Creep failure in highly oriented nylon 6 fibers

Creep failure in oriented nylon 6 fibers has been studied. The results suggest that the variations in the lifetime under various loading histories are inherent, but statistical, characteristics of the material itself. The treatment of experimental data by a stochastic theory shows that the creep failure can be regarded as a nucleation process. An interpretative analysis of the structural changes during creep indicates that the nucleation is brought about by bond rupture in the amorphous regions of the fiber structures.     

Birefringence of highly oriented fibers

A spectrophotometric method was developed for measuring the birefringence of highly oriented synthetic fibers. This method surmounts the low birefringence limit of the standard quartz compensator method and the difficulties in interpretation of the photographic fringe method. A highly oriented aramid fiber gave birefringence values of 0.60–0.75 by this method, compared with 0.25 for polyester and 0.06 for nylon by other conventional methods. The operating principles and excellent results of this new method provide a basis for the extension of routine birefringence characterization to highly oriented fibers.     

Fracture behavior in nylon 6 fibers

A reaction rate model of fracture in polymer fibers is described. This model assumes that bond rupture is governed by absolute reaction rate theory with a stress-aided activation energy. It is demonstrated that the key in obtaining good agreement between the model and experiment lies in taking proper account of the variation of stress on the tie-chain molecules. The more taut chains rupture first, and the load is redistributed among the remaining unruptured tie chains. The effect of varying the temperature both in the model and in experiments on fracture in fibers is explored. Good agreement between predictions of the model and experiment is possible only with an undeterstanding of the distribution in stress on the tie chains. The distribution in stress on the chains was experimentally determined by monitoring the kinetics of bond rupture with electron paramagnetic resonance (EPR) spectroscopy. Temperature is found to have two effects on macroscopic strength. (1) The thermal energy aids the atomic stress in breaking the atomic bonds; as a consequence the rate of bond rupture of a family of bonds under a given molecular stress is increased. In this respect temperature might be viewed as decreasing the “strength” of a bond. (2) Temperature also serves to “loosen” the molecular structure and in this way modify the distribution in stress on the tie chains. To explain bond rupture and macroscopic fracture behavior quantitatively, account must be taken of both effects.     

Domain structure of nylon 6 fibers

Stained and unstained sections of nylon 6 fibers are examined by means of transmission electron microscopy. Data are presented regarding dimensions and shape of macrofibrils, microfibrils, amorphous, and crystalline domains of the microfibril and the spacing between the microfibrils. The new results support the conclusions of a previous SAXS and diffusion study carried out with the same fibers.     

A phenomenological study of light transmitted through oriented nylon fibers

The polarization behaviour of light transmitted through a single layer of oriented parallel set of nylon fibers immersed in a neutral oil, has been experimentally investigated. A phenomenological theory based on the Mueller matrix algebra has been proposed. The mechanism of dipolar radiations from the polymeric fibers, has been explained with the help of the matrix formulation. Experimental results are in good agreement with the proposed theory.     

Mechanical relaxations and moduli of oriented nylon 66 and nylon 6

The mechanical relaxations of dry and wet nylon 66 and nylon 6 with draw ratios λ = 1–3 have been studied from ?180 to 160°C and in the frequency range of 1 Hz to 10 MHz. The five independent elastic moduli C_11, C_12, C_13, C_33, and C₄₄ have also been determined by an ultrasonic method at 10 MHz. Wide-angle x-ray diffraction and birefringence measurements reveal that the crystalline orientation rises sharply at low λ and becomes saturated near λ = 3; the amorphous orientation function increases continuously, reaching values of 0.3–0.5 at λ = 3. The alignment of molecular chains and the presence of taut tie molecules in the amorphous regions lead to a lowering of segmental mobility, thereby reducing the magnitude and increasing the peak temperature and activation energy of the α relaxation. Water absorption weakens the interchain bonding and so gives rise to effects opposite to those of drawing. At low temperature, the development of mechanical anisotropy is largely determined by the overall chain orientation, with the c-shear mechanism contributing a small additional effect. However, above the α relaxation, where the amorphous region is rubbery, the stiffening effect of taut tie molecules becomes dominant and leads to increases in all moduli.     

Drawing of nylon 6 fibers (bristles)

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.     

Bond rupture in highly drawn nylon 6 fibers influence of strain rate on radical concentration and permanent deformation

Summary In high-drawn nylon 6 fibers the radical concentration caused by the rupture of the tie molecules was measured by ESR at different strains and strain rates, viz. 2.5-500% min^–1.The spectral shape of the radical was temperature-dependent. This was tentatively explained from the fact that the -methylene protons of the secondary radical of nylon 6, (-CO-NH-(aHa-CoH2a) stop rotating at liquid nitrogen temperature.Furthermore the radical concentration depends on the strain rate, and consequently on time, which is explained by assuming that the tie molecules have a length distribution. So it follows that the process is of a viscoelastic nature and has little to do with thermally activated bond rupture. The assumption that the tie molecules have a length distribution is also supported by the value of the elastic modulus which, in contrast with what is sometimes assumed (28), is already accounted for by the relatively few taut tie molecules.Permanent deformation which, like chain rupture also an irreversible process, has been measured as a function of strain and strain rate. Roughly, a linear correlation seems to be present between the number of radicals and the magnitude of the permanent deformation.

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Zusammenfassung In hochverstreckten Nylon-6-Fasern wurde die durch das Zerreißen von Molekülketten entstandene Radikalkonzentration mit Hilfe von ESR bei einer unterschiedlichen Dehnung und bei Dehnungsgeschwindigkeiten zwischen 2,5 und 500% min^–1 gemessen. Die Form des Radikalspektrums war temperaturabhängig. Dies ließ sich aus der Tatsache erklären, daß die -Methylenprotonen des sekundären Radikals von Nylon 6, (-CO-NH-CH_ga-CßH₂), bei der Temperatur von flüssigem Stickstoff zu rotieren aufhören.Weiter ist die Radikalkonzentration von der Dehnungsgeschwindigkeit und somit von der Zeit abhängig. Dies konnte aus der Annahme erklärt werden, daß die Tie-Moleküle eine Längenverteilung haben.So folgt, daß der Prozeß viskoelastischer Natur ist und er wenig mit thermisch aktivierter Bindungsspaltung zu tun hat. Das Vorhandensein einer Längenverteilung für die Tie-Moleküle wird auch durch den Wert des elastischen Moduls unterstützt, der anders, als manchmal angenommen wird (28), schon aus den relativ wenig gespannten Verbindungsmolekülen erklärt werden kann.Die bleibende Deformation, ebenso wie die Kettenspaltung ein irreversibler Prozeß, wurde als Funktion von Dehnung und Dehnungsgeschwindigkeit gemessen. Annähernd gibt es eine lineare Korrelation . zwischen der Radikalkonzentration und der Größe der bleibenden Deformation.

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With 6 figures 2 tables     

Structure and formation mechanism of melt‐drawn highly oriented polymer fibers

Well‐separated and parallel aligned fibers of various polymers have been prepared by a simple but effective melt‐drawing procedure, and their structural features have been studied with field‐emission scanning electron microscopy. The results show that the resulting polymer fibers, with diameters ranging from tens of nanometers to hundreds of nanometers, consist of highly oriented lamellar or fibrillar crystals with the molecular chains aligned in the drawing direction. Scanning electron microscopy images of the drawing process indicate that drawing a thin polymer molten layer at temperature far above its melting point leads to the formation of elongated microcracks. The microcracks embedded in the polymer thin film propagate along the drawing direction and result in the formation of polymer microfibers, which split continuously under high instantaneous stresses and produce well‐separated polymer fibers with diameters on the nanometer scale. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2703–2709, 2004     

Polymerization in the crystalline state. X. Solid-state conversion of 6-aminocaproic acid to oriented nylon 6

When single crystals of 6-aminocaproic acid (ACA) are heated about 30°C below their melting point, polycondensation to nylon 6 takes place. The polymer crystallites are biaxially oriented towards each other and the relation between their orientation and that of the parent monomer crystal has been clarified. The kinetics of the process are characterized by three stages, (a) an induction period, (b) a stage in which monomer disappears at a constant rate while polymer of relatively low molecular weight is formed, and (c) a slow polycondensation of the polyamide chains after exhaustion of the monomer. Oligomer concentrations were below detectable limits at all stages of the process. Addition of monomer to the polyamide was retarded when ACA was kept from reaching its equilibrium vapor pressure (0.12 mm Hg at 170°C) by condensation on a cool surface or when an inert gas was admitted to the system. This was interpreted as suggesting that ACA is transported through the vapor phase to the propagating polyamide. A number of surfaces catalyzed the polycondensation of ACA vapor, but nylon 6 formed in this way on KCl crystals exhibited no preferred orientation. The linear dimer and trimer of ACA were also found to condense to nylon 6 in the crystalline state, although at a slower rate than the monomer. The solid-state polycondensation of these oligomers was accelerated when they were exposed to the vapor of the monomer. Solid-state polycondensation of single crystals of the linear dimer led also to biaxially oriented nylon 6.     

Wideline NMR studies of oriented nylon 66

Wideline NMR techniques have been applied to both singly and doubly oriented specimens of nylon 66. Variations in the spectra obtained are observed for different orientations of the specimens relative to the applied field. These variations demonstrate that the zigzag chain axis is essentially parallel to the draw direction and that motion occurs in all of the sample. The motion is of two types: random in the mobile regions and rotation of segments about the chain axis in the rigid regions.     

Production and properties of fibers spun from nylon 6/lithium chloride mixtures

The possibility of producing high-modulus nylon 6 fibers by incorporation of lithium chloride (LiCl) in the polymer prior to spinning and drawing has been examined. Samples containing 2% and 4% LiCl (w/w) together with an unsalted control were studied. Particular attention was given to optimizing the spinning process by varying the melt temperature and the draw-down. The spun fibers were subsequently drawn in a tensile testing machine at 135°C, preliminary studies having established that this was desirable for the production of high-modulus material. The influence of annealing after drawing was also examined. Drawn fiber moduli in the range 8–9 GPa were obtained, compared with ca. 5–6 GPa for unsalted material. Limited structural studies (birefringence and wide-angle x-ray diffraction) suggest that the enhancement of modulus is due to an increase in the stiffening effect of extended molecules in the noncrystalline regions. Dynamic mechanical measurements show that there is reduced chain mobility in the disordered regions of the polymer, suggesting strong polymer-ion interactions. The salt can be readily removed by washing the fibers in boiling water, with significant reduction in moduli. This militates against commercial application of the salted fibers.     

High-temperature annealing of drawn nylon 66 fibers

The structure and morphology of highly oriented fibers were modified by thermal treatments at varying tensions. The structural changes were characterized by wide- and small-angle x-ray diffraction, broadline nuclear magnetic resonance, and electron microscopy of surface replicas. Increasing temperatures caused increases in local ordering, number of regular chain folds, mobile fraction or amount of fluidlike mobility, and surface recrystallization in localized areas. Each of these changes was also a function of the amount of tension on the fiber during the annealing; each change was maximized when the fiber was free to shrink but minimized when the fiber was stretched.     

Annealing experiments on drawn nylon 66 fibers

Drawn, nylon 66 yarns have been annealed in oil under relaxed conditions. Shrinkage and tensile modulus were measured and the yarns were examined by wide- and small- angle x-ray diffraction. The results are similar to those obtained by Dismore and Statton, but there are significant differences. The data indicate a model for a drawn nylon 66 fiber in which substantial amounts of folds remain.     

Effect of processing consitions on the development of morphology in clay nanoparticle filled nylon 6 fibers

The effect of melt temperature on the phase behavior and preferential orientation development in Nylon 6/montmorillonite nanocomposites were investigated at melt spinning temperatures ranging from 230° to 250°C. The fibers were found to exhibit mostly γ crystalline form that is typical of Nylon 6 filled with montmorillonite nanoparticles. At higher take-up speeds α-crystals begin to appear in the crystalline phase. The presence of nanoparticles was found to impart substantial chain orientation levels even at low to moderate take up speeds reaching a plateau at moderate take up speeds. This was attributed to the increased spin line stress in the presence of nanoparticles that increase the overall viscosity due to their large contact areas with the polymer chains. This increased spinline tension was found to cause fiber breakup at moderate speeds. Increasing melt temperature from 230° to 250°C alleviated this problem.     

The spectral dispersion curves of highly oriented fibres

Spectral dispersion curves of the refractive indices and birefringence of highly oriented fibres [poly(ethylene 2,6-naphthalene-dicarboxylate) (1000 denier/248 filaments, PEN-Q50M4; PEN), poly(ethylene terephthalate) (PET), Seun yarn (meta-aramid fibres, Teijin Japan; CONEX) and Technora T-240 (1000 denier/667 filaments aramid fibres, Teijin Japan; TECHNORA)] have investigated using the automatic variable-wavelength interferometric (VAWI) technique. This technique is especially recommended for measuring the refractive indices of highly oriented fibres. The polarizabilities per unit volume are calculated for these fibres and the molecular orientation function of PEN and PET are determined. Microinterferograms are given for illustration.     

Interatomic correlation in partially oriented polymer fibers

The atomic ordering in materials possessing cylindrical symmetry can be investigated by an approximation of the cylindrical distribution function in a given direction. The approximation gives correctly the extent of the interatomic correlation for a wide class of polymer fibers and reduces considerably the amount of the experimental and computational times.     

The glass transition behaviour of salted nylon 6

Summary Glass transition measurements of nylon-6/lithium halides mixtures have been carried out in wide range of frequency with the aid of different experimental techniques.The results show an increase of the glass transition temperature when the salt is present and prove the larger effectiveness of lithium chloride with respect of lithium bromide.This effect, in line with the large reduction of the specific volume caused by the salt, is due to the formation of a pseudo-cross-linking between lithium ions and the carbonyl-oxygen groups of the polyamide.