degree of initial crystallinity the defects-based crystallization mechanism started to dominate over the classical classic-nucleation-theory-based nucleation-growth crystallization mechanism. Increased amount of defects-based crystallization centers formed during the primary pre-crystallization appears to be the key factor for this change of the kinetic mechanism, rather than decreased activation energy (associated with the already existing crystal/glass interface) accelerating the actual crystal growth micro-mechanism.

Nucleation and crystal growth of amorphous nilutamide – unusual low temperature behavior

Crystallization from organic undercooled melts is an important topic of investigation, in particular for pharmaceuticals. Nilutamide, a small organic molecule used to treat prostrate cancer, was found to exhibit interesting glass forming and crystallization behavior. Nilutamide readily formed an amorphous solid following melt quenching, despite having a very high crystal growth rate. The good glass forming ability of this compound was attributed to its reluctance to nucleate. Interestingly, when samples were cooled to deep within the glassy region, nucleation to a previously unreported metastable polymorph was observed. Surprisingly, nucleation was found to be more likely when samples were held far below the glass transition temperature (Tg) than when annealed above the Tg. At higher temperatures, the stable polymorph was observed to have the faster growth rate, while at low temperatures, the metastable form grew faster. Conformational analysis suggests that the stable crystal form contains a higher energy conformer which may explain the slow nucleation from the supercooled melt as well as the cross-over in the growth rates of the stable and metastable polymorphs at lower temperatures. This study highlights the complex crystallization behavior of organic supercooled liquids and glasses.

Relationship between the crystallization behavior and the warpage of film‐insert‐molded parts

AbstractThe dimensional variation of an injection‐molded, semicrystalline polymer part is larger than the variation of an amorphous polymer part because the shrinkage of a crystalline polymer is generally greater than the shrinkage of an amorphous one. We investigated the warpage of film‐insert‐molded (FIM) specimens to determine the effect of the crystallization behavior on the deformation of FIM parts. More perfect crystalline structures and higher crystallinity developed in the core region of the FIM specimens versus other regions. Relatively imperfect crystalline structures and low crystallinity developed in the adjacent regions of the inserted films, whereas a thin, amorphous skin layer developed in the adjacent regions of the metallic mold wall. The crystallizable substrate in the FIM specimens caused very large warpage because nonuniform shrinkage occurred in the thickness direction of the specimens. Therefore, the warpage of an experimentally prepared FIM poly(butylene terephthalate) specimen was greater than that predicted numerically because of its complex crystallization behavior. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Synthesis and Characterization of Triblock Terpolymers with three Potentially Crystallisable Blocks: Polyethylene‐b‐poly(ethylene oxide)‐b‐poly(ε‐caprolactone)

AbstractA first attempt was made to produce novel ABC triblock terpolymers with three potentially crystallisable blocks: polyethylene (PE), poly(ethylene oxide) (PEO), and poly(ε‐caprolactone) (PCL). Polybutadiene‐b‐poly(ethylene oxide) diblock copolymers were synthesized by living anionic polymerization. Then, a non‐catalyzed thermal polymerization of ε‐caprolactone from the hydroxyl end group of the PB‐b‐PEO diblock precursors was performed. Finally, hydrogenation by Wilkinson catalyst produced PE‐b‐PEO‐b‐PCL triblock terpolymers. Side reactions were detected that lead to the formation of undesired PCL‐b‐PEO diblock copolymers, however, these impurities were successfully removed by purification. A range of triblock terpolymers with PCL and PEO minor components were prepared. Topological restrictions on the PEO middle block prevented this block from crystallizing while the complex crystallization behavior of the PE and PCL blocks was documented by DSC and WAXS measurements.

Crystallization behavior of supercooled smectic cholesteryl myristate nanoparticles containing phospholipids as stabilizers

Supercooled smectic nanoparticles based on physiological cholesterol esters are under investigation as a potential novel carrier system for lipophilic drugs. The present study investigates the very complex crystallization behavior of such nanoparticles stabilized with the aid of phospholipids. Phospholipid and phospholipid/bile salt stabilized cholesteryl myristate dispersions were prepared by high-pressure melt homogenization and characterized by particle size measurements, differential scanning calorimetry, X-ray diffraction and electron microscopy. To obtain fractions with very small smectic nanoparticles, selected dispersions were ultracentrifuged. A mixture of cholesteryl myristate and the phospholipid used for the stabilization of the dispersions was also investigated by light microscopy. The nanoparticles usually display a bimodal crystallization event which depends on the thermal treatment and cannot be attributed to crystalline polymorphism. The ratio of the particle fractions crystallizing in the two successive steps strongly depends on the particle size of the dispersions. The presence of larger particles leads to an increased fraction crystallizing at higher temperature and a higher recrystallization tendency upon storage. The observed peculiarities of the crystallization behavior seem to be mainly caused by the presence of particles with different shapes (cylindrical and spherical) as observed in electron microscopy. Alterations in the composition of the nanoparticles may also play a role.

Complex Isothermal Crystallization and Melting Behavior of Nylon 6 Nanoclay Hybrids

The complex crystallization and melting behavior of neat Nylon-6 and two Nylon-6-nanoclay hybrids with different montmorillonite concentration was studied using several experimental techniques. The differential scanning calorimetry (DSC) traces were taken as a reference and experiments were made at several isothermal crystallization temperatures followed by linear heating. The results, depending on the crystallization temperature, indicated two sequences of melting for neat Nylon-6 and four for the nanoclay hybrids. After isothermal crystallization at low temperatures, neat Nylon-6 behaved different from the hybrids, however, after high crystallization temperatures there was a common melting behavior for all samples. Molecular orientation, molecular weight, and nanoclay concentration were sequentially considered in order to determine the origin of the regimes behavior in the DSC traces. Molecular weight was found as the main cause of the thermal behavior with a minimum influence of molecular orientation and nanoclay concentration. X-ray diffraction measurements indicated the formation of asymmetric γ and incomplete α and γ crystalline structures as a result of the nanoclay presence. These last structures were called truncated. Modulated melting was useful to clear mechanisms and corroborated the similar behavior for all three samples after isothermal crystallization at high temperatures. A melting mechanism was proposed on the basis of the previous behavior which involved the evolution of a quiescent low concentration morphology. This last was associated with crystallization of nonbounded macromolecules.

Formation of New Ti-based Metallic Glassy Alloys

New Ti-based bulk glassy alloys were synthesized in a multi-component alloy system of Ti-Cu-Ni-AI-Zr-Si-B. In a wide composition range of Ti > 50 at %, single-phase glass rods with diameters up to 2.5 mm were prepared using a conventional copper-mold casting method. The new Ti-hascd glassy alloys show compressive strength above 1900 MPa. The plastic (serrated) strain under compression is lager than 0.01. The new Ti-hased glass phase exhihits relatively high thermal stability with a large supercooled liquid region (ΔT x ) above 50 K. These glassy alloys. however, exhibit complex crystallization behavior characterized by multiple exothermic-reaction peaks on their differential scanning calorimeter (DSC) curves. The glass-forming ability of this complex system is discussed on the basis of knowledge available for bulk glass formation.

Synthesis and thermal properties of diblock copolymers utilizing non‐covalent interactions

AbstractDiblock copolymers of poly(styrene) and poly(ethylene oxide) were prepared utilizing a bisterpyridine ruthenium complex as non‐covalent interaction for the connection of the two blocks. Apart from the synthesis and characterization of four metallo‐supramolecular block copolymers, first studies on the thermal properties of the block copolymers have been performed. A complex crystallization behavior was observed and is described in a qualitative fashion. The influence of the metal complex on the thermal stability of the metallo‐supramolecular block copolymers remains a question for further investigation.

Phase equilibrium and crystallization behavior of mixed lipid systems

AbstractAs complex lipid systems, the phase and crystallization behavior of mixtures of a high‐melting milk fat fraction with a low‐melting milk fat fraction or canola oil was studied. A turbidity technique was developed to estimate solubility and metastability conditions of these lipid mixtures. Both solubility and metastability of the high‐melting milk fat fraction in liquid lipids increased exponentially with temperature. At a given equilibration temperature, liquid phases and solid fractions with nearly identical melting profiles and TAG compositions were obtained regardless of the original concentration of the lipid mixture. The maximum melting temperature (MMT), as measured by DSC, of the liquid phase increased dramatically in the equilibrium temperature range of 27.5–35.0°C but did not change at temperatures below and above this range (down to 25.0°C and up to 40°C in this study). The content of long‐chain TAG (C46−C52) increased and short‐chain TAG (C36−C40) decreased in the liquid phases as the equilibrium temperature increased. A plot of the TAG group ratio (i.e, long‐short‐chain TAG) vs. equilibrium temperature was generated to illustrate the phase behavior of the complex lipid system and to represent a solubility curve, from which the supersaturation level for crystallization kinetics was determined. Higher supersaturation and lower temperature resulted in higher nucleation and crystallization rates. Compared to the system with a low‐melting milk fat fraction, mixtures of the high‐melting milk fat fraction with canola oil had higher nucleation and crystallization rates due to the lower solubility found for this system.

Crystallization behavior of calcium phosphate glass powder

The crystallization behavior of calcium phosphate glass powder with the molar ratio [CaO]/[P2O5] = 0.88, to which 6.38 mol% TiO2 and 10 mol% Al2O3 were added as nucleation agents, was investigated. The results indicate complex crystallization behavior which depends on the powder particle size and the crystallization temperature. In the crystallization temperature range Tc 900°C the dominant crystallization mechanism depends on the particle size. In the size range 0.15–0.5 mm the surface mechanism of crystallization is replaced by the volume one. In the range >0.5 mm the volume mechanism of crystallization is dominant. The phases β-Ca2P2O7, TiP2O7 and AlPO4 are formed in that temperature interval for all particle sizes. The additives TiO2 and Al2O3 influence the nucleation and formation of TiP2O7 and AlPO4 but do not influence the formation of β-Ca2P2O7.

Effects of Sorbed Water on the Crystallization of Indomethacin from the Amorphous State

The water sorption isotherm and the crystallization rates for amorphous indomethacin were determined at 30°C as a function of relative humidity (RH), along with the effects of water content on the glass transition temperature (Tg). Below 43% RH, only the stable γ crystal form appears, whereas at higher RH, only the metastable R crystal form appears. The tendency for the α crystals to form at higher RH is consistent with the Ostwald step rule. The crystallization rate of the α form continuously increased with increasing RH due to increasing molecular mobility. The crystallization mechanism of the γ form changed from surface-initiated to bulk-initiated crystallization at 21% RH, and although crystallization rates of the γ form increased with increasing RH in both cases, they were higher when crystallization was surface-initiated. The complex crystallization behavior of the γ form is explained by the higher water content and molecular mobility of the surface relative to the bulk and the general effect of water on α or γ crystal form selection as described by the Ostwald step rule.