Melting behavior and morphology of drawn nylon 6

Thermal analysis has been carried out on drawn nylon 6 filaments annealed at various temperatures between 150 and 210°C and then methoxymethylated to various degrees. It is shown that the melting point inherent to the morphology of drawn nylon 6 can be obtained from samples in which the reorganization of defect crystallites in the course of thermal analysis is prevented by a proper degree of methoxymethylation of amorphous regions. The melting point thus obtained is in linear relation with the reciprocal crystallite size in the direction of fiber axis which has been obtained from small-angle x-ray data and crystallinity. The extrapolation and the slope of this linear relation give the equilibrium melting point of nylon 6 as 245°C and an end-surface free energy of 42 erg/cm². The results seem to provide strong support for the presence of chain-fold surfaces in the drawn and annealed polymers.     

Origin of double melting peaks in drawn nylon 6 yarns

The differential scanning calorimetry (DSC) melting curves of drawn nylon 6 were studied from the standpoint of reorganization of the crystals during the heating process. A new method was presented to obtain the DSC curve associated with the growth and melting of the original crystals, and that with the recrystallization and final melting process, separately. The results obtained show that, in the case of a heating rate of 10°C/min, the original crystals in the sample start perfecting themselves at temperatures far below their initial melting temperature and melt out below 222°C, recrystallization starts at about 210°C, and the newly emerged crystals melt out at 228°C. The superposition of two such constructed DSC curves reproduces the observed DSC curve well. Therefore, the double melting peaks of the sample are considered to be the result of superposition of three processes which occur successively during heating; perfection of the original crystals, melting of the perfected crystals concurrently with recrystallization, and melting of the recrystallized crystals.     

Effect of annealing conditions on the structure of drawn nylon 66 yarns

Differences in structural parameters, shrinkage, and retractive forces have been compared for nylon 66 tire yarns annealed at several temperatures in silicone oil and in air. It is found that significantly different structural changes occur for oil-annealing and air-annealing. Retractive forces measured in hot oil are characterized by a high initial force, followed by a rapid decay and a second, more gradual, increase which is also followed by a decay at sufficiently high temperatures. In heated air, the first, short-term, retractive force is absent. It is postulated that these differences are due to the different rates of heat transfer at gas–solid and at liquid–solid interfaces, and that rapid heat transfer (as in the case of oil-annealing) promotes two mechanisms of molecular change which are characterized by different degrees of structural parameter change, and by different amounts of shrinkage.     

Melting behavior of nylon 6 fiber in textiles

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.     

Melting behavior of drawn polypropylene

The melting behavior of drawn, compression-molded isotactic polypropylene has been ex-amined in terms of the infuence of drawing conditions on the observed properties. Two endothermic peaks were observed on differential scanning calorimetry (DSC) for samples when high draw ratios and high heating rates were used during DSC tests. The peak at lower temperature is influenced by draw ratio, temperature, and rate, and exhibits a strong superheating effect. The species associated with this peak can partially recrystallize into another species associated with the peak at higher temprature during DSC measurements. The position of the peak at higher temperature depends only on draw ratio. It is proposed that the doubel-melting peaks at lower and higher temperature result from extremely thin quasi-amorphous or crystalline layers between microfibrils and the lamellar crystals within microfibrils, repectively. © 1993 John Wiley & Sons, Inc.     

Solid-state 13C NMR characterization of molecular orientation of hot drawn nylon 6

Two-dimensional cross polarization (CP), magic angle spinning (MAS) rotor synchronization NMR spectroscopy has been used to determine quantitatively the molecular orientational distribution function on hot-drawn Nylon 6. Both films and fibers are studied that had been thermally deformed at temperature above T_g, from 60 to 175°C at draw ratios in the range of 1-5.5. In the two-dimensional NMR spectrum, the sidebands that intrinsically originate from the chemical shift anisotropy reveal the degree of molecular orientational order. No preferential orientational order is detected for the sample without thermal deformation, and the highest degree of order is observed for samples which have been hot drawn above T_g at ratios ca. 5. Based on the aggregate model the maximum achievable order parameters are determined. © 1994 John Wiley & Sons, Inc.     

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.     

Five-line model for the description of radial x-ray diffractometer scans of nylon 6 yarns

Types of nylon 6 yarn which contain large γ crystals cannot be described by a model comprising three Pearson VII curves, as we formerly proposed. Hence, a new extended computer program has been developed containing five Pearson VII lines. Two lines are related to the α phase, two to γ-phase crystals, and the fifth describes the amorphous contribution. Some physical relations between parameters could be derived and were incorporated into the computer program. This program provides extremely good fits to all equatorial diffractometer scans which can be obtained from nylon 6 yarns, including those containing large γ crystals. The parameter accuracy is quite satisfactory for diffractometer traces with well-resolved peaks from both α and γ crystals. However, if the traces are poorly resolved or the samples do not contain a sufficient amount of the γ modification, resulting in a lack of detailed information, the parameter accuracy can be rather poor. For those cases special versions of the computer program were developed which contain further empirical interrelations between parameters. With these programs crystal sizes and molecular distances within the crystals of both the α and β modifications can be determined. The method has also been applied to radial scans at various small azimuthal angles with respect to the equator. In that way the relative amounts of the α and γ crystals as well as their orientation factors could be determined. Various aspects of this investigation are illustrated by some examples.     

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.     

Melting behaviour of gelspun/drawn polyolefins

The melting behaviour of gelspun/drawn UHMW-PE and UHMW-PP fibres was investigated. Unconstrained UHMW-PE and UHMW-PP fibres melt at 142°C and 170°C, respectively. Upon constraining, by holding the fibres at a fixed length or by embedding the fibres in a matrix, an increase in the melting temperature of both fibres is observed. In the case of UHMW-PE fibres a solid-solid phase transition in polyethylene at 155°C from the orthorombic to the hexagonal crystal structure occurs. Above 155°C, the fibres can not sustain any load. This solid-solid phase transition at 155°C sets an upper limit to both the maximum curing and continuous use temperature of PE-fibre reinforced composites. In gelspun/drawn UHMW-PP, such a detrimental solid-solid phase transition is absent, and therefore the increase in melting temperature can be utilized effectively. For example, heating of UHMW-PP fibres for 30 minutes at 200°C does hardly affect the room temperature Young's modulus and tensile strength if the fibre is constrained during heating.     

Microstructural rearrangement during heat treatment of drawn nylon 66 fiber

Melt-spun nylon 66 fibers were drawn and subsequently heat treated isothermally and quenched. The heat-treated fibers were then examined by wide-and small-angle x-ray scattering (WAXS and SAXS), by differential scanning calorimetry (DSC), and by static mechanical testing. These measurements allow one to follow microstructural changes taking place during the course of the heat treatment. WAXS results show that as the treatment progresses, the crystallites become both more perfect and more disoriented with respect to the fiber axis. SAXS results show crystallite thickening. DSC results show that the melting point increases, goes through a maximum, and then decreases as the heat treatment progresses. The tensile modulus decreases with time to an asymptotic level. The changes in crystallite perfection and thickness occur more rapidly than do the changes in crystallite orientation, modulus, and melting point. A model is proposed whereby the two time frames are related to intracrystalline and intercrystalline processes, respectively.     

Melting behavior of drawn films from polyethylene single-crystal mats

Drawing of mats of linear polyethylene single crystals prepared from dilute solution is possible at temperatures above about 90°C. The structure and properties of the drawn specimens are much different from those ordinary drawn bulk polymer. Drawn mats have been investigated by differential scanning calorimetry. The characteristic experimental results are: (a) a broad melting curve, (b) considerable superheating depending on the rate of heating, (c) constancy of the melting point and the heat of fusion with annealing, (d) deviation from the relation between the heat of fusion and the density obtained for the drawn bulk specimens, (e) appearance of two melting peaks in samples annealed at temperatures above about 130°C. These results imply that the structure of the drawn mat is characterized by a larger number of the tie chains connecting the neighboring crystals (the structure postulated in earlier papers) than is the case in ordinary drawn bulk polymer. It can be concluded that the transformation of a fringed micellar type of structure to the folded lamellar structure may be difficult during annealing unless crystals melt and then recrystallize during cooling.     

On the refractive indexes and birefringence of nylon 6 yarns as a function of draw ratio and strain

A systematic study of the dispersion curves of the refractive indexes of nylon 6 yarns was made. The parameters were the draw ratio and strain. The measurements show that the dispersions of the refractive indexes n_∥ and n_⊥, parallel and perpendicular to the fiber axis, are equal, independently of draw ratio and strain. The average dispersion equals n_F ? n_C = 109 × 10^?4. Consequently, the birefringence is, within experimental accuracy, independent of the wavelength. The refractive indexes and the birefringence show a change in trend at 10–12% strain. This point corresponds to the yield strain in the stress–strain diagrams. The inference is that beyond the yield point the overall molecular orientation must increase less strongly with strain than before. An analysis shows that the Lorentz–Lorenz relation holds for the average refractive index n? = ? (n_∥ + 2n_⊥). So the change in n? versus draw ratio is mainly due to the change in density. By applying the Lorentz–Lorenz relation to the change of n? on straining, a value of Poisson's ratio (μ) could be derived. The average value found for nylon 6 yarns was μ = 0.48, which means that the density hardly changes with strain.     

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     

NMR observations of drawn polymers. VIII. Doubly oriented nylon 66

A wide-line NMR study of chain segmental motion in nylon 66 has been made on a rolled sheet having “double orientation.” In this sheet the crystallite c axis, i.e., the molecular chain axis, is oriented preferentially along the roll direction, and the crystallographic (010) plane lies predominantly parallel to the roll plane, or the plane of the sheet. The direction of the applied magnetic field with respect to the sheet is characterized by two angles, the polar angle γ subtended by the roll direction and the magnetic field, and an azimuthal angle ?. NMR spectra were taken at various values of the angles γ and ? and at three temperatures ?196°C, 20°C, and 180°C. The second moments of the absorption spectra taken at 180°C were compared with theoretical predictions of second moments based on two models for the high-temperature segmental motion (called the α_c process) in crystalline regions of nylon 66. One model consists of rotational oscillation with amplitudes δ of segments around their axies. The second model is denoted 60° flip-flop motion and consists of rotational 60°C jumps of the segments around their axes between two equilibrium sites with the possibility that the segments also oscillate with a general amplitudes δ around each site. The experimental results are consistent with fairly large amplitudes δ, in which case both models approach the limiting case of full segment rotation. For this reason the experiments do not allow a distinction between the two models. From the second moments at ?196°C and 20°C the decrease in second moment due to the low temperature segmental motion, the γ process, is obtained. This motion occurs in noncrystalline regions of nylon 66 and is found to cause a decrease in second moment which is strongly dependent on the two angles γ and ?, implying double orientation of the noncrystalline segments. It is suggested that at low temperatures the noncrystalline segments become immobilized in sites dictated by the crystallite orientation through the extensive hydrogen bonding known to exist in nylon 66.     

Solution crystallization of nylon 6

Electron microscopy and x-ray diffraction data have been obtained on nylon 6 which has been crystallized from solutions in 1,6-hexanediol and 1,2,6-hexanetriol. Lamellar single crystals and spherulites of the γ form are obtained by crystallization from 1,2,6-hexanetriol. The morphology of the single crystals is different from that obtained from glycerine solutions. The spherulites of the γ form are composed of larger lamellae. Sheaflike crystals of the α form are obtained from both solvents. α-form and γ-form crystals both grow from 1,2,6-hexanetriol at appropriate crystallization temperatures. α-form crystals alone are obtained from 1,6-hexanediol solution at every crystallization temperature. The long periods measured by small-angle x-ray diffraction for the solution-grown crystals are in the range 56 to 66 Å. The melting behavior of the solution-grown crystals is examined and discussed. Effects of solvent on growth of the two crystalline forms from solution are investigated.     

Annealing of melt-crystallized nylon 6

Effect of annealing on thermal behaviour and crystalline structure of meltcrystallized nylon 6 has been investigated.The annealing process is found to be characterized by an incubation period followed by a more or less doubling of the SAXS long spacing and of the crystallinity.The extrapolated heat of melting of the crystalline phase of nylon 6 in the-modification is 188 Jg^–1 and its extrapolated equilibrium melting temperature is 260 °C.Presented in part at 28th IUPAC Symposium on Macromolecules, Strasbourg, July, 1981.     

Strain‐rate dependence of the microstructure and mechanical properties for hot‐air‐drawn nylon 6 fibers

A hot‐air (HA) drawing method was applied to nylon 6 fibers to improve their mechanical properties and to study the effect of the strain rate in the HA drawing on their mechanical properties and microstructure. The HA drawing was carried out by the HA, controlled at a constant temperature, being blown against an original nylon 6 fiber connected to a weight. As the HA blew against the fiber at a flow rate of 90 liter/min, the fiber elongated instantaneously at strain rates ranging from 9.1 to 17.4 s⁻¹. The strain rate in the HA drawing increased with increasing drawing temperature and applied tension. When the HA drawing was carried out at a drawing temperature of 240 °C under an applied tension of 34.6 MPa, the strain rate was at its highest value, 17.4 s⁻¹. The draw ratio, birefringence, crystallite orientation factor, and mechanical properties increased as the strain rate increased. The fiber drawn at the highest strain rate had a birefringence of 0.063, a degree of crystallinity of 47%, and a dynamic storage modulus of 20 GPa at 25 °C. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1137–1145, 2000     

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.