Secondary Crystallization Kinetics Research Articles

Dual Nakamura model for primary and secondary crystallization applied to nonisothermal crystallization of poly(ether ether ketone)

AbstractNon‐isothermal crystallization kinetics has been traditionally predicted using the Nakamura crystal growth model, which is an integral form of the Avrami equation under the assumption that the non‐isothermal kinetics can be explained using a series of infinitesimal isothermal steps. However, the Nakamura model often overpredicts crystal fraction near the end of crystallization because of ignoring slower secondary crystallization kinetics. In this study, the dual Nakamura model is proposed to capture both primary and secondary crystallization kinetics of poly(ether ether ketone) (PEEK). First, non‐isothermal crystallization kinetics of PEEK is monitored at five constant cooling rates of 10, 30, 50, 70, and 90 K/min via differential scanning calorimetry. The dual Nakamura model, which contains a smaller secondary Avrami exponent () to account for the secondary crystallization, is fitted to the exothermal heat flow curves. The dual Nakamura model successfully describes the final stage of crystallization at all cooling rates by reducing the crystal growth dimension from = 3 for primary crystallization to .

Effect of Thermal Treatment on Crystallinity of Poly(ethylene oxide) Electrospun Fibers.

Post-process thermal treatment of electrospun fibers obtained from poly(ethylene oxide) (PEO) water and methanol solutions was examined. PEO fibers from methanol solution showed larger diameters as observed by scanning electron microscopy. Fibers both from water and methanol solutions exhibited a significant dimensional stability and surface cracking during the specific exposure time after thermal treatments at 40, 50, and 60 °C. Changes in crystallinity after the thermal treatment were studied by wide-angle X-ray diffraction. The kinetics of secondary crystallization were positively influenced by the as-processed level of the amorphous phase and temperature of thermal treatment. Samples treated at 60 °C were degraded by thermooxidation within the time.

Cover Image: Secondary crystallization kinetics

Cover Image: Secondary crystallization kinetics

Secondary crystallization kinetics

The overlap between primary and secondary crystallization is the cause of the repeated observations of fractional constant n values, of no theoretical significance, in the analysis of polymer crystallization by the Avrami equation. The time dependence of the growth nuclei in primary and secondary crystallization are explained in terms of reptation and nucleation theory. A composite equation involving both primary and secondary crystallization has been derived to account for the development of the total fractional crystallinity, Xt, with time, Xt = Xp,∞ (1–exp (–Zptn).(1 + ks t1/2) and procedures derived for determining the rate constants, Zp and ks, and the limiting fractional crystallinity of the primary process, Xp,∞, such that they are not treated as adjustable parameters. The fit of the equation to experimental crystallinity/time data was within experimental error. This interpretation accounted for n values in excess of those expected for the crystallization mechanisms adopted in deriving the Avrami equation but values >3.5 and <4.0 are attributed to the presence of induction times due to homogeneous primary nucleation of the lamellae. Evidence is presented for the presence of two simultaneous processes, primary and secondary, in the crystallization of polymers, which occur with different mechanisms and are readily distinguished by their dependence on time: an exponential increase and an increase with the square root of time. Differences arise from the critical growth nuclei, whose growth is limited by chain entanglements in the case of primary and local reptation in the case of secondary.

Magnetic properties and crystallization kinetics of (Fe100 − xNix)80Nb4Si2B14 metal amorphous nanocomposites

Fe-Ni based metal amorphous nanocomposites (MANCs) are investigated in the pseudo-binary alloys (Fe100−xNix)80Nb4Si2B14. To optimize the soft magnetic properties of the nanocomposites, primary and secondary crystallization kinetics must be understood. As such, primary and secondary crystallization temperatures are determined by differential scanning calorimetry, and activation energies are calculated, along with the resulting crystalline phases. Time-temperature-transformation diagrams for primary and secondary crystallization in (Fe70Ni30)80Nb4Si2B14 are presented. Saturation magnetization and Curie temperature are determined. Lastly, the shape of magnetization vs. time curves for (Fe30Ni70)80Nb4Si2B14 at various temperatures suggest that the secondary crystal product often consumes some of the primary crystalline product.

Characterization of the primary and secondary crystallization kinetics of a linear low-density polyethylene in quiescent- and flow-conditions

The primary and secondary crystallization kinetics of a homogeneous linear low-density polyethylene were characterized as function of cooling rate, pressure and flow strength. Our approach to describe primary crystallization is based on nucleation and growth of spherulites, quantified well below the melting temperature using small-angle light scattering. The description of the two-step secondary process is coupled to primary crystallization using a convolution integral, for which the parameters were determined from (fast-) differential scanning calorimetry. Extended-dilatometry was used to investigate the effect of different thermomechanical histories. Parameters were determined for an existing model that couples molecular stretch to both nucleation rate and fibrillar growth rate. Excellent agreement is shown between calculated and experimentally obtained crystallization kinetics in conditions representative for those found in real-life processing conditions. This opens the possibility to calculate in detail the evolution of and the final crystallinity structure in products such as blown film or extruded tape.

Experimental study of crystallization of PolyEtherEtherKetone (PEEK) over a large temperature range using a nano-calorimeter

The recently developed fast scanning differential calorimetry is used for the first time to determine the crystallization kinetics of Poly(EtherEtherKetone) (PEEK). In our experiments, crystallization is studied in isothermal conditions over a large temperature range from 170 °C to 310 °C. Two different measurement protocols were employed. Between 200 °C and 300 °C the heat flow was directly measured during isothermal crystallization. Outside this temperature range we measured the heat of fusion on heating after interrupted isothermal crystallization. We show that data can be analyzed with the Avrami approach incorporating a term describing secondary crystallization. The crystallization half-times are measured. The Avrami kinetic coefficient KAv associated with primary crystallization is evaluated from isothermal crystallization between 170 °C and 310 °C where data were not previously available. The kinetics of crystallization of PEEK has only one maximum located around 230 °C and its Avrami exponent is close to 3, suggesting instantaneous nucleation with subsequent spherical growth. The whole isothermal crystallization process is modeled in terms of Hillier's model since it takes secondary crystallization kinetics into account. Finally, it is shown that the double melting peak behavior observed after isothermal crystallization (below 260 °C) is a consequence of the reorganization process during heating.

Secondary crystallization in (Fe65Co35)79.5+xB13Nb4−xSi2Cu1.5 and (Fe65Co35)83B10Nb4Si2Cu1 nanocomposite alloys

Here, secondary crystallization kinetics of high induction, low loss HTX002-type nanocomposite alloys with the compositions (Fe65Co35)79.5+xB13Nb4−xSi2Cu1.5 (x = 0-4) and (Fe65Co35)83B10Nb4Si2Cu1 are reported. The magnetization of the alloys was measured through the thermal cycle of 50 °C-700 °C-300 °C-800 °C-300 °C-900 °C-200 °C by vibrating sample magnetometry. In (Fe65Co35)79.5+xB13Nb4−xSi2Cu1.5 alloys, the stability of the (Fe,Co,Nb)23B6 (23-6) phase is increased with increasing Nb content. In the x = 4 alloy, (Fe,Nb)2B is the only secondary crystalline phase to form, demonstrating that Nb is necessary for the 23-6 phase to form. The (Fe65Co35)83B10Nb4Si2Cu1 alloy forms the 23-6 phase more readily than the x = 0 alloy, likely due to the lower B content. The kinetics of secondary crystallization are important to assess long-term ageing effects on the metastable microstructure at elevated temperatures.

Effects of thermal aging on isotactic polypropylene crystallinity

AbstractThe effects of elevated temperature aging on the microstructural changes of isotactic polypropylene matrix in a composite have been studied using wide‐angle X‐ray scattering (WAXS) and Fourier‐transform infrared spectroscopy (FTIR). The objective was to quantify small and slow changes in crystallinity due to thermal aging. To minimize sample variability, polypropylene resin was extracted from the molded composite plaque. Changes in crystallinity level and crystalline form were detected using WAXS after prolonged aging at 90 and 140°C. FTIR was utilized to monitor in‐situ crystallinity changes and to detect oxidation products due to thermal decomposition. The level of crystallinity was monitored by changes in the absorbance ratio of A997/A973 and A841/A973; the former ratio was found to be more sensitive for detecting crystallinity changes. Aging at 140°C resulted in oxidation. The kinetics of secondary crystallization for the aging conditions studied was characterized using Avrami plots. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers

Morphology of semicrystalline oxyethylene/oxybutylene block copolymer thin films on mica

The morphology of as-cast and annealed thin films of four symmetric semicrystalline block copolymers on mica was investigated by tapping mode atomic force microscopy (AFM) and grazing incidence X-ray diffraction (XRD). It is found that the morphology of the thin films is dependent on chain length of oxyethylene/oxybutylene block copolymers. The as-cast thin films of the shorter EmBn block copolymers on mica exhibit a multi-layered lamellar structure parallel to the surface, in which the stems of the E crystals in the first half polymer layer contacting mica are parallel to the mica surface and perpendicular to the mica surface in the upper polymer layers. In contrast, the as-cast thin film of longer E224B114 exhibits a structure with mixed orientations of lamellar microdomains on a half polymer layer parallel to the surface. After annealing, the multi-layered structure on mica is transformed into a half-layered, densely branched structure, which is formed following a diffusion-limited aggregation mechanism, opposed to the featureless half-layered structure on silicon. Upon annealing, the upper polymer layers gradually retreat and the remaining area becomes thicker, but in contrast the first half polymer layer contacting mica becomes thinner due to wetting and the parallel orientation of the E crystal stems. The densely branched structure and the different chain orientations of the E crystal stems in the first half polymer layer contacting mica are attributed to the strong interaction between the E block and mica, as revealed by our previous work. The width of branches was employed to analyze the kinetics of secondary crystallization. It is also found that the width of the branches and the velocity of crystal front decrease as the chain length increases.

Investigation of secondary crystallization of polymers by means of microbeam X-ray scattering

The kinetics of secondary crystallization during spherulite growth of isotactic poly(propylene) (iPP) and poly(vinylidene flouride) (PVF 2) is studied using a novel technique that employs a micron size X-ray beam. The data are combined with separate conventional simultaneous on-line SAXS/WAXS measurements and optical microscopy studies. In our experiments, crystallization takes place at low undercooling so that slowly growing large single spherulites are obtained. The data reveal that the main mechanism of secondary crystallization is the growth of new lamellae stacks within remaining amorphous regions. It is shown that a substantial amount of crystallites form as a result of secondary crystallization while the spherulite is growing. Furthermore, secondary crystallization is strongest directly behind the boundary of the spherulite and is independent of its size or growth state. A separate, off-line microfocus study on a quenched spherulite sample confirms this observation; the crystallinity is higher in the main body of the spherulite and lower near the boundary, where crystallization progressed to a lesser degree.

Bridging the PE lifetime under fatigue and creep conditions with its crystallization behavior

AbstractThe service lifetime for several linear polyethylene copolymers was studied by fatigue‐type accelerated tests. The material morphology and crystallization behavior were correlated with the lifetime and the failure modes. The correlation is based on the rate constant of material degradation (RCMD) recently introduced by the authors within a mathematical model for crack layer growth kinetics. RCMD is found to depend on the loading conditions (e.g., creep or fatigue) and on material morphology reflected in crystallization kinetics. The ratio of RCMDs for fatigue and creep is a scaling factor that allows one to correlate fatigue and creep lifetimes. The dependence of the RCMD's ratio on the morphological features associated with the primary and secondary crystallization kinetics is also reported in this paper. © 1993 John Wiley &amp; Sons, Inc.

Low temperature thermal aging of ethylene vinyl alcohol copolymers

AbstractThe crystallites of semicrystalline polymers formed at low temperatures are subject to partial partial melting and recrystallization resulting in a systematic increase in the size and perfection of the nascent crystallites. Differential scanning calorimetry (DSC) and x‐ray studies suggest that ethylene‐vinyl alcohol copolymers (EVOH), in particular, may undergo these secondary crystallization processes upon storage at temperatures far below Tm. DSC scans have been run on “as received” and quenched samples; the mole percent of vinyl alcohol in the copolymers studied was 0, 62, 70, 72, 82 and 100 percent. Tests on secondary crystallization kinetics, performed at different temperatures, further confirm the hypothesis of an increased degree of perfection (lamellar thickening) of the bulk crystallites achieved upon long term storage at temperature between Tg and Tm.