Primary Crystallization Stage Research Articles

The comparative kinetic analysis of the non-isothermal crystallization process of Eu3 + doped Zn2SiO4 powders prepared via polymer induced sol–gel method

The comparative kinetic analysis of the non-isothermal crystallization was performed for Eu3+ doped Zn2SiO4 samples, prepared by the polymer induced sol–gel procedure and thermally treated in the conventional furnace and in a microwave oven. This comparison is done using various thermal analysis techniques and is supported by SEM investigation. It was found that low values of Avrami constant (n), identified in the primary crystallization stage resulted from a not constant radial growth rate. One possible mechanism that could alter the linear crystal growth rate is a diffusion-controlled transition, with instantaneous nucleation mechanism. In the secondary crystallization stage, observed for both samples, it was found that the probability for two-dimensional crystal growth increases, especially at higher heating rates. It was found that the substitution of zinc ions by europium ions in Zn2SiO4 matrix can lead to some defects due to different radius and charges, which induces autocatalytic behavior of the tested systems. It was shown that the two-parameter Šesták–Berggren (SB) autocatalytic model best describes the non-isothermal crystallization of doped samples. It has been shown that SB model is adequate to describe the crystallization kinetics for the fine powder samples (crystallite size of 60–65nm).

The effect of secondary crystallization on melting

The effect of isothermal crystallization temperature and time on the melting endotherms of poly (ethylene terephthalate), PET, has been measured using differential scanning calorimetry. Restricting the study to the primary stage of crystallization the melting endotherms were broad, frequently exhibiting multiple peaks. The temperature of the last trace of crystallinity increased linearly with crystallization temperature which could be interpolated to determine the equilibrium melting point of PET under equilibrium conditions.Heating beyond the primary stage of crystallization and into the secondary stage produced a narrowing of the melting endotherms with time with a loss of the low melting components and a progressive shift to higher temperatures. This was interpreted as an increase in thickness of the initial lamellae laid down in the primary stage by growth of the “fold surface” into adjacent amorphous material. By deriving the increase in lamellae stem length with time from the melting endotherms the growth was observed to be diffusion controlled, since it was dependent on the square root of time and a thermally activated process in that it increased with temperature following an Arrhenius dependence.The amorphous segments in the inter-lamellar regions were considered to have restricted mobility due to the presence of adjacent rigid crystalline regions and chain entanglements which were slow to diffuse from the growth front. An alternative suggestion which could also account for these observations was that growth was restricted by the diffusion of non-crystallizable low molecular weight impurities from the growth front.

Isothermal Crystallization Kinetics and Melting Behavior of a Luminescent Conjugated Polymer, Poly(9,9-Dihexylfluorene-Alt-2,5-Didodecyloxybenzene)

A study of the isothermal crystallization behaviors of poly(9,9-dihexylfluorene-alt-2,5-didodecyloxybenzene) (PF6OC12) was carried out using differential scanning calorimetry (DSC). The crystallization kinetics under isothermal conditions could be described by the Avrami equation. The Avrami exponent n ranges from 3.43 to 3.71 for PF6OC12 at crystallization temperatures between 100.0°C and 90.0°C, indicating a three-dimensional spherical crystal growth with homogeneous nucleation in the primary crystallization stage for the isothermal melt crystallization process. In the DSC scan, after the isothermal crystallization, multiple melting behavior was found. The multiple endotherms could be attributed to melting of recrystallized materials produced originally during different crystallization processes. According to the Arrhenius equation, the activation energy was determined to be 211.29 kJmol−1 for the isothermal melt crystallization of PF6OC12.

The morphology and non-isothermal crystallization kinetics of PET/BaSO4 hybrid fibre

AbstractThe addition of BaSO4 whose mass fraction was less than 0.052%, could improve the mechanical property and thermal stability of the fibre apparently. It was found that, the smoothness of the hybrid fibre was changed by BaSO4 appreciably, though allowable in actual production; The BaSO4 did not obviously affect the crystallization index ( a ) and the relative degree of crystallization. With the content of BaSO4 increasing, the crystallization rate firstly became smaller slightly and then restored to the original level at the primary crystallization stage, but the crystallization rate revealed a small fluctuation at the secondary crystallization stage. In addition, the Ozawa equation was fit for pure PET well, and the Mo equation was fit for pure and hybrid PET.

Nonisothermal crystallization behavior of biodegradable poly(butylene terephthalate‐co‐butylene adipate‐co‐ethylene terephthalate‐co‐ethylene adipate) copolyester

AbstractA series of biodegradable aliphatic‐aromatic copolyester, poly(butylene terephthalate‐co‐butylene adipate‐co‐ethylene terephthalate‐co‐ethylene adipate) (PBATE), were synthesized from terephthalic acid (PTA), adipic acid (AA), 1,4‐butanediol (BG) and ethylene glycol (EG) by direct esterification and polycondensation. The nonisothermal crystallization behavior of PBATE copolyesters was studied by the means of differential scanning calorimeter, and the nonisothermal crystallization kinetics were analyzed via the Avrami equation modified by Jeziorny, Ozawa analysis and Z.S. Mo method, respectively. The results show that the crystallization peak temperature of PBATE copolyesters shifted to lower temperature at higher cooling rate. The modified Avrami equation could describe the primary stage of nonisothermal crystallization of PBATE copolyesters. The value of the crystallization half‐time (t1/2) and the crystallization parameter (Zc) indicates that the crystallization rate of PBATE copolyesters with more PTA content was higher than that with less PTA at a given cooling rate. Ozawa analysis was not suitable to study the nonisothermal crystallization process of PBATE copolyesters, but Z.S. Mo method was successful in treatingthis process. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers

Preparation, melting, glass relaxation and nonisothermal crystallization kinetics of a novel dendritic nylon-11

A novel dendritic nylon-11, DPA-11, is prepared from the melting copolycondensation of 11-aminoundecanoic acid and the 2nd generation poly(propyleneimine) dendrimer as the dendritic unit, with its melting, glass-transition and nonisothermal crystallization behaviors highlighted. DPA-11 is characterized using FTIR, TGA, WXRD and DSC. The results show that PDA-11 has excellent thermal stability, at room temperature its δ′-form crystal is predominant, and the dual melting processes and glass transition occur during the heating. The subsequent crystallization kinetic analysis with JMA, Ozawa and Mo methods demonstrates the mechanisms and crystallizability changing with the relative crystallinity, α, especially, during the primary and secondary crystallization stages. Moreover, applying the Vyazovkin method yields the dependence of the effective activation energy on crystallinity, from which the crystallization mechanisms are discussed in detail. Then the equilibrium melting temperature is determined, being 200.9 °C. Furthermore, the spherulitic growth analysis with the extended Hoffman–Lauritzen method produces the Regime I/II transition temperature of 158° C.

Non-isothermal crystallization behaviors of poly(4-methyl-pentene-1)

Non-isothermal crystallization of isotactic poly(4-methyl-pentene-1) (P4MP1) is studied by differential scanning calorimeter (DSC), and kinetic parameters such as the Avrami exponent and the kinetic crystallization rate (Zc) are determined. From the cooling and melting curves of P4MP1 at different cooling rates, the crystalline enthalpy increases with the increasing cooling rate, but the degree of crystalline by DSC measurement shows not much variation. Degree of crystalline of P4MP1 calculated by wide angle X-ray diffraction pattern shows the same tendency with crystalline enthalpy, indicating that re-crystallization occurs when samples heated above the second glass transition temperature of P4MP1. By Jeziorny analysis, n1 value suggests that mainly spherulites’ growth at 2.5 K min−1 transforms into a mixture mode of three-dimensional and two-dimensional space extensions with further increasing cooling rate. In the secondary crystallization process, n2 values indicate that the secondary crystallization is mainly the two-dimensional extension of the lamellar crystals formed during the primary crystallization process. The rates of the crystallization, Zc and t1/2 both increase obviously with the increase of cooling rate, especially at the primary crystallization stage. By Mo’s method, higher cooling rate should be required in order to obtain a higher degree of crystallinity at unit crystallization time.

Improvement of soft magnetic properties in Fe 38Co 38Mo 8B 15Cu amorphous and nanocrystalline alloys by heat treatment in external magnetic field

The influence of both longitudinal and transverse magnetic field applied during heat treatment on the soft magnetic behavior in the amorphous and nanocrystalline Fe 38Co 38Mo 8B 15Cu alloys has been investigated. Sheared loops with good field linearity were achieved after annealing in transverse magnetic field. The value of induced anisotropy constant K u after such annealing increased from 440 J/m 3 for thermally relaxed amorphous samples up to 935 J/m 3 for alloys in advanced stage of primary crystallization. The nanocrystallization under a presence of the longitudinal magnetic field results in squared hysteresis loops that are characterized by reduction of coercivity H c to 3–7 A/m. Such low H c values surpass those previously reported for FeCo-based nanocrystalline alloys.

A cryogenic heat capacity study of the crystallization process in Fe-based nanocrystalline alloys

Nanocrystalline alloys have attracted wide attention because of their excellent soft magnetic properties such as high saturation induction and permeability, low magnetostriction, coercive field and anisotropy. In this paper, the nanocrystalline material is prepared by annealing a melt-spun Fe 73.5 Si 13.5 Cu 1 B 9 Nb 3 amorphous ribbon with various annealing conditions. This annealing process produced a series of samples with different crystalline fractions. The primary crystallization stage was investigated by cryogenic heat capacity and x-ray diffraction. The x-ray diffraction results showed that the sample began to crystallize at 490 °C and nanocrystalline Fe 3 Si particles were formed in all the annealed samples. The crystalline fraction obtained by the cryogenic heat capacity measurement indicated that the crystalline volume fraction can be controlled by selecting the annealing temperature or annealing time. The crystallization process is almost complete (99.31%) when the sample is annealed at 600 °C for 60 min.

Non-isothermal crystallization kinetics of partially miscible ethylene-vinyl acetate copolymer/low density polyethylene blends

The non-isothermal crystallization kinetics of ethylene-vinyl acetate copolymer (EVA, 14 wt% vinyl acetate content), low density polyethylene (LDPE) and their binary blends with different blending ratio were investigated via differential scanning calorimetry. Jeziorny theory and Mo’s method were utilized in evaluating the crystallization behavior of both neat materials successfully. In the primary crystallization stage both EVA and LDPE had three-dimensional spherulitic growth mechanism. Apparently the crystallization rate of LDPE was faster than that of EVA at a low cooling rate. Increase in cooling rate limited the spherulites’ growth, which narrowed their rate difference. Influences from blending on the crystallization kinetics of each component in EVA/LDPE mixture were evaluated by Kissinger’s activation energy (ΔE) and Khanna’s crystallization rate coefficient (CRC). Inter-molecular interaction in the melt increased the ΔE of both EVA and LDPE components at the beginning of cooling. During the primary crystallization stage of LDPE, dilution effect from EVA facilitated the crystal growth in LDPE. Co-crystallization between EVA component and the secondary crystallization stage of LDPE component also increased the CRC of EVA. In blend of EVA/LDPE = 7/3, LDPE obtained the maximal CRC value of 174.2 h–1. Results obtained from various approaches accorded well with each other, which insured the rationality of conclusion.

Effect of Nb on Transformation Kinetics and Mechanical Properties in Zr-Al-Ni-Cu Metallic Glasses

The effect of Nb addition on the nucleation and grain growth behaviors has been investigated in the monolithic Zr65Al7.5Ni10Cu12.5Nb5 metallic glass. Nb has been well-known as an element for icosahedral quasicrystalline (QC) phase formation in the primary crystallization stage as well as for improving mechanical properties and corrosion resistance. The grain growth rate of the QC phase at near the crystallization temperature is 1.2×10−9 m/s, which is approximately 10 times smaller than that of the primary fcc Zr2Ni phase in the Zr65Al7.5Ni10Cu17.5 metallic glass. In contrast, nucleation rate increases drastically by Nb addition. It can be calculated as 1.2×1021/m3 s, which is approximately 105 times larger than that of the Zr65Al7.5Ni10Cu17.5 alloy. The results indicate that Nb is a very effective element for controlling grain growth and nucleation rates. Good plasticity is exhibited in the as-cast alloy, however, it is lost drastically even after the structural relaxation as well as the QC precipitation. Such significant change may be attributed to the local ordering and/or phase separating tendencies by a positive chemical affinity of the Zr-Nb pair.

Studies on the phase structure, mechanical, rheological properties, and nonisothermal crystallization kinetics of the PEN/SCF composites

AbstractShort carbon fiber reinforced poly(ethylene 2,6‐naphthalate) composites (PEN/SCF) were prepared by twin‐screw extruder. The structure, mechanical, rheological properties, and nonisothermal crystallization kinetics of the composites were investigated by scanning electron microscope, universal tester, and differential scanning calorimetry. The results suggest that there is better interaction between SCF and PEN matrix, which leads to an increase in the tensile strength, Young's modulus, and impact strength of the composites with proper contents of SCF. Rheological behavior of the PEN/SCF composites melt is complicated, combining a dilate fluid at lower shear rate and a pseudoplastic fluid at higher shear rate. Moreover, the flow activation energy of the composites suggests that the melt with more SCF has higher sensitivity to the processing temperature. In conclusion, the composite with 5–10 wt % content of SCF has better properties. The Avrami equation modified by Jeziorny and Ozawa theory was used, respectively, to fit the primary stage of nonisothermal crystallization of various composites. The Avrami exponents n are evaluated to be 2.6–3.1 for the neat PEN and 3.4–4.8 for PEN/SCF composites, and the SCF served as nucleation agent accelerates the crystallization rate of the composites, and more the content of SCF faster the crystallization rate. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Influence of molten status on nanoquasicrystalline-forming Zr-based metallic glasses

Quasicrystals, formed during the primary crystallization stage of annealing Zr70−XCu25Pt5AlX metallic glasses, decrease in size from 100 to 5 nm as the Al content is increased from 8 to 10 at. %. The glass transition and crystallization processes during annealing at fixed heating rate were found to be influenced by the thermal history of the melt. Pair distribution function analyses of neutron scattering data revealed that the broad peak from the nearest atomic pairs separates into multiple peaks with lower melt quenching temperatures (TL). These results suggest that the short-range order is enhanced with decreased TL. Such ordered regions remain in the glass upon quenching and contribute to nanoquasicrystal formation upon heating.

Similarities in the kinetics of photocrystallization and photodarkening in a-Se

The onset time of photocrystallization (PC) as a function of temperature in a-Se films designed for avalanche photodetectors is explored using Raman scattering experiments. The PC onset time τon is compared to the time-constant τIPD for irreversible photodarkening (IPD) studied in earlier work. Both τon and τIPD exhibit activated thermal behavior mediated by an energy barrier EB∼0.7–0.95eV. We suggest that the formation kinetics of PC and IPD in a-Se are governed by the same energy surface and configuration changes, occurring via photoinduced defects during the primary stage of crystallization.

Local structural fluctuation in Pd–Ni–P bulk metallic glasses examined using nanobeam electron diffraction

Local structural fluctuation in relation to the medium range order (MRO) in a Pd–Ni–P bulk metallic glass was examined using nanobeam electron diffraction (NBED) technique. We found diffraction spots with strong intensities in the NBED patterns taken from any observation sites in the specimen. This indicates a presence of MRO regions densely formed in the specimen. The diffraction spots in NBED were normally dispersed around positions corresponding to the first halo-diffraction ring in selected area diffraction. A low-temperature annealing led to form a nanocrystalline microstructure consisting of unidentified phosphides. In the course of annealing, the MRO structures deduced from the NBED patterns have no structural similarity to the phosphides found in the primary crystallization stage. The MRO structure changes into a similar structure with the primary crystals just before the crystallization. A discussion is made for the MRO structure and its relation to the glass stability and also to the phosphide nucleation in the primary crystallization.

Nonisothermal and isothermal crystallization kinetics of nylon‐12

AbstractThe isothermal and nonisothermal crystallization behavior of Nylon 12 was investigated using differential scanning calorimetry (DSC). An Avrami analysis was used to study the isothermal crystallization kinetics of Nylon 12, the Avrami exponent (n) determined and its relevance to crystal growth discussed and an activation energy for the process evaluated using an Arrhenius type expression. The Lauritzen and Hoffman analysis was used to examine the spherulitic growth process of the primary crystallization stage of Nylon 12. The surface‐free energy and work of chain folding were calculated using a procedure reported by Hoffmann and the work of chain folding per molecular fold (σ) and chain stiffness of Nylon 12 (q) was calculated and compared to values reported for Nylons 6,6 and 11. The Jeziorny modification of the Avrami analysis, Cazé and Chuah average Avrami parameter methods and Ozawa equation were used in an attempt to model the nonisothermal crystallization kinetics of Nylon 12. A combined Avrami and Ozawa treatment, described by Liu, was used to more accurately model the nonisothermal crystallization kinetics of Nylon 12. The activation energy for nonisothermal crystallization processes was determined using the Kissinger method for Nylon 12 and compared with values reported previously for Nylon 6,6 and Nylon 11. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Nonisothermal crystallization kinetics of poly(ethylene terephthalate)/antimony‐doped tin oxide nanocomposites

AbstractPoly(ethylene terephthalate) (PET)/antimony‐doped tin oxide (ATO) nanocomposites were prepared by in situ polymerization. The nonisothermal crystallization behaviors of neat PET and PET/ATO nanocomposites were investigated with differential scanning calorimetry. The nonisothermal crystallization data were analyzed with the Avrami analysis modified by Jeziorny, the Ozawa method, and a method developed by Liu et al. The modified Avrami equation could describe only the primary stage of nonisothermal crystallization of PET and PET/ATO nanocomposites. The Ozawa analysis, when applied to the polymer systems studied here, failed to describe their nonisothermal crystallization behavior. The kinetic method developed by Liu et al. was successful in describing the nonisothermal crystallization of neat PET and PET/ATO nanocomposites. According to the Kissinger equation, the activation energies were determined to be −205.3, −220.0, and −243.7 kJ/mol for neat PET and 99/1 and 95/5 PET/ATO nanocomposites, respectively. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Crystallization of Lightly Sulfonated Poly(ethylene terephthalate) Ionomer. I. Isothermal Crystallization from the Melt

Thermal properties and overall rates of isothermal crystallization from the melt of a commercial ionic copolyester (K‐X/SPET) based on poly(ethylene terephthalate) (PET) were analyzed in detail over a composition range from pure PET to a copolymer containing 10.1 mol% of potassium‐neutralized sulfonated PET. For measurements, differential scanning calorimetry (DSC) was used. Copolyesters with the ionic group content of 4.4 mol% or more were unable to crystallize. The isothermal melt crystallization of the copolyesters was analyzed using both the Avrami and the modified Lauritzen‐Hoffman equations. It was found that both the overall rate constant, as well as the Avrami parameter for the primary crystallization stage, varied with the sulfonated unit percentage—but surface free energy and work of folding were practically independent of them. The observed changes in the thermal properties and the kinetic parameters of crystallization were attributed to the comonomer effects and the intermolecular aggregation of the ionic groups.

Nonisothermal crystallization kinetics of poly(vinylidene fluoride) in a poly(vinylidene fluoride)/dibutyl phthalate/di(2‐ethylhexyl)phthalate system via thermally induced phase separation

AbstractThe nonisothermal crystallization kinetics of poly(vinylidene fluoride) (PVDF) in PVDF/dibutyl phthalate (DBP)/di(2‐ethylhexyl)phthalate (DEHP) blends via thermally induced phase separation were investigated through differential scanning calorimetry measurements. The Ozawa approach failed to describe the crystallization behavior of PVDF in PVDF/DBP/DEHP blends, whereas the modified Avrami equation successfully described the nonisothermal crystallization process of PVDF. Two stages of crystallization were observed in this analysis, including primary crystallization and secondary crystallization. The influence of the cooling rate and DBP ratio in the diluent mixture on the crystallization mechanism and crystal structure was determined by this method. The Mo approach well explained the kinetics of primary crystallization. An analysis of these two methods indicated that the increase in the DBP ratio in the diluent mixture caused a decrease in the crystallization rate at the primary crystallization stage. The activation energy was determined according to the Kissinger method and also decreased with the DBP ratio in the diluent mixture increasing. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Quasicrystallization of (Zr<sub>65</sub>Al<sub>7.5</sub>Cu<sub>27.5</sub>)<sub>95</sub>Ti<sub>5</sub> Glassy Alloy

Phase transformation in the melt-spun (Zr65Al7.5Cu27.5)95Ti5 glassy alloy was investigated by a combined use of X-ray diffraction (XRD), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). It was found that the crystallization mode of the Zr65Al7.5Cu27.5 glassy alloy was altered by the addition of Ti, and nanometer scaled icosahedral phase (I-phase) formation was observed in the primary crystallization stage of the (Zr65Al7.5Cu27.5)95Ti5 quaternary glass, which transformed to the stable Zr2Cu-type phase upon further annealing. The crystallization behavior of (Zr65Al7.5Cu27.5)95Ti5 glass was further discussed in terms of the activation energy as compared to that of Zr65Al7.5Cu27.5 alloy.