Bulk Crystallization Kinetics Research Articles

Desupersaturation dynamics in solutions with applications to bovine and porcine insulin crystallization

Evolution of crystal ensembles in supersaturated solutions is studied at the initial and intermediate stages of bulk crystallization. An integro-differential model includes fluctuations in crystal growth rates, initial crystal-size distribution and arbitrary nucleation and growth kinetics of crystals. Two methods based on variables separation and saddle-point technique for constructing a complete analytical solution to this model are considered. Exact parametric solutions based on these methods are derived. Desupersaturation dynamics is in good agreement with the experimental data for bovine and porcine insulin. The method based on variables separation has a strong physical limitation on exponentially decaying initial distribution and leads to the distribution function increasing with time. The method based on saddle-point technique leads to a dome-shaped crystal-size distribution function decreasing with time and has no strong physical limitations. The latter circumstance makes this method more reasonable for describing the kinetics of bulk crystallization in solutions and melts.

Polymer co-crystallization from LLE: Crystallization kinetics of POCB hydrate from two-phase mixtures of POCB and water

Polyoxacyclobutane (POCB, –[CH2CH2CH2O−]n) has the rare ability to co-crystallize with water to form a hydrate. Above the melting point of the hydrate (∼37 °C), mixtures exist in liquid-liquid equilibrium (LLE) between a POCB-rich and a water-rich liquid phase over a wide range of POCB:water ratios. Such phase behavior, which combines liquid-liquid equilibrium (LLE) and co-crystallization of a polymer with a small molecule, appears to be unique to POCB-water mixtures. Here, we examine the kinetics of isothermal hydrate co-crystallization when mixtures that are initially in liquid-liquid equilibrium (LLE) are cooled to below the hydrate melting point. Hydrate co-crystallization requires both POCB and water, and hence, it is coupled with transport of species between the liquid phases. Optical microscopy shows that hydrate crystallization occurs within the POCB-rich domains, and hydrate spherulites grow at constant speed. The temperature-dependence of the speed of hydrate growth front is consistent with the Hoffman Lauritzen model for homopolymer crystallization. Dilatometric measurements of bulk co-crystallization kinetics are conducted with constant stirring to avoid gravitational separation of the liquid phases. The temperature-dependence of bulk crystallization kinetics of these stirred mixtures in LLE is found to be far weaker than that of spherulitic growth kinetics, or of the quiescent bulk crystallization kinetics of mixtures starting from homogeneous conditions.

Effect of Overcooling and Stoichiometry Deviations on Kinetics and Mechanism of Crystallization of As2Se3 Glass

A comparative study of the kinetics of massive bulk crystallization of As2Se3, As1.9Se3, and As2.1Se3 bulk glasses at 240°C is performed using the Kolmogorov–Avrami model. A theoretical analysis of the experimental dependences of the linear growth rate of As2Se3 crystals on temperature and viscosity of the As2Se3 glass is performed. It is shown that the dislocation growth of crystals in the form of spherulites appears to be probable in the As2Se3 glass under overcooling at ∆T = 30–135°С. For individual lamellar crystals As2Se3 with dislocation-free faces inside the spherulites, a growth mechanism with surface two-dimensional nucleation (2Dsg model) appears to be probable at ∆T = 75–135°С.

Polymer crystallinity and crystallization kinetics via benchtop 1H NMR relaxometry: Revisited method, data analysis, and experiments on common polymers

Semi-crystalline polymers play an enormously important role in materials science, engineering, and nature. Two thirds of all synthetic polymers have the ability to crystallize which allows for the extensive use of these materials in a variety of applications as molded parts, films, or fibers. Here, we present a study on the applicability of benchtop 1H NMR relaxometry to obtain information on the bulk crystallinity and crystallization kinetics of the most relevant synthetic semi-crystalline polymers. In the first part, we investigated the temperature-dependent relaxation behavior and identified T = Tg + 100 K as the minimum relative temperature difference with respect to Tg for which the mobility contrast between crystalline and amorphous protons is sufficient for an unambiguous determination of polymer crystallinity. The obtained bulk crystallinities from 1H NMR were compared to results from DSC and XRD, and all three methods showed relatively good agreement for all polymers. In the second part, we focused on the determination of the crystallization kinetics, i.e., monitoring of isothermal crystallization, which required a robust design of the pulse sequence, precise temperature calibration, and careful data analysis. We found the combination of a magic sandwich echo (MSE) with a short acquisition time followed by a Carr-Purcell-Meiboom-Gill (CPMG) echo train with short pulse timings to be the most suitable for monitoring crystallization. This study demonstrates the application of benchtop 1H NMR relaxometry to investigate the bulk crystallinity and crystallization kinetics of polymers, which can lead to its optimal use as an in-situ technique in research, quality control, and processing labs.

Sequestration of Carbon Dioxide with Simultaneous Formation of Fine Calcium Carbonate Particles in Liposomes

Abstract1‐Palmitoyl‐2‐oleoyl‐sn‐glycero‐3‐phosphocholine (POPC) liposomes containing CaCl2 were prepared in Tris solution. The optimal initial pH and temperature were determined based on the kinetics of bulk crystallization of CaCO3 and to depress the leakage of Ca2+ from liposomes. The CaCO3 particles were formed by mixing the liposome suspension and CO2‐saturated water, as analyzed with the scanning transmission electron microscopy and energy‐dispersive X‐ray spectroscopy. The liposomal reaction system can be utilized for the formation of fine CaCO3 particles through sequestration of CO2.

Polymer induced changes of the crystallization scenario in suspensions of hard sphere like microgel particles

We investigated the crystallization scenario of highly cross linked polystyrene particles dispersed in the good solvent 2-ethylnaphtalene and their mixtures with non-adsorbing low molecular weight polysterene polymer using time resolved static light scattering. The samples were prepared slightly below the melting volume fraction of the polymer free system. For the polymer free samples, we obtained polycrystalline solids via crystallization scenario known from hard sphere suspensions with little competition of wall crystal formation. Addition of non-adsorbing low molecular weight polystyrene polymer leads to a considerably slowing down of the bulk crystallization kinetics. We observed a delay of the precursor to crystal conversion for the bulk crystallization while the induction times for the wall nucleation are reduced. The increased polymer concentration thus shifts the balance between the two competing crystallization pathways giving the possibility to tune the relative amount of wall based crystals.

Influence of supercritical CO2 and initial melting temperature on crystallization of polypropylene/organoclay nanocomposite

Polypropylene (PP)/clay nanocomposite with maleic anhydride modified polypropylene (PP-MA) was prepared using a twin-screw extruder. The effect of supercritical carbon dioxide (scCO2) on mixing was investigated. Isothermal crystallization of the nanocomposites was investigated by differential scanning calorimetry (DSC) and also by optical microscopy as a function of initial melting temperature. Increasing initial melting temperature causes a gradual decrease in bulk crystallization kinetics, with the exception of the 240–260 °C temperature range for the system without CO2. Optical microscopy revealed a large number of small spherulites for the system without CO2 after initial melting at 250 °C. After 28 min initial induction period of crystallization many small spherulites appeared in the vicinity of large spherulites for the system with CO2, indicating the beginning of homogenous nucleation. X-ray diffraction (XRD) and direct observation of the samples after tensile testing revealed better dispersion of nanoclay for the system without CO2.

Isothermal crystallization in polypropylene/ethylene–octene copolymer blends

Crystallization kinetics of polypropylene (PP)/ethylene–octene copolymer (EOC) blends was measured by differential scanning calorimetry (DSC) and by optical microscopy at various temperatures (123–140 °C). By DSC it was found that small amounts of EOC (10–30%) increase the bulk crystallization kinetics; in some cases even 80% increase was observed. Larger amounts of EOC (50–80%) have caused a decrease in crystallization kinetics. Optical microscopy has revealed that crystallization kinetics of a single spherulite is decreased by the presence of EOC, and at the same time there are more spherulites in the blend. EOC acts as a nucleation agent increasing the bulk crystallization rate and at the same time it acts as an obstacle to the growing front of a spherulite when the PP lamellae have to go around the EOC obstacle. This was confirmed by detailed analyses according to Avrami and Hoffman–Lauritzen. Structure was also observed by transmission electron microscopy (TEM). At low PP content (20%) PP forms very small isolated particles with size smaller than 1 micrometer. At 40% of PP a structure resembling a co-continuous one was found. When the PP is a majority phase (20–40% of EOC) rather large EOC particles were found. TEM also revealed presence of PP lamellae in EOC regions that together with T g shift of PP and also EOC suggested partial miscibility. While at low temperatures the crystallization is very fast, at higher temperatures there is a competition of phase separation and crystallization.

Crystallization kinetics, mineralization and crack propagation in partially crystallized bioactive glass 45S5

Thermal treatment of bioactive glass ceramics dictates many important features such as microstructure, degree of crystallinity, mechanical properties, and biological response. This report investigates the heat treating conditions and the Avrami crystallization kinetics of melt cast bioactive glass 45S5 at 680 °C. Glass discs were found to follow three dimensional bulk crystallization kinetics (Avrami exponent n = 3). Partially crystallized bioactive glass samples were subjected to in vitro immersion testing to assist a comparative study of the adhesion capabilities of the mineralized layer formed on crystalline and amorphous regions. Higher adhesion of the mineralized layer to the amorphous region was observed as compared to its crystalline counterpart. Furthermore, Palmqvist crack propagation in amorphous and partially crystallized bioactive glass was studied. The crack paths in amorphous bioactive glass were straight, yet crack deflections were observed in the crystalline regions, likely attributed to different crystallographic orientations of crystals or residual thermal mismatch strains present in the bioglass ceramic. Hence, the mineralized layer interfacial fracture toughness and bulk fracture toughness of bioglass ceramics are different from their amorphous counterpart.

Impact of plasticizers and tackifiers on the crystallization of isotactic poly(1-butene)

Crystallization of a semi-crystalline polyolefin in the presence of low molecular weight modifiers was quantified by differential scanning calorimetry and optical microscopy. The polyolefin was a commercial grade of isotactic poly(1-butene) (iPB). Two modifiers were used: an oligomeric plasticizer, designated HOAO, which decreased the glass transition temperature ( T g ) of the system, and an oligomeric tackifier, designated HOCP, which increased T g . Binary iPB/modifier blends containing 10% or 20% by weight of HOAO or HOCP were examined to determine how their addition affects T g , while ternary iPB/HOAO/HOCP blends containing 10% or 20% by weight of total modifier were examined to determine the effects of dilution by using a ratio of HOAO to HOCP that matched the T g of iPB. The addition of modifier decreased the nucleation rate, spherulitic crystal growth rate, and final crystallinity of each blend. However, only the nucleation rate showed a dependence on the type of modifier, with nucleation retarded more by HOCP than by HOAO. A Hoffman–Weeks analysis of the melting point as a function of crystallization temperature confirmed that the driving force for nucleation was reduced, and that the effect was larger for HOCP. An Avrami analysis of the bulk crystallization kinetics was consistent with these observations, as the Avrami exponents were in the range of 3–4.

Sinter-crystallization of a glass obtained from basaltic tuffs

Glass-ceramic materials, obtained by sinter-crystallization of melted alkaline-olivine basaltic tuffs, were investigated. The kinetics of bulk crystallization was evaluated by differential thermal analysis (DTA) at different heating rates. The phase formation and the sintering behavior of glass powders (<75μm) were studied in air and in nitrogen atmospheres by DTA and dilatometry, respectively. The crystalline phases formed were identified by X-ray diffraction. The DTA traces showed an unusual phase formation behavior with a higher crystallization trend for the bulk samples. The crystallization activation energy was evaluated as 590±20kJ/mol in the range 1080–1110K. A value of about 3 of the Avrami constant, corresponding to three-dimensional growth on a fixed number of nuclei, was evaluated by Ozawa and Augis–Bennet methods. The densification at low-temperatures is reduced by the intensive crystallization process in both air and nitrogen atmospheres. The sintering starts again at 1150–1250K. In air atmospheres, due to the FeO oxidation, the sintering temperature increases and the percentage of formed crystal phase decreases by 15–20%.

Influence of phase segregation on the mechanical properties of binary blends of polyethylenes that differ considerably in molecular weight

Phase segregation in melt-extruded blends of polyethylenes that differ considerably in molecular weight was found to exert diverse degrees of influence on various measured mechanical properties. The development of phase segregated morphology was found to depend on blend component viscosity in conjunction with blend composition. The instantaneous tensile deformation properties were insensitive to phase segregation while the high-strain tensile deformation behavior was strongly and adversely influenced by phase segregation. In certain instances, phase segregation was found to favor plane stress fracture resistance. Finally, the bulk crystallization kinetics of such blends also revealed signatures of phase segregation in the form of incomplete co-crystallization of the blend components.

Characterization of chemically treated bacterial ( Acetobacter xylinum) biopolymer: Some thermo-mechanical properties

Bacterial cellulose prepared from pellicles of Acetobacter xylinum ( Gluconacetobacter xylinus) is a unique biopolymer in terms of its molecular structure, mechanical strength and chemical stability. The biochemical analysis revealed that various alkali treatment methods were effective in removing proteins and nucleic acids from native membrane resulting in pure cellulose membrane. The effect of various treatment regimens on thermo-mechanical properties of the material was investigated. The cellulose in the form of purified cellulose membranes was characterized by differential scanning calorimetry (DSC), thermo-gravimetric analysis (TGA) and dynamic mechanical thermal analysis (DMTA). The glass transition temperature ( T g) of the native cellulose (untreated, compressed and dried pellicle) was found to be 13.94 °C, in contrast, the chemically treated cellulose membranes has higher T g values, ranging from 41.41 °C to 48.82 °C. Investigations on isothermal crystallization were carried out to study the bulk crystallization kinetics. Thermal decomposition pattern of the native as well as alkali treated cellulose was determined by obtaining thermo-gravimetric curves. At higher temperatures (>300 °C), the biopolymer was found to degrade. Nevertheless, the alkaline treated cellulose membrane was more stable (between 343.27 °C and 370.05 °C) in comparison to the native cellulose (298.07 °C). Further, the percentage weight loss in case of native cellulose was found to be 26.57%, in comparison to 6.45% for the treated material, at 300 °C. The DMTA revealed complex dynamic modulus of the material, at different temperatures and fixed shear stress, applied at a frequency of 5 Hz. The study delineated the effect of alkali treatment regimens, on the thermo-mechanical properties of bacterial cellulose for its application over a wide range of temperatures.

Nonisothermal bulk crystallization of high‐density polyethylene via a modified depolarized light microscopy technique: Further analysis

AbstractThe quiescent nonisothermal bulk crystallization kinetics of high‐density polyethylene was investigated with a modified depolarized light microscopy technique, which allowed for studies at average cooling rates of approximately 5–2500 °C min−1. All of the samples crystallized at a pseudoisothermal temperature (i.e., the plateau or crystallization temperature), despite the nonisothermal nature of the cooling conditions. The rate of the crystallization process increased monotonically with increasing the cooling rate and decreasing the crystallization temperature. Moreover, the apparent crystallinity content was a certain decreasing function with the cooling rate. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 1009–1022, 2002

Bulk Crystallization Kinetics of Metallocene Polyethylenes with Well-controlled Molecular Weight and Short Chain Branch Content

Collaborating with Phillips Petroleum Company, we obtained a series of metallocene polyethylenes (mPE) with narrow molecular weight distribution and uniform short chain branching. The overall crystallization data of these mPEs and two other linear PEs determined from differential scanning calorimetry (DSC) was analyzed and compared using the well-known Avrami equation. The crystallization temperature and the crystallization rate were found to depend on the molecular weight and short chain branch (SCB) content as well. However, the SCB content played a comparatively dominant role. The secondary-nucleation-theory-based regime transition concept was applied to analyze the kinetic data, too. The crystal growth regimes and the regime transition temperatures observed were discussed in terms of the effect of SCB on the relative rates of surface nucleation and lateral spreading.

Kinetics of Bulk Crystallization of Salts from Aqueous Solutions of Ethanol

The kinetics of crystallization of a number of salts from aqueous solutions of ethanol was studiedunder isothermal conditions on a setup enabling simultaneous automated recording of the electrical conduc-tivity, optical transmission, light scattering (at an angle of 90° to the incident light beam) of a super-saturated solution in salt crystallization. The rate coefficients of crystal growth were determined.

Thermal stability, crystallization kinetics and morphology of a new semicrystalline polyimide based on 1,3-bis(4-aminophenoxy) benzene and 3,3′,4,4′-biphenyltetracarboxylic dianhydride

This work investigates the crystallization kinetics and thermal stability of a new melt processable semicrystalline polyimide (T g =210° C, T m =395° C) based on 1,3-bis(4-aminophenoxy) benzene and 3,3′,4,4′-biphenyltetracarboxylic dianhydride, and endcapped with phthalic anhydride. Earlier studies have demonstrated that this polymer is a strong candidate as a structural adhesive for high-temperature and high-performance applications. In this study, an Avrami Analysis was utilized to study the bulk crystallization kinetics as a function of crystallization temperature, melt time and melt temperature. For the crystallization temperatures studied, the Avrami parameter K increased substantially for small increases in undercooling or small decreases in melt temperature and residence time. Spherulitic growth rates were measured microscopically at different crystallization temperatures. The effect of varying the melt time and temperature on the growth rate at a particular temperature was determined. The variation in the Avrami parameters is correlated with independent microscopic observations of the nucleation density and the spherulitic growth rates. Melt viscosity was followed as a function of melt temperature, residence time in the melt and deformation rate. Higher melt temperatures result in a faster increase in the isothermal melt viscosity indicating chemical changes in the polymer at such high melt temperatures. Followed rheologically, onset of crystallization requires greater undercooling when cooling from higher melt temperatures. Non-isothermal crystallization was followed using DSC and optical microscopy, both of which gave evidence of a distinct ‘catastrophic nucleation’ process at temperatures in the vicinity of 330°C during cooling from the melt.

Nucleation kinetics of emulsified triglyceride mixtures

AbstractThe purpose of this study is to determine character istic nucleation parameters such as the surface free energy for nucleus formation in mixtures of fully hydrogenated palm oil (HP) in sunflower oil (SF). These parameters will be used to model the bulk crystallization kinetics of the same mixtures. This was achieved by determining the crystallization kinetics in emulsified triglyceride mixtures using differential scanning calorimetry, proton nuclear magnetic resonance, and ultrasound velocity measurements. The latter technique appeared to be very sensitive for monitoring the crystallization kinetics of fat dispersions containing triglycerides with a simple phase behavior. Isothermal crystallization of emulsified HP stabilized by sodium caseinate started at 7 K below the α clear point, and the kinetics were best fitted assuming heterogeneous nucleation. Isothermal crystallization of emulsified 10% HP in SF stabilized by caseinate started at 14 K below the α melting point, and the kinetics were best fitted assuming homogeneous nucleation. If the same dispersion was stabilized by Tween 20, crystallization started at 11 K below the α melting point, and the kinetics were fitted best using heterogeneous nucleation. Analysis of the temperature dependency of the fit parameters yielded a surface free energy of a nucleus of about 4 mJ.m−2 in the case of homogeneous nucleation. Pre‐exponential nucleation frequencies indicated that a large proportion of the triglyceride molecule should be in the right conformation to be incorporated in a nucleus.

Nonisothermal bulk crystallization studies of high density polyethylene using light depolarizing microscopy

The quiescent nonisothermal bulk crystallization kinetics of two high-density polyethylene resins were investigated by a modified light-depolarizing microscopy (LDM) technique. The technique allows studies at average cooling rates up to 2500°C/min. The polymer was found to crystallize at a pseudo-isothermal temperature even at these very high cooling rates. The overall bulk crystallization rate increased rapidly as the cooling rate and supercooling increased. Crystallization kinetics was analyzed by Avrami analysis. Avrami exponents near 3 suggested spherical growth geometry and instantaneous nucleation at predetermined sites. Observation of spherulites by optical microscopy together with a number density of spherulites that changed little with increase in cooling rate or supercooling supported this model of crystallization behavior. Analysis of the half-time of crystallization based on the Lauritzen and Hoffman secondary nucleation theory indicated that the regime II-III transition was found to occur at a degree of supercooling of approximately 22°C. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 681–692, 1998

Bulk Crystallization Behavior of Poly(ε-caprolactone) with a Wide Range of Molecular Weight

Bulk crystallization kinetics and crystallizability of poly(ε-caprolactone) (PCL) with number average molecular weight (Mn) ranging from 1900 to 64700 were investigated. Most PCL samples crystallized rapidly at 30 and 40°C, such that the crystallization started before the equilibration of DSC signal. As a consequence, the relative crystallinity required for Avrami analysis could not be calculated accurately from the recorded DSC exotherms. An alternative method is proposed to evaluate the Avrami crystallization rate constant from the peak time of the isothermal crystallization exotherm. The calculated crystallization rate constants of PCL displayed a maximum in variation with molecular weight (MW). Such behavior was attributed to interplay between the MW effects on the thermodynamic driving force and the segmental mobility associated with crystallization. A kinetic formula proposed by Hoffman was used to obtain the crystal surface free energy product (σσe). In contrast to the conventional Lauritzen-Hoffman analysis based on crystallization rates measured at different crystallization temperatures (Tc) for a given MW, the present analysis was based on rates measured for different MW at a given Tc. MW variation of degree of crystallinity also displayed a maximum rather than a monotonic drop. Crystallizability was suggested to be controlled by the presence of uncrystallizable short chains and the entanglements in the polymer. The fraction of the uncrystallizable short chains played the main role in controlling the crystallinity for the low MW samples, while entanglement density was the principal factor for the high MW samples.