Summary: The crystallization behavior of nylon‐6 and nylon‐6/montmorillonite nanocomposite films with different heat histories was investigated by wide‐angle X‐ray diffraction (WAXD). For nylon‐6 films isothermally crystallized above 170 °C or annealed at 200 °C and then quenched in ice water, a crystalline peak appeared at 2θ = 28.5°. This crystalline peak was strong in intensity for the former and weak for the latter. However, for nylon‐6 films cooled in air after isothermal crystallization or annealing, no crystalline peak at 2θ = 28.5° was observed in the WAXD patterns. For nylon‐6/montmorillonite nanocomposite films annealed above 140 °C, a crystalline double peak was observed between the α1 and α2 peaks. The possible origins of the peak at 2θ = 28.5° and the crystalline double peak are discussed.
WAXD patterns of isothermally crystallized nylon‐6/montmorillonite nanocomposite films. 相似文献
Structure of PAN fibers during pre-oxidation and carbonization was studied using two dimensional small angle X-ray scattering/wide angle X-ray diffraction(2D SAXS/WAXD).The SAXS results show that during pre-oxidation between 180 ℃ and 275 ℃,the volume content of microvoids increases with the temperature increasing,which may be one of reasons for the decrease of tensile strength of pre-oxidized fibers.253 ℃ was the critical transition temperature,the length,diameter,aspect ratio and orientation distribution of microvoids increased with temperature before this temperature and decreased after this temperature.After the high temperature carbonization,lots of spindly microvoids formed.WAXD patterns demonstrate that the crystallite size of PAN fibers first increased before 230 ℃ and then decreased with the increase of temperature during the pre-oxidation.The diffraction peak of PAN fibers at 2θ≈ 17° almost disappeared at the end of preoxidation while the diffraction peak of aromatic structure at 2θ≈ 25° appeared at 253 ℃.During carbonization,the peak intensity at 2θ≈ 25° increased apparently due to the formation of graphite structure.The results obtained give a deep understanding of the microstructure development in the PAN fibers during pre-oxidation and carbonization,which is important for the preparation of high performance carbon fibers. 相似文献
The melting process of constrained nylon 6 fibers has been studied to estimate the true melting point of its original crystals.
The melting peak became simpler in shape and shifted to higher temperature with increasing fiber-axis restricting force. When
heating rate, β, was increased, the temperature where the melting curve initially departs from its baseline, Tsm, decreased
steeply in the range of 45 to 60°C min-1, and increased linearly with increasing β above 60°C min-1. By linear extrapolation of Tsm to 0°C min-1, the temperature of ca 190°C was obtained for the melting temperature of the original nylon 6 crystals. This seems to correspond
to the zero-entropy-production melting of the most imperfect crystallites of the nylon 6 fabric.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
The effect of melt annealing over a wide range of temperatures (240–300°C) and durations (0.5–120 min) on the induction period and the heat of crystallization of nylon 6 was studied by means of differential scanning calorimetry. It was found that the crystallization characteristics are determined by the total annealing time experienced by the melt at a given temperature; i.e., a cumulative effect on the erasure of the melt memory is observed. Complete destruction of the crystalline nuclei is achieved after annealing for 90 min at 280°C, the equilibrium melting point of nylon 6; the nuclei cannot be regenerated by nonisothermal crystallization. All the effects observed are explained by the existence of a wide distribution of size and perfection of nuclei. 相似文献
Electrospun nylon-6 fibers were prepared from its polyelectrolyte solution in formic acid with different concentrtaions. In situ Fourier transform infrared (FTIR), wide-angle X-ray diffraction and small-angle X-ray scattering (SAXS) were performed on the nylon-6 fibers heated to various temperatures until melting. For comparison, stepwise annealing of the solution-cast film having exclusively the α-form was also carried out to elucidate the structural evolution. Our results showed that Brill transition in the electrospun fibers occurs at a lower temperature than that in the solution-cast film due to the crystal size difference. Differential scanning calorimetry heating traces on the as-spun fibers exhibited a unique crystalline phase with a melting temperature of ~235?°C, higher than the equilibrium melting temperature of nylon-6. The content of high melting temperature (HMT) phase increased with increasing nylon-6 concentration; a maximum of 30?% of the fiber crystallinity was reached for fibers obtained from the 22?wt.% solution regardless of the heating rates used. Based on the SAXS and FTIR results, we speculated that the HMT phase is associated with thick α-form crystals developed from the highly oriented nylon-6 chains that are preserved in the skin layer of the as-spun fibers. A plausible mechanism for the formation of the skin/core fiber morphology during electrospinning was proposed. 相似文献
A commercial main-chain liquid-crystalline, naphthalene-based polyesteramide, was studied by three experimental techniques: extrusion capillary rheometry, dynamic viscoelasticity, and differential scanning calorimetry (DSC). From capillary rheometry a maximum at 360°C was observed in viscosity temperature curve. This result is compared with literature data for other thermotropics, and the possibility of a transition from a nematic to an isotropic phase is considered. The results obtained from dynamic viscoelasticity and DSC agree, and reveal the existence of a glass transition at 128°C (by DSC) and 137–147°C (by viscoelastic measurements, depending on frequency) as well as melting at 282°C. Annealing below 230°C produces two types of crystals, whereas annealing above this temperature gives rise to only one type of crystal, the melting temperature of which can be, depending on annealing time, as high as 340°C. The results are compiled in a phase diagram with six regions, four of them corresponding to the solid state, one to a liquid mesophase, and one to an isotropic phase. 相似文献
Summary: Wide‐angle X‐ray diffraction (WAXD) was used to investigate the crystal transition of nylon‐12,12 under annealing and drawing. The triclinic α‐form could be obtained by crystallizing from a melted state or by annealing the γ‐form at high temperature (above 150 °C). The crystal structure of the α‐form annealed at 90 °C didn't change with time except for the perfection of crystals and an increase in the degree of crystallinity. The pseudo‐hexagonal γ‐form could be produced by crystallizing from the melted state at low temperature (90 °C) or by drawing at 90 and 160 °C. This is the first time a Brill transition has been observed under drawing conditions, instead of under the traditional conditions of continuous heating and cooling. Experimental results also confirm that drawing inducement is preferable to produce the γ‐form and plays an important role in determining the crystal structure; there is a competition between drawing inducement and thermal inducement.
The WAXD patterns of the nylon‐12,12 α‐form drawn different drawing ratios at 90 °C. 相似文献
By means of electron microscopy of surface replicas and both small-angle and wide-angle x-ray scattering, nylon 6 fibers were investigated in the as-spun state, after drawing at 180°C to a draw ratio up to 4.95, and after subsequent annealing. As spun, the fiber exhibits a small fraction of row-nucleated cylindrites and a great many spherulites (with an average diameter of a few microns) side by side. Drawing deforms the spherulites into spindle-shaped structures (λ = 2) and subsequently produces well-aligned microfibrils. Small-angle x-ray scattering yields a two-point diagram at small λ and a fourpoint diagram at high λ. The long period seems to decrease slightly with draw ratio. Annealing at temperatures above the temperature of drawing increases the long period to a greater extent with samples of lower λ. The crystal lattice orientation is nearly complete at λ = 4.95. 相似文献
Structural changes in poly(L -alanine)(PLA), a model compound related to tussah silk fibroin, induced by heat treatment have been studied by differential thermal analysis, x-ray diffraction, and infrared spectroscopy. PLA heated below 300°C shows x-ray patterns very similar to those of the α-helix crystalline phase, in addition to the diffraction patterns due to the β structure. Samples heated at 368°C exhibit predominantly the diffraction patterns due to the β crystalline phase. From infrared spectra, PLA samples heated below 280°C are found to be composed of all three molecular conformations: β sheet, random coil, and α helix. The intensity of the infrared absorption at 650 cm?1 (amide V) assigned to the random coil conformation is decreased by heat treatment above 325°C. The content of the β-sheet structure remains almost constant when the specimen is heated below 325°C, and increases abruptly on heating to around 335°C, while the random coil content is decreased abruptly by heat treatment above 335°C. The α-helix content does not change, regardless of heat treatment. It is suggested therefore that the random coil conformation of PLA transforms directly into the β sheet on heat treatment above 335°C. 相似文献
Temperature-reversible and -irreversible morphological events could be separated in the case of linear polyethylene during quasi-isothermal crystallization by using simultaneous temperature-modulated synchrotron SAXS and WAXD. Crystallization and subsequent annealing was followed at 126 °C for 90 min while applying a temperature modulation with an amplitude of 1 °C and a period of 2 min. The crystal growth rate associated with the irreversible part of the crystallization decreases with increasing temperature in a cycle. The crystalline lamellae irreversibly thicken with time. The actual crystallite thickness, however, exhibits a superimposed modulation out of phase with that of the temperature modulation. Melting was studied during heating at 1 °C/min after cooling at 10 °C/min. A temperature modulation was superimposed with an amplitude of 2 °C and a period of 3 min. Once again temperature-reversible crystal thickness changes and irreversible crystal thickening could be observed. 相似文献