Dynamic Crystallization Behavior Research Articles

Dynamic changes in the triacylglycerol composition and crystallization behavior of cocoa butter

Dynamic changes in triacylglycerol composition during complete crystallization were investigated by separating crystals from molten cocoa butter (CB) over different lengths of crystallization time. The dynamic crystallization behavior of CB was described by solid fat content (SFC), crystal morphology, and microstructure. The crystallization process was divided into stages, based on the growth mode of crystals: the induction, primary crystallization, and secondary crystallization stages. As crystallization progressed, the relative content of 1,3-dipalmitoyl-2-oleoylglycerol (POP) increased from 12.1% to 29.8%; that of 1,3-distearoyl-2-oleoylglycerol (StOSt) decreased from 37.1% to 20.2%; and that of tristearin (StStSt) decreased from 4.9% to 0.2%. Circular crystals were formed at the induction and primary crystallization stages, and β′-III form and β′-IV form polymorphisms were exhibited. When crystallization entered the secondary crystallization stage, the crystal morphology changed from circular to acicular, and polymorphic transformation from the β′-III form and β′-IV form to the β-form was observed.

Chebyshev Spectral Collocation Method for Population Balance Equation in Crystallization

The population balance equation (PBE) is the main governing equation for modeling dynamic crystallization behavior. In the view of mathematics, PBE is a convection–reaction equation whose strong hyperbolic property may challenge numerical methods. In order to weaken the hyperbolic property of PBE, a diffusive term was added in this work. Here, the Chebyshev spectral collocation method was introduced to solve the PBE and to achieve accurate crystal size distribution (CSD). Three numerical examples are presented, namely size-independent growth, size-dependent growth in a batch process, and with nucleation, and size-dependent growth in a continuous process. Through comparing the results with the numerical results obtained via the second-order upwind method and the HR-van method, the high accuracy of Chebyshev spectral collocation method was proven. Moreover, the diffusive term is also discussed in three numerical examples. The results show that, in the case of size-independent growth (PBE is a convection equation), the diffusive term should be added, and the coefficient of the diffusive term is recommended as 2G × 10−3 to G × 10−2, where G is the crystal growth rate.

Effects of molecular architecture on the rheological and physical properties of polycaprolactone

The molecular modification of polycaprolactone (PCL) is of great importance for producing optimum physical properties for a given application. Linear polycaprolactone (L-PCL) and 4-arm star polycaprolactone (4-PCL) with similar molecular weights were prepared, and their rheological, thermal, and morphological properties were investigated in relation to their molecular architecture. In dilute solutions, L-PCL exhibited a higher intrinsic viscosity than 4-PCL. In the molten state, the former displayed a higher viscosity and greater temperature dependence of molecular relaxation time than the latter. DSC thermograms showed that molecular architecture had little effect on the melting/crystallization temperature and crystallinity. Thermogravimetric analysis indicated that the introduction of a branched structure deteriorated the thermal stability of PCL, which might be associated with the increased number of hydroxyl end groups in the polymer chains. In isothermal crystallization under shear at two different temperatures, 4-PCL exhibited longer crystallization times than L-PCL. A more notable difference in dynamic crystallization behavior caused by the chemical architecture was observed at 40°C than at 45°C. Examination with a wide angle X-ray diffractometer revealed that shear and temperature applied during isothermal crystallization, as well as chemical architecture, had little effect on the crystal structure.

Crystallization behaviors of polyarylene ether nitrile filled in multi-walled carbon nanotubes

The isothermal and dynamic crystallization behaviors of PEN filled in CNT are studied. The PEN filled in CNT cannot crystallize without shearing. By shearing, the shear stress transmits from CNT to the PEN and induces the crystallization of it.

Investigation on the dynamic crystallization and melting behavior of β‐nucleated isotactic polypropylene with different stereo‐defect distribution—the role of dual‐selective β‐nucleation agent

The selectivities of different β‐nucleating agents might be quite different from each other, which is important in determining the crystallization and properties of the obtained β‐isotactic polypropylene (β‐iPP). However, the relationship between molecular structure and dynamic crystallization behavior of β‐iPP nucleated by dual‐selective β‐nucleating agent (DS‐β‐NA) is still not clear. In this study, the dynamic crystallization and melting behavior of two β‐iPP with nearly same average isotacticity but different stereo‐defect distribution, nucleated by a DS‐β‐NA (N,N′‐dicyclohexyl‐2,6‐naphthalenedicarboxamide; trade name TMB‐5), were studied by differential scanning calorimetry, wide‐angle X‐ray diffraction, and scanning electronic microscopy. The results indicated that in the presence of TMB‐5, the dynamic crystallization and melting behavior of the samples are quite different because the joint effects of the dual selectivity of TMB‐5 and stereo‐defect distribution of the iPP under different cooling rates. Two important roles were observed: (i) slow cooling rate favors the formation of high β‐fraction; and (ii) high crystallization temperature favors the crystallization of α‐phase accelerated by TMB‐5. Generally, the dual selectivity of the DS‐β‐NA, the stereo‐defect distribution of iPP, and the cooling rate were important factors in determining the formation of β‐crystal. Copyright © 2013 John Wiley & Sons, Ltd.

Dynamic crystallization and melting behavior of β-nucleated isotactic polypropylene polymerized with different Ziegler-Natta catalysts

Large amount of work has be published on the dynamic crystallization and melting behavior of β-nucleated polypropylene (β-PP). However, the relationship between molecular structure and dynamic crystallization behavior of β-PP is still not clear. In this study, the dynamic crystallization and melting behavior of two β-nucleated isotactic polypropylene (β-iPP) with nearly same average isotacticity but different stereo-defect distribution, were studied by differential scanning calorimetry (DSC), wide angel X-ray diffraction (WAXD) and temperature modulated DSC (TMDSC). The results indicated that stereo-defect distribution of iPP can significantly influence the dependence of the β-crystal content and thermal stability on the cooling rate. NPP-A with less uniform stereo-defect distribution favors the crystallization at higher temperature region and the formation of β-crystal with high thermal stability in all cooling rates concerned, moreover, the β-crystal content is influenced by cooling rate; for NPP-B with more uniform distribution of stereo-defect, the crystallization temperature and the regular insertion of molecular chains can be reduced in a larger extent. NPP-B is more suitable for the formation of high proportion of β-crystal in both low and high cooling rates, meanwhile, the thermal stability of crystal is sensitive to the cooling rate. This work provides a new insight into the design of β-iPP in dynamic crystallization.

Dynamic Crystallization Kinetics and Nucleation Parameters of a New Generation of Nanocomposites Based on Isotactic Polypropylene and MoS2 Inorganic Nanotubes

Differential scanning calorimetry (DSC) and time-resolved synchrotron X-ray diffraction have been used to investigate the dynamic crystallization behavior and crystalline structure of novel nanocomposites based on isotactic polypropylene (iPP) and molybdenum disulfide inorganic nanotubes (INT-MoS(2)). The influence of the INT-MoS(2) content and different cooling rates on the crystallization behavior has been studied. The crystallization exothermic peak shifted to higher temperature, and the overall crystallization time was reduced by increasing the INT-MoS(2). The dynamic crystallization kinetics was analyzed using the Ozawa-Avrami method, which was successful in describing the dynamic crystallization behavior of these new nanocomposites. On the other hand, study of the nucleation activity using the Dobreva method revealed that the INT-MoS(2) had an efficient nucleation effect on the monoclinic crystal form of iPP. Moreover, this effect was corroborated by the results of the crystallization activation energy, calculated using Kissinger and Takhor methods, which also confirmed the fact that the addition of INT-MoS(2) made the molecular chains easier to crystallize and increased the crystallization rate of iPP.

Dynamic crystallization behavior, morphology, and physical properties of highly concentrated poly(vinylidene fluoride)/silver nanocomposites

AbstractThe effects of the inclusion of silver (Ag) nanoparticles on the physical properties, the crystallization behavior under shear, and the consequential crystalline morphology of poly(vinylidene fluoride) (PVDF) were investigated. Ag nanoparticles were melt compounded with PVDF in weight fractions of 20, 50, and 90 wt % (15.3 vol %). In the melt rheology, the presence of 20 wt % Ag nanoparticles had little effect on the dynamic viscosity of PVDF, but further addition increased it with the loading level. In Cole–Cole plot, all the melts gave a single master curve independent of the presence of Ag nanoparticles. As Ag loading level was increased, the overall crystallization process under shear was accelerated by reducing both induction time and crystallization time. The degree of acceleration was more notable at higher crystallization temperatures. The induction time and the crystallization time of 90 wt % loaded nanocomposites were promoted by 53.5 and 3.7%, respectively, at 145 °C and by 62 and 26.3%, respectively, at 160 °C, compared with those of pure PVDF. For the isothermal crystallization measured by differential scanning calorimeter, the critical Ag concentration, where overall crystallization was not promoted by further addition, occurred between 50 and 90 wt %. Both wide angle X‐ray diffraction profiles and Fourier transform infrared spectra of the samples crystallized under shear displayed the peaks for only α‐form crystals without new peak or peak shift regardless of the Ag loading and crystallization temperature. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2010

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 139kJ/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.

Effect of silver nanoparticles on the dynamic crystallization behavior of nylon‐6

AbstractThe effect of introducing silver nanoparticles on the rheological properties and dynamic crystallization behavior of nylon‐6 was investigated. The nanocomposites showed slightly higher viscosity than pure nylon‐6 in the low‐frequency range even at an extremely low loading level of the silver particles (0.5–1.0 wt %). The nanoparticles had a more noticeable effect on the storage modulus than on the loss modulus of a nylon‐6 melt and reduced its loss tangent. They increased the crystallization temperature of nylon‐6 by about 14 °C and produced a sharper crystalline peak. The silver nanoparticles promoted the crystallization of nylon‐6, and their effect on the dynamic crystallization of nylon‐6 at 200 °C was more notable at a lower shear rate and at 190 °C at a higher frequency. Nylon‐6 produced large spherulitic crystals, but the nanocomposites showed a grainy structure. In addition, the silver nanoparticles reduced the fraction of the α‐form crystal but increased that of the γ‐form crystal. The nanocomposites crystallized at 190 °C showed a lower melting temperature than nylon‐6 by about 3 °C, whereas the nanocomposites crystallized at 200 °C showed almost the same melting temperature. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 790–799, 2004

Study of the Dynamic Crystallization Behavior of GeSbTe Phase Change Optical Disk

Dynamic erasing process is of great importance to increase the data transfer rate of rewritable phase change optical disks. Two disk structures with the GeSbTe phase change layer were employed. The first one is a disk with the conventional four-layered structure while the other one is a five-layered disk with an thermal buffer layer of Si. The dynamic erasing process of the GeSbTe disks was investigated through studying the dynamic crystallization behavior of these disks, which can be characterized by the changes of oscilloscopy signals such as mark length and amplitude. The correlations between the erasing power and the mark length and the amplitude were established. It has been shown that for the shorter marks the mark edge growth from the boundary between the amorphous mark and the crystalline space dominated the whole erasing process. However, for the longer marks, both mark edge growth and nucleation in the mark center and subsequent growth contributed to the dynamic erasing process.

Crystallization kinetics of poly(ethylene terephthalate) with thermotropic liquid crystalline polymer blends

The isothermal and dynamic crystallization behaviors of polyethylene tere- phthalate (PET) blended with three types of liquid crystal polymers, i.e., PHB60- PET40, HBA73-HNA27, ((PHB60-PET40) - (HBA73-HNA27) 50 : 50), have been studied using differential scanning calorimetry (DSC). The kinetics were calculated using the slope of the crystallization versus time plot, the time for 50% reduced crys- tallinity, the time to attain maximum rate of crystallization, and the Avrami equation. All the liquid crystalline polymer reinforcements with 10 wt % added accelerated the rate of crystallization of PET; however, the order of the acceleration effect among the liquid crystalline polymers could not be defined from the isothermal crystallization kinetics. The order of the effect for liquid crystalline polymer on the crystallization of PET is as follows: (PHB60-PET40) - (HBA73-HNA27) (50 : 50); HBA73-HNA27; PHB60-PET40: This order forms the dynamic scan of the DSC measurements. q 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67: 1383-1392, 1998