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Mike J Jenkins - One of the best experts on this subject based on the ideXlab platform.
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The melting of poly (l-lactic acid)
European Polymer Journal, 2018Co-Authors: Azizan A. Aziz, Mike J JenkinsAbstract:Abstract The effect of Crystallization temperature and time on the melting of a stereo-copolymer of l -lactic acid has been measured using differential scanning calorimetry and hot stage microscopy. The temperature of the last trace of crystallinity increased with time and temperature. Increasing the heating rate to 50 °C min−1 and correcting for thermal lag enabled the m.pt of the as-crystallized material to be determined without premature melting and reCrystallization of the sample on heating. The measured melting points enabled the equilibrium m.pt. of the copolymer to be determined as 205 ± 2 °C. Changes due to lamellae thickening occurred by Secondary Crystallization from the initial onset of Crystallization increasing with increasing temperature consistent with the process being diffusion controlled The addition of small chain segments emerging from the top lamellar surface occurred by reptation and growth increased with the square root of the lapsed time. This is consistent with the process of Secondary Crystallization.
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the effect of a Secondary process on Crystallization kinetics poly є caprolactone revisited
European Polymer Journal, 2016Co-Authors: Kate Phillipson, Mike J JenkinsAbstract:Abstract It is suggested that the overlap of primary and Secondary Crystallization is a cause of the repeated observation of non-integer n values in polymer Crystallization. The possibility that two concurrent Crystallization processes occurring during the Crystallization of poly (є-caprolactone), and polymers in general, may account for anomalous fractional values of the Avrami exponent, of no theoretical significance, is reconsidered using data from the recent evaluation of the kinetics of Crystallization which placed emphasis on evaluating the Secondary Crystallization stage. In general constant n values in excess of that expected for the Crystallization mechanisms could readily be interpreted in terms of the additional crystallinity developed by the Secondary process and these values increased commensurate with the rate constant of Secondary Crystallization.
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The effect of a Secondary process on Crystallization kinetics – Poly (є-caprolactone) revisited
European Polymer Journal, 2016Co-Authors: Kate Phillipson, Mike J JenkinsAbstract:Abstract It is suggested that the overlap of primary and Secondary Crystallization is a cause of the repeated observation of non-integer n values in polymer Crystallization. The possibility that two concurrent Crystallization processes occurring during the Crystallization of poly (є-caprolactone), and polymers in general, may account for anomalous fractional values of the Avrami exponent, of no theoretical significance, is reconsidered using data from the recent evaluation of the kinetics of Crystallization which placed emphasis on evaluating the Secondary Crystallization stage. In general constant n values in excess of that expected for the Crystallization mechanisms could readily be interpreted in terms of the additional crystallinity developed by the Secondary process and these values increased commensurate with the rate constant of Secondary Crystallization.
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the effect of Secondary Crystallization on Crystallization kinetics polyethylene terephthalate revisited
European Polymer Journal, 2016Co-Authors: Ziyu Chen, Mike J JenkinsAbstract:Abstract The possibility that two concurrent Crystallization processes occurring during the Crystallization of polymers may account for anomalous fractional values of the Avrami exponent, of no theoretical significance, is reconsidered using data from the recent evaluation of the kinetics of Crystallization of poly (ethylene terephthalate) which placed emphasis on evaluating the Secondary Crystallization stage. In general constant n values in excess of that expected for the Crystallization mechanisms could readily be interpreted in terms of the additional crystallinity developed by the Secondary process and these values increased commensurate with the rate constant of Secondary Crystallization. The difference in mechanisms of primary and Secondary arises from differences in the mechanism of chain segment diffusion by reptation and the free energies of nucleation of the two growth steps.
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The effect of Secondary Crystallization on Crystallization kinetics – Polyethylene terephthalate revisited
European Polymer Journal, 2016Co-Authors: Ziyu Chen, Mike J JenkinsAbstract:Abstract The possibility that two concurrent Crystallization processes occurring during the Crystallization of polymers may account for anomalous fractional values of the Avrami exponent, of no theoretical significance, is reconsidered using data from the recent evaluation of the kinetics of Crystallization of poly (ethylene terephthalate) which placed emphasis on evaluating the Secondary Crystallization stage. In general constant n values in excess of that expected for the Crystallization mechanisms could readily be interpreted in terms of the additional crystallinity developed by the Secondary process and these values increased commensurate with the rate constant of Secondary Crystallization. The difference in mechanisms of primary and Secondary arises from differences in the mechanism of chain segment diffusion by reptation and the free energies of nucleation of the two growth steps.
Benjamin S Hsiao - One of the best experts on this subject based on the ideXlab platform.
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Reversible de-intercalation and intercalation induced by polymer Crystallization and melting in a poly(ethylene oxide)/organoclay nanocomposite.
Langmuir, 2005Co-Authors: Ethan A. Ertel, Benjamin S Hsiao, Carlos A. Ávila-orta, Igors SicsAbstract:Semicrystalline polymer/layered silicate nanocomposites were prepared by solution blending of a low molecular weight poly(ethylene oxide) (PEO) with an organically modified montmorillonite, Cloisite 10A (C10A). The intercalation morphology was studied by temperature-dependent synchrotron wide-angle X-ray diffraction (WAXD). Unlike PEO homopolymers, significant Secondary Crystallization was observed in the PEO/C10A nanocomposites. Reversible de-intercalation and intercalation processes were detected during Secondary Crystallization and subsequent melting of Secondary crystals. On the basis of two-dimensional WAXD results on oriented samples, an interphase layer between the silicate primary particles and PEO lamellar crystals was proposed. Secondary PEO Crystallization in the interphase regions was inferred to be the primary driving force for polymer chains to diffuse out of the silicate gallery. This study provided a useful method to investigate polymer diffusion in nanoconfined spaces, which can be contro...
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effect of miscible polymer diluents on the development of lamellar morphology in poly oxymethylene blends
Journal of Polymer Science Part B, 1999Co-Authors: Benjamin S Hsiao, Bryan B Sauer, Edmund Arthur FlexmanAbstract:The development of lamellar morphology in poly(oxymethylene) (POM) and its miscible blends was studied by synchrotron time-resolved small-angle X-ray scattering (SAXS), during primary and Secondary Crystallization at temperatures near 150°C. The blends contained two different diluents: poly(vinyl 4-hydroxy styrene) [common name poly(vinyl phenol), (PVP)], which had a high glass temperature (Tg = 150°C), and styrene-co-hydroxy styrene oligomer (PhSO), which had a low glass temperature (Tg = −37°C). The SAXS data were analyzed by correlation function analysis to extract several lamellar parameters: long period (L), lamellar crystalline thickness (lc), amorphous layer thickness (la), and invariant (Q). The variation in Q defined the region where spherulites quickly grew and filled the entire space, and was referred to as the primary Crystallization dominant regime. A rapid drop in L and lc was observed at early times, and this can be explained by defective lamellar stacks filling in space between primary stacks, as Secondary crystals form during the nominal primary Crystallization dominant regime. Lamellar thickening with time in the long-time Secondary Crystallization region was observed in neat POM and the blend with 10 % low Tg diluent, while this process was inhibited with the high Tg diluent due to the higher Tg of the interlamellar species. A decrease in la at long times confirmed the lamellar thickening. We refer to the lamellar thickening process as a type of Secondary Crystallization. Interlamellar inclusion or trapping was detected to different degrees with the high Tg diluent, while exclusion was found for the low Tg diluent. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 3115–3122, 1999
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the nature of Secondary Crystallization in poly ethylene terephthalate
Polymer, 1999Co-Authors: Zhigang Wang, Benjamin S Hsiao, Bryan Benedict Sauer, William G KampertAbstract:Abstract The nature of Secondary Crystallization in poly(ethylene terephthalate) (PET) was examined during isothermal Crystallization and subsequent melting by time-resolved synchrotron small-angle X-ray scattering (SAXS), differential scanning calorimetry (DSC) and temperature modulated DSC (MDSC) techniques. In one experiment, the process of isothermal Crystallization was sustained over 72 h to induce a relatively large crystallinity (46%, by weight). The purpose of this experiment was to resolve the issue of controversial assignment for the crystal lamellar thickness (lc) by the correlation function analysis of the SAXS data. Results suggest that a two-stage decrease mechanism exists in both long period (L) and lc during isothermal Crystallization: (1) a significant decrease in the initial stage (primary Crystallization dominant), and (2) a much slower decrease in the later stage (Secondary Crystallization dominant) that is nearly linear with log time. We attribute this behavior to the formation of thinner separate stacks of lamellae between the primary stacks by Secondary Crystallization. Both Secondary and primary stacks can undergo a great deal of crystal perfection and rearrangement with time. From DSC measurements, a triple-melting behavior was observed in the samples crystallized at 205 and 215°C for 1 h, and a double-melting behavior at higher temperatures of 225 and 231°C for 2 h. Temperature scanning SAXS and MDSC directly characterize aspects of crystal perfection and melting. Consistent with some of the literature, we confirm that for short annealing (∼hour) at 200–220°C, the first (low) endotherm is related to melting of Secondary crystals, the middle endotherm is due to melting of primary crystals, and the third endotherm is due to melting of crystals reorganized during heating. With prolonged Crystallization at 231°C for 24 and 72 h, a single higher melting endotherm was observed even though SAXS experiments indicate a slight decrease in average lamellar thickness. In PET, ester exchange reactions contribute to unusual high mobility, allowing chains to avoid topological constraints such as entanglements and tie chains. The results suggest that the change in population of tie molecules in the non-crystalline phase reduces the entropy of melting causing an increase in Tm, and that this overwhelms the contribution of the decrease in lc.
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morphological changes during Secondary Crystallization and subsequent melting in poly ether ether ketone as studied by real time small angle x ray scattering
Macromolecules, 1996Co-Authors: Ravi Verma, Herve Marand, Benjamin S HsiaoAbstract:In this paper, we present results of morphological studies during long time melt Crystallization and subsequent melting in poly(aryl ether ether ketone) (PEEK). Morphological changes were monitored via small angle X-ray scattering (SAXS). SAXS data were analyzed via a combination of the correlation and interface distribution functions. Our analysis indicates the following: (1) The semicrystalline morphology is best described by a three-phase, dual lamellar stack model. Stacks of a finite number of lamellae and interlamellar amorphous layers are separated from each other by interstack regions of amorphous material (liquid pockets). (2) Secondary Crystallization occurs via the formation of Secondary lamellar stacks within the liquid pockets. Secondary lamellae are thinner than primary lamellae (70 A vs 120 A), and the amorphous layer thicknesses are about 47 A in both stacks. (3) The low endotherm observed during a heating scan is associated with the melting of the Secondary lamellae. (4) At room temperatu...
Ziyu Chen - One of the best experts on this subject based on the ideXlab platform.
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the effect of Secondary Crystallization on Crystallization kinetics polyethylene terephthalate revisited
European Polymer Journal, 2016Co-Authors: Ziyu Chen, Mike J JenkinsAbstract:Abstract The possibility that two concurrent Crystallization processes occurring during the Crystallization of polymers may account for anomalous fractional values of the Avrami exponent, of no theoretical significance, is reconsidered using data from the recent evaluation of the kinetics of Crystallization of poly (ethylene terephthalate) which placed emphasis on evaluating the Secondary Crystallization stage. In general constant n values in excess of that expected for the Crystallization mechanisms could readily be interpreted in terms of the additional crystallinity developed by the Secondary process and these values increased commensurate with the rate constant of Secondary Crystallization. The difference in mechanisms of primary and Secondary arises from differences in the mechanism of chain segment diffusion by reptation and the free energies of nucleation of the two growth steps.
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The effect of Secondary Crystallization on Crystallization kinetics – Polyethylene terephthalate revisited
European Polymer Journal, 2016Co-Authors: Ziyu Chen, Mike J JenkinsAbstract:Abstract The possibility that two concurrent Crystallization processes occurring during the Crystallization of polymers may account for anomalous fractional values of the Avrami exponent, of no theoretical significance, is reconsidered using data from the recent evaluation of the kinetics of Crystallization of poly (ethylene terephthalate) which placed emphasis on evaluating the Secondary Crystallization stage. In general constant n values in excess of that expected for the Crystallization mechanisms could readily be interpreted in terms of the additional crystallinity developed by the Secondary process and these values increased commensurate with the rate constant of Secondary Crystallization. The difference in mechanisms of primary and Secondary arises from differences in the mechanism of chain segment diffusion by reptation and the free energies of nucleation of the two growth steps.
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the effect of Secondary Crystallization on melting
European Polymer Journal, 2013Co-Authors: Ziyu Chen, Mike J JenkinsAbstract:Abstract 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.
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The kinetics of Crystallization of poly(ethylene terephthalate) measured by FTIR spectroscopy
European Polymer Journal, 2013Co-Authors: Ziyu Chen, Mike J JenkinsAbstract:Abstract The kinetics of Crystallization of poly(ethylene terephthalate), PET, have been measured by FTIR spectroscopy using the absorbances of the crystalline and amorphous phase carbonyl bands at 1717 and 1727 cm −1 to determine the fractional crystallinity as a function of time and isothermally over the temperature range 230–240 °C. Both primary and Secondary Crystallization could be measured with similar accuracy limited only by the length of time over which the measurements were made. The primary and Secondary Crystallization time dependences were fitted to Avrami equations with n values of 2.0 and 1.0, respectively. The initial primary process was interpreted as due to two-dimensional spherulites restricted early in the Crystallization by the thickness of the film sample, 1.5–3.0 μm. Secondary Crystallization was considered to be due to one dimensional thickening of lamellae produced in the initial production of the spherulites. FTIR spectroscopy was considered to have some advantages in measuring the development of crystallinity over other techniques, such as DSC and DTA, which measure the rate of Crystallization and are limited by the sensitivity of the calorimeter to measure the rate of heat evolution.
Christian Riekel - One of the best experts on this subject based on the ideXlab platform.
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investigation of Secondary Crystallization of polymers by means of microbeam x ray scattering
Polymer, 2001Co-Authors: R Kolb, Christoph Wutz, Norbert Stribeck, G Von Krosigk, Christian RiekelAbstract:Abstract 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.
Ravi Verma - One of the best experts on this subject based on the ideXlab platform.
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morphological changes during Secondary Crystallization and subsequent melting in poly ether ether ketone as studied by real time small angle x ray scattering
Macromolecules, 1996Co-Authors: Ravi Verma, Herve Marand, Benjamin S HsiaoAbstract:In this paper, we present results of morphological studies during long time melt Crystallization and subsequent melting in poly(aryl ether ether ketone) (PEEK). Morphological changes were monitored via small angle X-ray scattering (SAXS). SAXS data were analyzed via a combination of the correlation and interface distribution functions. Our analysis indicates the following: (1) The semicrystalline morphology is best described by a three-phase, dual lamellar stack model. Stacks of a finite number of lamellae and interlamellar amorphous layers are separated from each other by interstack regions of amorphous material (liquid pockets). (2) Secondary Crystallization occurs via the formation of Secondary lamellar stacks within the liquid pockets. Secondary lamellae are thinner than primary lamellae (70 A vs 120 A), and the amorphous layer thicknesses are about 47 A in both stacks. (3) The low endotherm observed during a heating scan is associated with the melting of the Secondary lamellae. (4) At room temperatu...